JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 102, NO. D24, PAGES 28,731-28,769, DECEMBER
26, 1997
BOREAS in 1997: Experiment overview, scientific results, and future
directions
Piers J. Sellers,• Forrest G. Hall, • Robert D. Kelly,2 Andrew Black,3 Dennis Baldocchi,4 Joe Berry,s Michael Ryan,6 K. Jon Ranson,• Patrick M. Crill, 7 Dennis P. Lettenmaier,s Hank Margolis,9 Josef Cihlar,•ø Jeffrey Newcomer,• David Fitzjarrald,TMPaul G. Jarvis,•2 Stith T. Gower,•3 David Halliwell,TMDarrel Williams,• Barry Goodison,•s Diane E. Wickland,•6 and Florian E. Guertin •ø Abstract. The goal of the Boreal Ecosystem-Atmosphere Study(BOREAS) is to improve our understandingof the interactionsbetween the boreal forest biome and the atmospherein order to clarify their roles in global change.This overviewpaper describes the sciencebackgroundand motivationsfor BOREAS and the experimentaldesignand operationsof the BOREAS 1994 and BOREAS 1996 field years.The findingsof the 83 papersin thisjournal specialissueare reviewed.In section7, important scientificresults of the project to date are summarizedand future researchdirectionsare identified. The magnitudesand dynamicsof these exchangesand many of their controllingprocessesare poorly understoodfor most of Persuasivearguments indicate that there will be global the Earth's ecosystems.The boreal forest is a good example: warmingresultingfrom the continuingincreasein atmospheric this ecosystemencircles the Earth above 48øN, is second in CO2 concentration[Houghtonet al., 1995;Hasselmann,1997]. areal extent only to the world's tropical forests,occupiesabout However, there are uncertainties about the magnitude and 21% of the forested land surface[Whittakerand Likins, 1975], regionalpatternsof projectedglobalchangebecauseof short- and contains 13% of the carbon stored in biomass and 43% of comings in the atmospheric general circulation models the carbon stored in soil [Schlesinger,1991]. Several climate (AGCMs) usedfor climatesimulation.There is a real need to model projectionsindicatethat the greatestwarmingwill occur improve (1) our understandingof basicclimatic physicaland at these high latitudes (43ø-65øN) with the most marked efdynamic processesso that we can enhance the realism and fects within the continental interiors [Houghtonet al., 1995; accuracyof AGCMs and (2) our abilityto quantifyglobal-scale Mitchell, 1983; Schlesingerand Mitchell, 1987; Sellers et al., climate variablesand parametersto better initialize and vali- 1996a]. Large perturbationsin northern continental climates date models. Success in these two research areas should result are expectedto lead to changesin the carbon cycle and the in improvedclimate modelsand data sets,which in turn should ecologicalfunctioningof the boreal forest, which could feedprovide more credible and useful climate projections[Tren- back onto the globalclimate.Thus the sizeof the boreal forest, berth, 1992;Sellerset al., 1997]. its sensitivityto relatively small climatic variations, and its The exchangesof energy, water, and carbon between the importanceto the climate of the northern hemisphereand the atmosphereand the continentsrepresentthe lower boundary globalcarboncyclecombineto make it an important biome to condition for the atmosphericphysicalclimate systemand the understandand to representcorrectlyin global models. climatic forcing to terrestrial biota and biogeochemicalcycles. The Boreal Ecosystem-AtmosphereStudy (BOREAS) was initiated as a large-scaleinternationalinvestigationfocusedon •NASA GoddardSpaceFlightCenter,Greenbelt,Maryland. improving our understandingof the exchangesof radiative 2University of Wyoming,Laramie. energy,sensibleheat, water, CO2, and other radiatively active 3University of BritishColumbia,Vancouver, Canada. 4NationalOceanographic and AtmosphericAdministration,Oak trace gasesbetween the boreal forest and the lower atmosphere.A primary objectiveof BOREAS was to collect the Ridge, Tennessee. SCarnegie Institution,Stanford,California. data needed to improve computer simulation models of the "UnitedStatesDepartmentof Agriculture,Fort Collins,Colorado. important processescontrolling these exchangesso that the 7University of New Hampshire,Durham. effects of global change on the biome can be anticipated, in "Universityof Washington, Seattle. '•Centrede Recherchcen BiologieForestigre,Sainte-Foy,Quebec, particularthe effectsof altered temperatureand precipitation, Canada. as well as providingAGCMs with better land surfaceprocess •øCanada Centerfor RemoteSensing, Ottawa. submodelsand data setsfor the boreal zone. The field phaseof • •Atmospheric Sciences Resource Center,Albany,New York. the experimentextendedfrom 1993 to 1997 and included two •2University of Edinburgh, Edinburgh, Scotland. seriesof intensivefield campaignsin 1994 and 1996. Monitor•3University of Wisconsin, Madison. •4Forestry Canada,Edmonton,Alberta,Canada. ing of the BOREAS studyareawill continueon a reducedbasis •5Atmospheric Environment Service,Downsview, Ontario,Canada. through at least 2001 and someinfrastructuremay be retained •'NASA Headquarters, Washington, D.C. for possiblefollow-up studies.This journal specialissuebrings Copyright 1997 by the American GeophysicalUnion. together resultsfrom field work conductedin 1994, and to a lesserextentin 1996,and somepreliminaryfindingsfrom modPaper number 97JD03300. 0148-0227/97/97J D-03300509.00 eling studies.A 2-year program of follow-on analysesof the 1.
Introduction
..
28,731
28,732
SELLERS ET AL.: BOREAS IN 1997
PHYSICAL CLIMATE
SYSTE•M BIOPHYS•
B•OGEO••
Hy•mlogy I X •
/
I
•bpn
Temperature,
ECOSYSTEM STRUCTURE
AND FUNCTION
Species Composition,Structure Pedology
Figure 1. Important interactionsbetween the boreal forest and the atmospherewith respectto global change.(a) Influence of changesin the physicalclimate systemon biophysical processes.These may feedback to the atmospherethrough changesin energy,heat,water, and CO2 exchange;(b) changes in nutrient cyclingrates;releaseof CO2 and CH 4 from the soil carbonpool back to the atmosphere;(c) changesin biogeochemicalprocessesand water and nutrient availabilityinfluencecommunitycompositionand structure;(d) changein species composition results in changes in surface biophysical characteristicsand biogeochemicalprocessrates.
BOREAS
data set will be initiated
in late 1997. Other
work
a lengtheningof the approximately150-daygrowingseasonby about 6 daysat higherlatitudes.Keelinget al.'s [1996] resulthas recently been supportedby analysisof a global satellite data record which suggeststhat the growing seasonand photosynthetic activity increasedover large areas of Europe, northern Eurasia, Alaska, and Canada for the same period [Myneni et al., 1997]. The land surfaceparameterizations(LSPs) and surfaceparameter setsusedin AGCMs have improvedconsiderablyover the last decade [Sellerset al., 1997]. Climate modelsuse many of the same formulations,submodels,and parametersas numericalweatherprediction(NWP) models;the latter alsobenefit from a continuousprocessof operationalverification.Recent experiencehas shownthat the resultsof large-scalefield experimentsare usuallytransferredfirst to NWP models[Betts et al., 1993, 1996, 1997, 1998] and later to climate models [Sellerset al., 1995a, 1997]. In any case,if a processsubmodel is responsiblefor systematicerrors in an NWP model, it will certainlylead to even larger biasesin a climate model which is not reinitialized at short time intervals.Analyseshave demonstrated that until very recently, even the best NWP models consistentlyoverpredictedthe evaporationrates and specified unrealistic winter albedo fields over the boreal region with seriousconsequences for forecastingskill [Sellera' et al., 1995b; Bettsand Ball, this issue].The reasonsfor these errors were directlyconnectedto misrepresentationsof important biophysical processesin LSPs,for example,controlson evapotranspiration, and inaccuraciesin specifyingmodel parameters,such asthe extent,type, and densityof forestbiomes.The main aims of the physicalclimate systemcomponentof BOREAS were therefore to provide data which could be used (1) to improve and validate LSPs and (2) to enhancemethodsfor deriving parameterfieldsfrom satelliteobservations, which are the only feasible,consistentmeansof providingglobal surfacedata sets. NWP modelshave alreadybenefitedfrom this work, and climate modelsare expectedto benefit in due course.
from BOREAS has been published elsewhere,notably in a specialissueof TreePhysiology [Margolisand Ryan, 1997].This 2.2. Carbon and Biogeochemistry overviewpaper describesthe BOREAS experiment,reviews The physicalclimatesystemis stronglycoupledto the global the constituentpapers of the special issue,summarizesthe important resultsof work done to date, and identifiesfuture carbon cycle. Temperature and precipitation anomalieshave research directions. been comparedwith seasonalvariations in atmosphericCO2 concentrationand isotopic analysesto show that warm years over the northern
2.
Science Background
continents
are associated
with a net terres-
trial carbon sink, while cold and/or dry years are associated The interactions between the boreal forest and the atmowith a net sourceof terrestrialcarbon[Keelinget al., 1995;Ciais spherecan be groupedinto physicalclimate systemprocesses, et al., 1995; Denninget al., 1995; Tans et al., 1990]. Together, carbonand biogeochemistry, and ecology(seeFigure 1). These theserecent studiessuggestthat the northern continentsacted are discussedin turn below, with particular emphasison the as a large sink for atmosphericcarbon,an averageof 1-2 Gt C yr-• overthe 1980s,or about15 to 30% of the anthropogenic issuesaddressedby BOREAS. CO2 flux from fossilfuel burning.The exactbiophysicalmech2.1. Physical Climate System anismsresponsiblefor this sink are unclear, although the hyAt the present rate of increase, atmosphericCO2 concen- pothesizedlengtheningof the growingseasoncouldbe a factor tration will double before the end of the next century [Hough- [Keelinget al., 1996;Myneni et al., 1997]. The boreal ecosystem ton et al., 1995]. Climate simulationsmade with AGCMs point is vast,between12 millionkm2 [Whittaker andLikins,1975] to large temperature increasesin the northern high-latitude and 20 millionkm2;the latter figurewe computefrom the continentalinteriors [Schlesinger and Mitchell, 1987;Houghton classification of DeFriesand Townshend[1994],aspublishedby et al., 1995; Sellers et al., 1996a], partly due to projected Meesonet al. [1995] and describedby Sellerset al. [1996b]. changesin the polar seaice climatologyand snow-albedofeed- Simple arithmetic implies then that on average,only 50-80 g backs.A significantwarming trend was observedin the boreal C m-2 yr-• needbe sequestered to accountfor a 1 Gt C yr- ' zone during the 1980s and early 1990s, reaching 1.25øCper global sink. As will be seen later in this paper, sucha number decade within the Canadian interior [Chapman and Walsh, is well within the range of annual carbon uptake values esti1993].Keelinget al. [1996] analyzedtime seriesof atmospheric mated from eddy correlation data acquired at the BOREAS CO2 concentrationsto showthat this warming may have led to tower sites. However, to extrapolate these measurementsto
SELLERS ET AL.: BOREAS IN 1997
the entire boreal zone, or into the future from a coupleof years worth of data, necessitatesa deeper understandingof the climatologicaland physiologicalprocesses controllingcarbonuptake, respiration,and fire frequency,how suchprocessesdepend on soil and land cover type, and how they might be affectedby change.From the outsetan important objectiveof BOREAS was to acquire the data needed to improve terrestrial carbonmodelsfor the boreal region. In particular, data are needed to improve our understandingof the dependence of carbonfluxeson physicalclimatevariationsand to develop methodsfor extractinguseful parametersfrom satellite data. 2.3.
Ecology
Carbon sequestrationin the boreal ecosystemamountsto the relatively small difference between gains from photosynthesisand lossesdue to respirationin the plants, roots, and soils.For roughlythe past 8000 years after the last glaciation, the boreal ecosystemhasbeen accumulatingcarbonin its soils, particularlyin deep layersof organicpeat where soil organic matter
accumulates
under water-saturated
conditions.
Harden
et al. [1992]placehistoricalcarbonaccumulationratesin these
peatsoilsin the rangeof 10-50 g C m2 yr-• throughsurface mossproduction,fine root turnover,and litter fall. On shorter timescales,primary carbonstoragemechanismsappearto be in abovegroundstandingbiomass.The progressivewarming that occurredduring the 1980sand early 1990scould have altered rates of photosynthesis, soil respiration,and fire frequencyin the region.In additionto drivingchangesin the ecophysiology of the biome, continuedwarming could eventually alter the spatial structure of the boreal ecosystem.There have been several attempts to map the future extent of the northern biomesbasedon the projectedwarmingand dryingregimedue to a "doubled-CO2" climate. Some of these suggestthat the North American boreal forestwould move north and perhaps split into two halves:one in Alaska and the CanadianNorthwest and the other in the Canadian Northeast [Rizzo and Wiken, 1992]. Such changesmay themselveshave significant feedbackson the climate systemthrough changesin winter albedosand energyfluxesover the altered land surfaces. The earliestdiscussions about BOREAS recognizedthe importance of the issuesreviewed above; they also recognized how these issuesare mutually dependent and that an integrated experimentalstrategythat addressedall three disciplinesofferedthe mostprofitablescientificapproach(seeFigure 1). BOREAS, however, has served to elaborate and sharpenthe detailed questionsrelated to these issuesand has provided partial resolutionto many of them.
3.
Experimental Objectives
BOREAS was designedto bridge a wide range of spatial scalesbecausesomeof the important governingprocessescan only be studied at very small spatial scales(e.g., the links amongleaf biochemistry,spectralproperties,and photosynthesis),but ultimately,the scientificgainsmustbe appliedwithin the contextof AGCMs, carboncyclemodels,or similar largescalestudies.A multi-scale-nested designwasdevelopedwhich permitsknowledgeat one scaleto be translatedand compared to that obtainedor inferred at different scales(see Plate 1). The governingobjectivesof BOREAS canbe statedasfollows: 1. Improve the processmodelsthat describethe exchanges of radiativeenergy,water, heat, carbon,and trace constituents between the boreal forest and the atmosphere.The approach
28,733
herewasto measurethe fluxesof energy(radiation,heat) and mass(water, CO2, and radiativelyimportanttrace gases)at severalscalesalong with observationsof the ecological,biogeochemical,and atmosphericconditions controlling them. Data collectionschemeswere designedto support development and testingof processmodelsthat will be applied to the global change issuesdescribedabove. The field observations that support processmodel development include measurementsof water, CO2, and trace gasfluxesat the small plot or leaf scale (chambers,porometers),the stand scale (towermountededdy correlationand profile instrumentation),and the mesoscaleand regionalscales(airborne eddy correlation; meteorologicalmeasurementsand analyses).At the smaller length scales,these measurementswere coordinated with a seriesof ecological,meteorological,and edaphicobservations to link thesefluxesto appropriatestatevariables(seePlate 1). 2. Develop methodsfor applyingthe processmodelsover large spatialscalesusingremote sensingand other integrative modelingtechniques.The processstudiesdescribedin objective 1 abovewere coordinatedwith remote sensinginvestigations using satellite, airborne, and surface-basedinstruments that focus on methodsfor quantifyingcritical state variables. These remote sensingstudies,combinedwith mesoscalemeteorologicalstudies,will allow us to scaleup and applyprocess modelsto regional and ultimately global scales.Some largescalevalidation techniqueswere incorporatedin the experiment designto test scale-integrationmethods directly; these techniquesincluded airborne flux and profile measurements, meteorologicalobservations,and modeling.
4.
Experiment Design and Resources
The objectivesof BOREAS relate to two spatial scalesthat had to be reconciledwithin the experimentdesign.The primary focusof objective1 is mosteasilyaddressedby local-scale (a few centimetersto a few kilometers)processstudiesthat involve detailed, coordinated in situ observations.These local-
scalestudieshad to be connectedto the larger-scalemeasurement and analysistools associatedwith objective2, which was directedtoward definingregional-scale(10-1000 km) fluxes and states.In BOREAS, as in previousfield experimentssuch as the First ISLSCP (International SatelliteLand SurfaceClimatology Project) Field Experiment (FIFE) [Sellerset al., 1992a; Hall and Sellers,1995] and the Hydrological AtmosphericPilot Experiment-Sahel,HAPEX-Sahel [Goutorbeet al., 1994], the scienceteam adopted a nestedmultiscalemeasurementstrategyto integrate observationsand processmodels over the scalerange, as shownin Plate 1. The resolutionof global AGCMs, of the order of hundreds of kilometers,defined the largestspatialdomain in BOREAS, the 1000 x 1000 km BOREAS region shownin Plate 1. This was the domain for meteorologicaland satellite data acquisition and large-scalemodeling.Sinceit is impractical,evenwith aircraft, to measure surface-atmospherefluxes over areas larger than a few tens of kilometerson a side,another scaleof investigationwasset at about50 x 50 km whichcorrespondsto the two BOREAS studyareas (Plate 1 and Figure 2). These two study areas were placed near the northern and southern ecotonesof the biome in order to study important processes associatedwith the controlling factors (temperature in the north, moisturein the south)whichare mostlikely to undergo significantchangewithin the biome as a whole. The northern study area (NSA) and the southernstudy area (SSA) were
28,734
SELLERS ET AL.' BOREAS IN 1997
Region 1000 x 1000km Satellites
Remote Sensing Aircraft Large-Scalemodels
Study Area ~ 100 x 50km
Modeling Subarea 40 x 40kin
Flux Aircraft Local Area Models
Tower Flux Measurements
Hydrology
Tower
Flux Site
~1 xlkm
PhysiologicalStudies Radiometric
Measurements Trace Gas Measurements Soil Processes
Process Study Site ~ 1 m2 ~ 100m2
Plate 1. Multiscale measurementstrategyused in BOREAS; see text.
located near Thompson, Manitoba, and Prince Albert, Saskatchewan,respectively.The studyareasare about 500 km apart, sufficientlyfar apart to resolvethe ecologicalgradient while permitting the routine ferry of research aircraft and people between them. The studyareas themselveswere small enoughto be characterizedby usingsurfaceand aircraft measurements,yet large enough to test scalinghypothesesusing models and remote sensingimagery. Within each study area, water vapor, heat, and CO2 fluxeswere measuredat the local
scale(about 1 km) usingeddycorrelationequipmentmounted on "flux" towers(five in the NSA, six in the SSA) whichwere located in patches of relatively homogenousvegetation and soils. Three boreal forest dominants were characterized by thesetower flux (TF) measurements: black spruce,jack pine, and aspen.Two wetland fens, one in the NSA and one in the SSA, and beaver ponds in the NSA and SSA were equipped with smaller towers (Figure 2). The tower-basedmeasurementshavebeen comparedwith aircraftflux data (up one scale level) and with the biophysicalcharacteristics of the constituent vegetationin the patch as definedby small-scalemeasurements made at small plots (down one scalelevel). Because thesefew TF sitescould not capture the range of variability in the region,an additional70 auxiliarysites(marked ascategory 3 sitesin Figure 2) were selected,stratified by speciestype, productivity,and age. In these small (approximately100 x 100 m) sites,biometry and optical techniqueswere used to measurebiomassdensity,leaf area index, net primary productivity (NPP), litter fall, etc.While processmodelswill be tuned
by usingprimarily the tower flux data, the auxiliarysitescan be used as independenttest sitesfor processmodel and remote sensingalgorithm validation. Some embeddedplots (marked as category2 auxiliarysites in Figure 2) were the focus of in situ measurement
work
which measured
the variables
needed
to drive stand-levelecophysiological models,for example,sitelevel biomass, leaf photosyntheticrate, litter decomposition and quality, soil moisture,and temperature.Trace gas studies were conducted in both study areas with some specialized investigationsin the NSA where somebeaverpond sitesand a collapsedpalsa were instrumentedto measuremethane flux. (Palsasare commonfeaturesin the boreal landscape;they are shallowdepressionsin the land surfacecausedby melting of subsurfacepermafrostand subsequentcollapseof the surface. Usually, palsashave high water tables which support typical bog vegetation cover such as sphagnummoss, sedges,bog birch, etc.) A remote sensingscienceprogram was implemented (see Table 4 and Plate 2) to characterizesurfacecomponentoptical propertiesfrom the leaf level(usinglaboratoryspectroradiometers),to the canopyand standlevel (usingtowerand helicopter-mounted spectrometers),to the study area and regional level (using aircraft and satellite platforms). Microwave scattering properties were determined at the canopy level using airborne radiometersand at the studyarea and regionallevels using aircraft and satellite-basedmeasurements. A large multidisciplinaryteam of scientistswas needed to cover the measurement and modeling requirements of the
SELLERS ET AL.: BOREAS IN 199'7
(a)
97 ø 30'
98 ø 00'
98ø430 '
28,'735
56 ø 15'
• MODELING • TUDY AREA • HARD SURF. SUB-AREA
U6W5S
U5W5S
NSA-Fen
NSA-OJP •T7R9S
56 ø 00'
ROADS
[• LOOSE SURF.
p NSA-BP
(ALL WEATHER)
"•1 LOOSE SURF • TRAIL OR r• TOWER SITES -• AUX. SITES C• AUX SITES • PROFILER, RADAR, (DRY WEATHER) CART TRACK
•11TOWlS T3U9S
(CATEGORY 1)
(TE TOWER)
T5Q7SNSA-OBS
,
(CATEGORY 2)
•.-.•.
(CATEGORY 3)
R8V8A
OR LIDAR
55• 45'
LAKES 20
KILOMETERS
570.000
Additional sites exist
alongthe transectand may not appear on this map
N
(b)
• 104ø30'
1
106 ø
54 ø 15'
!
+ % ß
I
ROADS
I H3DIM
(ALL WEATHER)
-OBS H2DlS
54ø
• MODELING • STUDY AREA • HARD SURF [• LOOSE SURF. /•1 LOOSE SURF. r• TRAIL OR •--• TOWER SITES SUB-AREA
I
+
(TETOWER)
(DRY WEATHER) CART TRACK
SSA-YJP
(CATEGORY 1)
"•r• ß
Aux SITES (CATEGORY 2) AUX.
SITES
(CATEGORY 3)
53 ø 45'
•
'PROFILER, RADAR, OR LIDAR
+
½•71• LAKES
Additional sites exist
alongthe transectand to the southof this study area.
Figure2. Mapsof (a) thenorthern(NSA) and(b) southern (SSA)studyareasof BOREAS.Towerflux(TF) sites,auxiliarysites,andmodelingsubareas are marked.TF sitesare identifiedby a studyareaprefix(NSA or SSA)anda vegetation-type identifier(OA, old aspen;YA, youngaspen;OBS,oldblackspruce; OJP,old jackpine;YJP,youngjackpine;FEN, fen;BP,beaverpond).Auxiliarysites(category 2) wereusedfor carbon modelingstudies;category3 siteswereprimarilyusedfor remotesensing validationwork.
28,736
SELLERS
ET AL.: BOREAS
IN 1997
project. In 1992, 85 science teams were selected from 229 proposalsand other solicitationsto take part in BOREAS. Most U.S. universityinvestigatorswere funded by either the National Aeronauticsand SpaceAdministration(NASA), the National Oceanographic and Atmospheric Administration
facilitate accessto the forest canopyfor Icaffbranchchamber
(NOAA), or the National ScienceFoundation(NSF). Canadian universityinvestigatorswere funded through a collaborative specialproject grant from the Natural Sciencesand Engineering Research Council (NSERC) and Canadian governmentinvestigatorsby their respectiveagencies.There was also significantparticipation by scientistsand research organizationsfrom the United Kingdom, France, and Japan. The individual projects were organized into six disciplinary groupsto facilitate coordinationduring the field phase. The objectivesof these six sciencegroupsare summarizedbelow and the namesof individualprincipal investigatorsare listed in
techniquesto characterizethe flux of trace gasesbetweenthe soil and the atmosphere, including CO2, CH 4 and nonmethanehydrocarbons(NMHCs). The TGB group alsostudied the long-term accumulationof carbon in boreal soils.
Table
4.1.
1.
Airborne Fluxes and Meteorology (AFM)
Four aircraft were used to measure turbulent fluxes; sound-
ing lidars and radars were also deployed (see Figure 3 and Table 2). One other aircraftwasusedfor operationssupportin 1994 and 1996 and for CO2 concentration measurement work in 1996. Ten meteorological stations and a dense array of upper air radiosoundingstationsoperatedover the regionduring 1994,with their data being transmittedto operationalmeteorological centers for assimilationvia the Global TelecommunicationsSystem.Severalinvestigators,includingsomewith close links to these centers,are using these data to improve mesoscaleand global-scaleatmosphericmodels.The surface network, a subsetof the aircraft, and a few of the upper air stationsalso operated during 1996. 4.2.
Tower Fluxes (TF)
The TF group'sprimary objectivewas to quantify the turbulent exchangesof energyand massbetweenthe atmosphere and a variety of boreal forest surfacecoversand to investigate the processes controllingthesefluxes.This work was designcd to be complementaryto chamberobservations and other process-oriented studieson smallerscales(TE, TGB) and aircraft studies (AFM, RSS) covering larger scales.The TF towers operated almost continuouslyduring the growing seasonof 1994,measuringradiation, heat, water, CO2, and in somecases CH 4 and other trace gasfluxes;see Figure 2 for their location. Two of the sites,one in the NSA and one in the SSA, operated more-or-less continuously from the fall of 1993 onward to characterizethe annual cycleof the energyfluxcs.Six TF sites operated during the growingseasonin 1996;see Figure 3 for the operating schedules. 4.3.
Terrestrial Ecology (TE)
Over 20 teamsexaminedthe biophysicalcontrolson carbon, nutrient, water, and energyfluxesfor the major ecosystems in the boreal landscapeand are developingmodels and algorithms to scale chamber measurementsto stand, landscape, and regionalscales.Much of the work wascarriedout at plots within the footprints of the TF sites, i.e., within the surface zonesthat contributefluxesmeasuredby the TF instrumentation, typically within a 200 m radius of the TF tower. An important focus for the TE group was the measurementof carbon cycle components.A number of free-standingtowers (TE towersin Figure 2) were installedin the studyareas to
measurements
4.4.
in situ work.
Trace Gas Biogeochemistry (TGB)
Ten
4.5.
and other
TGB
teams
used chamber
measurements
and other
Hydrology (HYD)
The HYD group consistedof eight teams. Five focusedon the measurementof snowhydrologycomponentsto support remote sensingalgorithm development;two worked on catchment hydrologicalprocessesin the SSA and NSA usingprecipitationgage networks,streamgages,and a rain radar; and one team operated a program of almost continuoussoil moisture measurementsat the TF sites during the 1994 and 1996 growingseasons. 4.6.
Remote Sensing Science (RSS)
The RSS group developcd linkagesbetween optical and microwaveremotesensingsignaturesand borealzonebiophysicalparametersat scalesthat includeleaf,canopy,andregional levelsusingficld, aircraft and satellite-bornescnsors,and radiativetransfermodels.The TE and RSS groupscollaborated in gatheringa wide range of biomctric and radiometric data at the auxiliarysites(Figure 2). Severalremote sensingaircraft were deployed in BOREAS 1994 and BOREAS 1996; see Table 4 and Figure 3. 4.7.
Staff
The scienceteamswere supportedby a staff of scientistsand supportcontractorsfrom the National Aeronauticsand Space Administration (NASA); the Atmospheric Environment Services(AES), Canada;the CanadaCenter for RcmotcScnsing (CCRS); ParksCanada;the Schoolof Forestry,Laval University; the Schoolof Forestry, University of Wisconsin;and the Canadian Forestry Service. The BOREAS staff ovcrsawthe componentsof the project which required significantlogistical effort, extendedand/or routine monitoringwork, or work that required the particular expertiseand resourcesof onc of thc participatingagencies.In the early stagesof the project, the staff carried
out the detailed
site reconnaissance
work
and
constructionplanning. During the field seasons,the staff dealt with the organizationof the tield logisticsand the day-to-day managementof field operations.The staffmonitoringprogram includedautomaticmeteorologicalstationnetwork;upper air network; hydrology,snow, and soil moisture; auxiliary site work; biometry and allomctry; radiometric calibration: stan-
dardgasesand gascalibration;thermalradianceintercomparison;global positioningsystem(¸PS) facilities.
The NASA staffwcrc alsoresponsible fi•r implementingthe BOREAS InformationSystem(BORIS) whichservesasa data organization,distribution,and short-termarchivingcenter for the project.All in all. some3(1(1 peoplewcrc workingwithin or abovethe studyareasin BOREAS 1994and around 15()people participated in BOREAS 1996.
5.
Experiment Execution
Many of the BOREAS measurementswcrc taken continuously;for examplc,the 1{)metc()rologicalstationstook 15-min
SELLERS ET AL.: BOREAS IN 1997
28,737
Table 1. PrincipalInvestigatorsand TasksAssociatedWith Each BOREAS ScienceGroup Aircraft
Sounding/Networks
Models
AirborneFluxesand Meteorology(AFM)," 14 Teams AFM-
1 Crawford
AFM-2 Kelly AFM-3
Lenschow
AFM-5
Atkinson
AFM-6 AFM-7
Martner Shewchuk
AFM-4 MacPherson/Desjardins
AFM-8 AFM-9 AFM-11 AFM-12
Betts
Dickinson Mahrt Pielke
AFM-13 Schuepp AFM-14
Sellers
AFM-15 Verseghy Vegetation Type
SSA
NSA
TowerFluxes(TF),• 1! Teams Old aspen(OA) Old black spruce(OBS)
TF-1 Black.... TF-2 den HaGtog TF-7 Desjardins TF-9
Old jack pine (OJP) Young jack pine (YJP) Fen (FEN) Soils, Forest Floor, Wetlands TE- 1 Anderson
.-. TF-3 Wofsy*
Jarvis
TF-5 Baldocchi TF-4 Anderson TF-11 Verma
TF-8 Fitzjarrald TF-10 Jelinsky/McCaughey TF-11 Jelinsky/McCaughey
EGophysiologyand Ground Carbon TerrestrialEcosystems(TE), c 22 Teams TE-2 Ryan TE-4 Berry TE-5 Ehleringer/Flanagan TE-6
Gower
Models
TE-13 Apps TE-14 Bonan TE-15 Bukata TE-16
Cihlar
TE-7 Hogg
TE-17 Goward
TE-8
TE-18
Kharuk
Hall
TE-9 Margolis
TE-19 HaGriss
TE-10
TE-20
Middleton
TE-11 Saugier TE-12 Walter-Shea
Knox
TE-21 Running TE-22 Shugart TE-23
Methane
Rich
Isotopes,Pesticides
NMHCs
TraceGasBiogeochemistry (TGB),J 10 Teams TGB-1 TGB-3 TGB-4
Grill Moore Roulet
TGB-6 TGB-7
Wahlen Waite
HYD-1
Cucnca
TGB-9
Niki
TGB-10 Westburg TGB-12
TGB-5 Zepp
Soil Moisture
TGB-8Monson
SnowProcesses, Remote Sensing Snowand Hydrology(HYD)," 8 Teams HYD-2 Chang HYD-3
Davis
HYD-4
Goodison
Trumbore
Hydrological Modeling HYD-8 HYD-9
Band Soulis
HYD-5 Harding HYD-6
Optical
Peck
Microwave
Algorithms/Modeling
RemoteSensing Science (RSS),t 20 Teams RSS-1 Deering
RSS-13 Gogenini
RSS-4 Curran
RSS-2 Irons RSS-3 Walthall
RSS-15 RSS-16
RSS-5 Goel RSS-6 Williams
RSS-10 Holben RSS-11 Markham RSS-12 Wrigley
RSS- 17 Way
RSS-14
Ranson Saatchi
Smith
RSS-7 Chen RSS-8 Running RSS-9 Strome RSS-18 RSS-19
Green Miller
RSS-20
Vanderbilt
Staff sciencesupport:infrastructureinstallationand maintenance,calibration of radiometric instruments,aircraft operations,satellite data acquisitionprogram,BOREAS Information System(BORIS), operationsmanagement. 'lAirborneeddycorrelationprofiling(5 aircraft), surfacemeteorologicalnetwork(10 stations),upper atmospheresoundings(radiosondes), lower atmosphereprofiling,modelingand analysis.
bTwolong-termtowers:*heat,H20, CO2,sometracegases; eightgrowingseason towers:heat,H20, CO2,sometracegases. CPorometryand chamberphotosynthesis, canopyand soil respiration,biometry,nutrient cycling,modeling.
dTracegaschambermeasurements including methaneandnonmethane hydrocarbons (NMHCs);studies of fire scars, beaverponds,soiland moisture gradients;isotopes. CSnowprocessesand snowremote sensing;catchmenthydrology;soil moisture, canopyinterception;hydrologicalmodeling.
•Opticalremotesensing (four aircraft),microwave/radar/gamma remotesensing (four aircraft),surface-based remotesensing studies,algorithm development.
28,738
[•[ [[ [•[
SELLERS
[ ,[ [
,-[
, ,
[ ,
[ [
ET AL.' BOREAS
IN 1997
SELLERS ET AL.: BOREAS IN 1997 Table
2.
28,739
Remote Sensing(R-Prefix) and Flux Measurement(F-Prefix) Aircraft Deployed in BOREAS
Aircraft
Type ER-2
BOREAS Identifier RE
Science Team RSS-18 HYD-2
Home
Primary Target/Role
Equipment
RemoteSensingAircraft (R-Prefixon BOREAS Identifier) airborne visible infrared imaging vegetationproperties spectrometer(AVIRIS) MODIS airborne simulator (MAS) snow
Institution
NASA
Ames
Research
Center
in FFC-W C-130
RC
RSS-2 staff
advancedsolid-statearray spectroradiometer(ASAS) thematic mapper simulator(TMS), MODIS
RSS-20
Piper Chieftain
RP
RSS-19
airborne
vegetation properties, SART
NASA
Ames
Research
Center
simulator
(MAS), airborne tracking Sun photometer (ATSP) Polarization and Directionality of Earth's Radiation (POLDER) compactairborne spectrographic imager (CASI)
vegetationproperties,SART
Ontario
Remote
SensingOffice, Canada
Helicopter (UH- 1)
RH
vegetation and surface properties
NASA Wallops Flight Facility
airborne syntheticaperture radar (AIRSAR); P, L, C-band; fully polarimetric airborne syntheticaperture radar (CCRS-SAR); X and C-band, polarimetric
vegetationand soil properties soil moisture
NASA Ames Center
vegetationand soil properties
Canada
microwave radiometers; 18, 37, and 92 GHz; H and V
snow
gamma ray equipment
snow and soil moisture
RSS-3, 13, 20
SpectronElectronics-590,Barnes
RSS-15, 16, 17
multiband modular radiometer, C-band scatterometcr, ATSP, POLDER
DC-8
CV-580
Twin Otter (DH-6)
RD
RV
RT
TE-16
HYD-2
soil moisture
RA
HYD-6
Electra
FE
Flux MeasurementAircraft (F-prefixon BOREAS Identifier) AFM-3 flux, H, LE, CO2, limited local but mainly regional flux atmosphericchetnistrylidar measurements,chemistry
FK
AFM-2
flux; H, LE, CO2
Twin Otter (DH-6)
FT
AFM-4
flux; H, LE, CO2, O•
Centre
for
Remote Sensing (CCRS) National
Research
Council, Canada
Aerocommander
Kingair
Research
local and regional scale (5600 km) flux measurements local and regional scale flux measurements
LongEZ
FL
AFM-1
flux; H, LE, CO2
local scale(5-60 kin) flux
PA-34
FB
AFM-14
CO2 conccntrations(1996 only)
profiles (0-10000'), regional gradients
NOAA National Weather Service
National
Center
for
Atmospheric Research, Colorado University of Wyoming National
Research
Council, Canada NOAA, Oak Ridge
measurements
NASA
GSFC
Scc also Figurc 3.
data from early in 1994 to the end of 1996, and satellitedata werecollectedregularlyoverthe sameperiod(Figure 3). Some flux towersran continuouslyand a few scienceteamswere in the field during the entire growingseasonsof 1994 and 1996, but for most investigationsinvolvingcomplexequipmentand moderate-sizeteams, it was not practical or necessaryto sustain a continuouspresencein the field, so special periods, intensivefield campaigns(IFCs), were definedfor coordinated
runningoperationsin 1994 [Sellerset al., 1994]. During IFC-93 and the five BOREAS 1994 field campaigns,operationswere coordinated out of two centers, one in the NSA and one in the
SSA. Each center was equipped with FM ground-to-ground and VHF ground-to-airradio links, telephones,fax machines, etc., to maintain real-time management of the airborne operations and related surfacework. Nightly meetings of the participating scientists,air crew, and managersanalyzedthe revisits. The IFCs, each of which lasted about 20 days, were sults of the day's completed operations and set up the next spacedto catch the major phenologicalevents,includinga day's activities. Weekly sciencesymposiaprovided a useful snow hydrology/RSScampaign in the winter, spring thaw, forum for sharing scientific results and reviewing recent green-up,peak greenness,and the beginningof senescence progress.This continuousexchangeof information and ideas betweenBOREAS staff and scienceteam memberspermitted (Figure 3). In August 1993, many of the BOREAS investigatorsand continuousrefinement of the experiment design and operastaff were in place in the studyareas as part of the "shake- tions and better integration of the various scientific compodown" intensivefield campaign,lFC-93. This 21-day IFC was nents in the project. The BOREAS 1994 fall field campaign, IFC-3, ended in used to test the experimentinfrastructureand investigator's instruments as well as to refine coordination and communicaSeptember 1994, and the first scienceworkshopwas held in tion procedures. All of theseexperiences were pooledto refine December of that year. On the basisof the resultsfrom early the four-volumeexperimentplan which servedas the basisfor analysespresentedat the workshop,it became apparent that
28,740 Table
SELLERS ET AL.: BOREAS IN 1997 3.
Organization of Papersin This SpecialIssue No. of
Overviewof Papers
Papers
Section6.1: Carbon-Water-Energy Fluxes small-scalefluxesand physiological measurements (chambersand enclosures) standand plot-levelfluxesand dynamics(flux towers) landscape-scale fluxesand surface-boundary layerinteractions(aircraft,sondes,profilers)
6.1.1 6.1.2 6.1.3
10 17 15
Section 6.2.' Trace Gas Fluxes 6.2.1
small-scalefluxes(chambersand enclosures) standand plot-levelfluxesand dynamics(flux towers)
6.2.2
Section6.3: Soil and SnowMoistureand Runoff point measurementsand modelingof soil moisture dynamics stand-levelsoil moistureand snowdynamics(modelsand measurements) landscape-scale precipitationand soil moisture(models,radar estimates
6.3.1 6.3.2 6.3.3
Section6.4: Remote SensingScience groundand aircraft measurements of biophysicaland opticalcharacteristics, and understoryand canopyreflectance radiative transfermodelsand algorithmdevelopment landscape-scale land coverand biophysicalcharacteristics algorithms radiation and atmosphericeffects
6.4.1
6.4.2 6.4.3 6.4.4
Each sectionis devotedto a specificsciencearea and is further subdividedby spatialscale.
there were a number of gapsin the 1993-1994 data set, and in addition the meteorologicalrecord showed1994 to be an unusualyear: a new recordhad been set for the longestfrost-free period at Prince Albert National Park, while the NSA had experiencedthe driest year on record. A full accountof the BOREAS 1994 field campaignsmay be found in the work of Sellerset al. [1995b]. During 1995 the decisionwas made to take additional field measurementsduring 1996 to fill the 1994 data gaps[Sellerset al., 1996c].The BOREAS 1996 campaignsinvolvedfewer aircraft and scientistson the groundthan BOREAS 1994 as the objectives were strictly defined by shortcomings in the BOREAS 1994 data sets and important findingsfrom early analyses.Particularlyimportant was the decisionto start TF measurements earlier (in March) andend later (in October)in 1996, comparedwith 1994, to better coverthe growingseason (Figure 3). This was done becausethe BOREAS 1994 model-
Table 4.
ing and measurementresultsindicatedthat variationsin spring thaw and fall temperatureswere critical in determininginterannualvariability in ecosystemproductivity.
6.
Overview of Papers in This Special Issue
The 83 papersin this specialissuewere contributedby the sciencegroupslisted in Table 1. The papershave been organized into four major sections(see Table 3). The first three sectionscoversurfaceenergyand massexchangeprocesses and the fourth final sectionis devotedto remote sensingscience. The findingsof thesepapersare summarizedbelow. 6.1.
Carbon-Water-Energy Fluxes
Carbon, water, and energy fluxes were measured in BOREAS usinga comprehensive rangeof techniquesranging from leaf-scaleobservations made with chambersand porom-
BOREAS RSS Ground, Aircraft, and Satellite Instruments With Measurements and Related Products Measurements
Products Ground-Based
PARABOLA
Sensors
Laboratory and field
3-bandbidirectionalradiance BRDF, spectralalbedo spectralradiancespectroradiometers understoryand tree componentreflectances
NASA
radar backscatter
DC-8/AIRSAR
NASA ER-2/AVIRIS,
spectralradiance
RemoteSensingAircraft/Sensor forest type, biomassdensity,canopymoisture forest type, canopymoisture,atmosphericproperties
MAS
Piper Chieftain/CASI
spectral radiance
BRDF
NASA
NASA helicopter
TMS, ASAS, MAS, POLDER MMR, SE-590, POLDER
Radar
C-band
spectralradiance,bidirectionalradianceBRDF, albedo,foresttype,FPAR spectralradiance BRDF, FPAR canopyscatteringprofiles
C-130
backscatter
AVHRR
Landsat SPOT
TM
ERS-1, 2 SAR JERS- 1 SAR Radarsat SIR-C/XSAR GOES
scatterometer
spectralreflectance,emittance spectralreflectance,emittance spectralreflectance
Remote SensingSatellites land cover, FPAR land cover,FPAR, LAI, biomassdensity land cover
radar backscatter radar backscatter radar backscatter
freeze/thaw
radar backscatter
land cover,biomassdensity PAR, albedo,downwellingirradiance
spectral radiance
land cover land cover
SELLERS ET AL.: BOREAS IN 1997
28,741
Leaf Chambers/Bags M•ddleton
et al.
Dang et al. (th•s •ssue; 1997) Flanagan et al. Flyan et al. Ryan et al. (1995) Sullivan et al. (1997) Saug•er et al. (1997)
Leaf spectra Middleton
et al.
Woody Respiration Ryan et al. Lavlgne and Ryan (1997)
Moss/Soil
Chambers
Normanet al.
Long Term Carbon
Rayment andJarvis
Turnover
Nakane et al.
Winston et al.
Harden et al.
Roots
Gouldenand Cnll(1997)
Ryanet al.
Steeleetal.(1997)
Trumbore and Harden
:: :::
Hardenet al.
Figure4. Schematic summarizing papers in section 6.1.1'small-scale fluxesandphysiology withchambers and enclosures.
the driersubplot,whichsuggests that near-surface andsurface decomposition ratescouldbe significantly enhancedundera warmer,drier climate.Winstonet al. [this issue],who worked mainlyin the NSA, extendedmanyof their CO2 fluxmeasure6.1.1. Small-scale fluxes and physiologywith chambers mentsthroughthe wintersof 1993-1994and 1994-1995.The of observed wintertimeCO2fluxesrangedfrom0.5 and enclosures. Six papersdescribesoil fluxesand four pa- magnitude to 1.0 g CO• m 2 d •. The lowestfluxeswerecorrelated with persdescribe lea[-andbranch-scale measurements (Figure4). the midwinterminimumtemperatures.Accumulatedover the A BOREAS specialissueof 77c½Physiology [MargeIls Ryan,1997]has1()papersthatrcporton a varietyof BOREAS longwinterperiod,thesefluxescontributean importantfrac-
ctcrs (section6.1.1), throughintermediate-scale studiesconductedwith fluxtowersandsupporting measurements (section 6.1.2),up to regional-scale studieswhichusedaircraft,meteorologicalarrays,andsatellites(sections 6.1.3and6.4.4).
ccophysiological studics; the resultsof someof thesearc dis- tion of the annual carbonbudget.The winter fluxesseem to agreewell with the eddycorrelationmeasurements takenat cussedhcrc for complctcncss. Norma•tel al. [this issue]comparedsoil CO2 fluxesmca- the northernblacksprucesite(NSA-OBS).Gouldenand Crill chambers at NSA-OBSto measure the surcdby sixdift'crcntchambermethodsas fieldedby partici- [19971usedautomated pantsin BOREAS(Figure5a). It wasdemonstrated thatsys- CO, fluxesfrom the feathermossand sphagnummosscovers tematic differences exist between the measurement methods. and deepersoilsbeneaththe blacksprucecanopy.Maximum occurredbetween5øCand 8øC,andmoss Adjustment factorsof bctwcen (1.8and1.4areneededto bring net photosynthesis wasestimatedto accountfor all the data into agreement.These differencesare actually and surfacenet photosynthesis uptakeand50 to 90% smallerthanwcrcanticipatedprior to the BOREAS intercom- between10and40% of wholeecosystem of the whole ecosystem respiration from above-canopy eddy parisonbasedon thedisparate techniques used.Biasesamong
the techniques wereconstantacrossthe rangeof fluxesmea- correlation measurements. Winston etal. [thisissue] reportthatthe •4Ccontentof the sured(1-8/•mol m 2s •).Rayment andJarvis[thisissue] used releases showed a progressive shiftto lighter,thatisolder, an "open"chamberthat maintainedan interiorpressure very CO,__ closeto externalatmospheric pressure, therebyreducingmass carbon as the winter wore on, which suggeststhat the source from recentlyfixed,modern flow effects on the soil GO, flux measurements. CO• fluxes materialfor the CO2fluxchanged locateddeeperin were measuredmore or lesscontinuouslyfrom a subploton carbonto olderorganicmaterial,presumably the floorof a blackspruceforest(SSA-OBS)overa periodof the soil.Hardenet al. [thisissue]alsousedisotopesto invesof carbonby surfacepeatsandmosses about 3 months in 1994; the fluxeswere found to be strongly tigatethe sequestering correlatedwith soiltemperatureat 5 cm depth.Nakaneet al. near the NSA-OBS site' these two surfacetypeshave sequesof 40-60 g C m-2 yr-• overthelast90years, [thisissue]measured soilCO,__ fluxesat wetanddrysubplots at teredanaverage --2 in deepersoilsreleased20-50 g C m the sameblacksprucesiteasRaymerit andJarvis[thisissue]. andnet decomposition Theyalsomeasured carbonstorage components (litterfall,soil yr • overthe sameperiodleadingto an averagenet exchange --2 carbon,etc.).Thedrierplothaddecomposition ratesthatwere of soilcarbonwithin the rangeof + 10 (efflux) to -50 g C m roughly doublethosefromthewetplot,presumably because of d • (uptake)over the last century.Hardenet al. [thisissue] differences aredueto (1) therateof the muchwarmersoiltemperatures and aerobicconditionsat pointoutthatsite-to-site _
28,742
SELLERS ET AL.: BOREAS IN 1997
.o
o•
o
o
decompositionof surfacepeats,(2) ratesand methodof lateral spreadof moss,and (3) the historyand rate of decomposition in the deeper soil layers.Sitescan therefore vacillatebetween sourceand sink statusbased on climate variability. The primary control on carbon accumulationin peats and moss is drainage,while deep carbonstorageseemsto be a functionof respiration,decomposition,drainage,and fire history. The isotopicrecord found in soil organicmatter has been usefulin determiningdecadal-scalecarbonaccumulationrates in the NSA soils. Analyses of soil carbon and its isotopic compositionrevealeddifferencesin carbondynamicsbetween sitesin the NSA [Trumboreand Harden, this issue].The jack pine siteshad the lowestsoil carbon concentrations,and the fastest turnover occurred in the surface organic and mineral layers.Black sprucesiteswith mosssurfacelayerswere found mainly on clay soilswith degradingpermafrost.Thesesiteshad net carbonuptake rates that were similar to thoseat the jack
pinesites(50-100 g C m-2 yr-•), but theyalsohad slower ([.%.wIOWri) eoueIonpuoo leIewoI$
decompositionrates, due to high soil moisture contentsand lower temperatures,which resulted in the accumulationof more carbon in their soils.The fen sites had the largest soil
carbonstores(1.2 x 105g C m-2) andthehighest productivities(200-400g C m-2 yr-•). Slowdeepsoilrespiration offset '
'
'
I
'
about 15% of the carbonuptake rates in wetlandsand about 45% in upland sites.Deep soil respiration/decomposition rates were observedto decreasewith depth. Deep soil carbon de-
composi, ratesasdetermined from •4C andsoilinventory
--
'
i
i
{
I
work agreewith the findingsof Winstonet al. [this issue]. Middletonet al. [thisissue]acquiredleaf-levelmeasurements of gas exchange,chemistry,morphology,and spectraloptical propertiesduring the BOREAS 1994 IFCs at all of the SSA forestcoverTF sites(Figure 5b). Leaf net photosynthetic rates were found to be comparablebetweenthe old (SSA-OA) and the young(SSA-YA) aspensites,but leaf transpirationrates were significantlydifferent; the SSA-YA leaves transpired about 30% more than the SSA-OA leaves.The black spruce shootsat SSA-OBS exhibited the lowest photosyntheticrates amongall the foresttypes.The conifers(SSA-OBS, SSA-OJP, SSA-YJP) were observedto have peak photosyntheticratesin late summerto early fall, while the deciduousspeciespeaked in midsummer(see alsoSullivanet al. [1997] and Saugieret al. [1997]).Dang et al. [1997,this issue]examinedthe profilesof photosynthetically activeradiation (PAR), leaf nitrogen,and photosyntheticcapacityat two jack pine sites,an aspensite and two black spruce sites in the NSA. Beer's law descriptions (exponentialextinction)fit the PAR profilesdownthroughthe canopyfairly well (r = 0.73-0.92) under both cloudy and clear-skyconditions.Leaf nitrogendecreasedwith the fraction of absorbedPAR in each of the forest stands(relationships were similaramongthe speciesexceptfor an alderunderstory),
Figure 5. (opposite)(a) Comparisonof soil CO2 fluxesmeasuredby different investigatorsand instrumentsat a common test site in the SSA in BOREAS 1994,from Norman et al. [this issue];(b) leaf assimilationratesasmeasuredby porometryfor severalspeciesat the SSA TF sites,from Middletonet al. [this issue].Note that understory(hazel, bog birch) as well as the dominant species(bs, black spruce;jp, jack pine, ws, white spruce)were sampledat each TF site (OA, old aspen;YA, youngaspen;OJP, old jack pine; YJP, youngjack pine; OBS, old blackspruce);(c) plot of leaf stomatalconductance against photosyntheticcapacityfor several speciesin the NSA, from Dang et al. [this issue].
SELLERS ET AL.' BOREAS IN 1997
28,743
Net Radiation, Albedo
Tower Flux (Eddy Correlation)
Hodges and Smith
Blanken et al. Baldocchiet al. Jarviset al.
(SSA-OA) (SSA-OJP) (SSA-OBS)
Patteyet al.
(SSA-OBS)
Betts and Ball Shewchuk
Local-Scale Kimball
Flux Models
et al.
Frolking et al. Bonan and Davis
...
Humidity Sensors
Goulden et al. (NSA-OBS) McCaugheyet al. (NSA-YJP) LaFleur et al. (NSA-Fen)
Suykeret al.
Betts et al.
(SSA-Fen)
Productivity Gower
et al.
Respiration Lavigneet al.
Sapflow Hogg et al.
Hogg and Hurdle(1997)
Figure6. Schematic summarizing papers in section 6.1.2;stand andplot-level carbon-water-energy dynamics. and photosynthetic capacitydecreased significantly with leaf thesis,averaged0.44, 0.29, and 0.43 for aspen,blackspruce, Differencesin CUE between nitrogenexceptfor thesinglecaseofjackpineduringearlyleaf and old jack pine, respectively. flush.Aspenleaveshadhigherphotosynthetic capacities than the NSA and the SSA sites were small for the conifers but of higherrootrespiration in theNSA. conifersfor the same leaf nitrogen. Good correlationswere largerfor aspenbecause 6.1.2. Stand and plot-levelcarbon-water-energy dynamics. found betweencanopyphotosynthetic capacityand remote sensing spectral vegetation indices(NDVI andsimpleratio), The firsttwo papersin thissectiondealwith radiationfluxes but the relationships varieddependingon the canopyscaling and albedo measurements measured over forested sites in procedure. In all cases it wasfoundthatphotosynthetic capac- BOREAS; most of the remaining 15 papers describeeddy from the flux towers(TF sites)and ity correlated wellwithstomatal conductance (Figure5c) but correlationmeasurements of patch-scale carbondynamics inferredfrom these decreased lesssteeplythandid PAR downthroughthe canopy. analyses observations takenwithinfootprintsof the TF Flanaganet al. [thisissue]correlatedleaf isotopiccontents, andsupporting specifically 13Cto I:C ratios,to inferthe degreeof stomatal sites(Figure 6). Hodges andSmith[thisissue]analyzedsurfacenet radiation restrictionon photosynthesis. Despitethe very low stomatal acrossthe BOREAS region.Three caliconductances observedamongthe forestspeciesin BOREAS (Rn) distributions bratednet radiometerswere comparedwith 22 Rn instruments of photosynthesis, in agreement withgasexchange andmete- at 21 sitesin BOREAS; R,fields generatedfrom GOES oborologicaldatapublished byDanget al. [1997].Measurements servationswere alsocomparedwith the in situ measurements.
their results indicated that there was little stomatal limitation
of the carbonisotopeconcentration of cellulosefrom individual tree growthringsvariedwith summerprecipitation and temperature, suggesting thattreeringcarbonisotopes maybe usedto infer the effectsof past environmentalconditionson
The results show that one make of radiometer, used at 15 out
photosynthesis.
to create time series of albedo estimates over the different
of the 21 sites,underestimated R,by about5% in the daytime
andby about45% at night.BettsandBall [thisissue]analyzed theBOREASmesonetdata(seenotebyShewchuk [thisissue])
Ryanet al. [thisissue]and Lavigneand Ryan [1997]esti- covertypessampledin BOREAS throughout1994and 1995. dailyaveragealbedosin summerrangedfrom matedannualautotrophicrespirationfrom chambermeasure- Representative mentsof foliage,woodytissue,andfine rootsof aspen,black 0.083 for coniferous sites, 0.15 for aspen, to 0.20 at grass
were0.13for conifers, 0.21for spruce,andjackpineforestsin the SSAandthe NSA during (airport)sites;winteralbedos the BOREAS 1994growingseason.Mean foliagerespiration aspen,and 0.75 for the grasssite (Figure7a). The winter per unit leaf areawasfoundto be similaramongexpanded albedosof the forestsitesvariedwith canopyclosureand with leavesfor all species at 10øC.Woodrespiration wasstrongly solar diffuse/direct flux ratios but never exceeded 0.3. These differentfrom thoseusedin the seasonal, with high ratesin midsummerthat coincidedwith numberswere significantly underestimawoodgrowth.Finerootrespiration declinedby abouta factor EuropeanNWP modelwhichled to a systematic of 3 throughoutthe season,but rates were similar among tion of the near-surfaceair temperature(Figure7b). The note species. Annualcostsof autotrophic respiration for thewhole of Bettset al. [thisissue]comparedsurfacehumiditymeasuresystem wereestimated to be 310-610g C m-2. Carbon use mentsfrom the operationalAES sensorat Meadow Lake, efficiency (CUE), the ratioof net production to net photosyn- Saskatchewan,with a colocated BOREAS mesonet sensor,
28,744
SELLERS ET AL.: BOREAS IN 1997
(a)
i
I
1994
/
-Grass#1
•1.
.
- Grass#2
, /
,I 11 d
........ Aspen
0.8
.....
Conifer
0.6
I
0.4 i
showingthat the operational instrument has significantbiases that could be important to surfaceclimate studies. Figure 8 summarizesthe flux data reported by the TF sites during BOREAS 1994; it can be seen that the coniferous vegetationis characterizedby low evaporativefractions(ratio of latent heat to the sumof the latent and sensibleheat fluxes) and low CO: uptake rates during the growingseason.Boreal deciduousland cover,while showinglarger transpirationand CO,_tiptake rates than its coniferouscounterpart,is still much lessactive than deciduousforestsin temperate North America [e.g., Baldocchiand l/ogel, 1996; Baldocchiand Harley, 1995]. The papers summarizedbelow provide more details on these results.
Blankene! al. [thisissue]presentedresultsfrom eddycovariance
measurements
of latent
and
sensible
heat
flux
made
aboveand below the aspenoverstoryat the SSA-OA site during 1994. Before leaf emergence,most of the availableenergy was converted to sensible heat flux, but latent heat flux dom0
60
120
(b)
180
240
300
360
DayofYear,1994
20[
1996
were330and113mmolm : s •, respectively (Figure9a).The
••
canopyconductancesof both speciesincreasedwith increasing photosyntheticphoton flux densityand decreasingsaturation deficit at the leaf surfaceand were not limited by soil moisture. Forest canopyconductancewas directly proportionalto forest leaf area index. The daytime average ratio of the latent heat flux to the equilibrium latent heat flux (the Priestley-Taylor alpha) suggests that the understoryhazelnuttranspirationwas mainly energy controlled, while the overstory aspen canopy transpirationrate was more limited by stomatal conductance. [togg et al. [this issue]measuredsap flows through aspen bolesat the SSA-OA site in BOREAS 1994 usingtwo different techniques(seealsoHoggand Hurdle [ 19971).Both techniques produced similar results and compared well with the eddy
o o
'•
•
inated after leaf emergence.During the full leaf period, daytime mean dry leaf aspen and hazelnut canopyconductances
10
lO
o
correlation -20
1
3
5
7
9
11
13
15
17
19
21
Day of Year, 1996
Figure 7. (a) Annual courseof surfacealbedoover different BOREAS cover types during 1994, from Bettsand Ball [this issue]. Note that the winter albedo over the grasslandsites varies around 0.7, while the winter albedos over the forest sites
are much lower, 0.12-0.30; see section 6.1.2: (b) effects of overestimatingsurfacealbedo on the performanceof the European numerical weather prediction (NWP) model. The [nodel
assumed
a winter
surface
albedo
of about 0.8 over the
boreal zone, while the true value was around 0.1-0.2; see
Figure 7a. Shownhere is the ECMWF (European Center for Medium-Range Weather Forecasting)model near-surfaceair temperaturefor the SSA site as predicted24 hours ahead of the start of the model run, comparedwith the mean observations from the BOREAS meteorological towers for the same time. (The NWP runs were initialized with observedtemper-
measurements
described
above in terms of diurnal
and seasonalpatternsand magnitudes,althoughthe sap flow time serieslagged the eddy correlation data by about 1 hour due to water storagechangesin the trees. All methodsshowed that as expected,transpirationincreasedwith air vapor pressure deficit
until
about
1 kPa but then
remained
almost
con-
stant for higher vapor pressuredeficits. Baldocchiet al. [this issue]report on the variationsof radiation, sensible, and latent heat fluxes above and below a boreal
jack pine canopy(SSA-OJP) during the 1994growingseason. The sum of the measuredhourly averagesensible,latent and soil heat fluxes, and canopy heat storagewas about 8% less than the net radiation, indicatinga systematicunderestimation of one or more of the measured energy fluxes. When the canopywasdry, daily evapotranspiration was lessthan 2.5 mm
d • withmostgrowingseason valuesbeingmuchlessthan1.5 mmd • (Figure9b).Thisextremely lowratewasattributed to
the low LAI (1.9-2.2) and low stomatalconductancesobserved at the site. Factors restrictingstomatal openingwere low soil atures).Note how the (24 hour) predictedsurfaceair tempermoisturesupply,limiting atmosphericsaturationdeficitsand ature is alwaysmuch lower than observationsbecausethe surface is calculatedto absorbonly a small fractionof the incident the low photosyntheticcapacityof the needles.Typically,20solar energy, exceptwhen the model predictsmelting condi- 40% of the total energy exchangedoriginated at the forest tions (marked by horizontal bar in figtire) in which case the floor underneaththe sparsejack pine canopy. Jarviset al. [this issue]summarizeeddycovarianccmeasurecalculatedalbedo is reset to a value representativeof a snowfree surface,around 0.2. which is actuallycloseto the observed ments of CO2, water vapor, and sensibleheat fluxesabove a winter albedo for the forest. ECMWF output are from Holl- black spruce (SSA-OBS) forest over a 120-day (May 23 to ingsworth(personal communication,1996) and the analysis September 21) period in 1994 (Figure 9c). Average midday andfigurearefrom Huemmrich(personalcommunication, 1996). evaporativefractions(latent heat flux dividedby the sumof the latent and sensibleheat fluxes) for the three IFCs were 0.34-
SELLERS ET AL.: BOREAS IN 1997
--
28,745
0
o -
O>-•LLO
I i.{-)
I o
1.1'3
0
o
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m.
o.
i.q.
o
o
o
o.
0
(s/•m/fim)xnl:l•00 Aepp!l/•
UO[JO'eJ-.I OA!J'eJOd'eA:l Al•pp!l/•
r-:. o
m. o
m. o
q o
UO!J01•J-i OA!Jl•JOdl•A 3 A•PP!PI
c,q. o
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I
I
I
o. o
i.q. o,
o. "'7
(s/•m/fim)xnl-t•OO A•pp!PI
28,746
SELLERS
ET AL.' BOREAS
(a) 800• • • • •' I20 (b) 4 SSA-OA i Aspen I_
400
o,
i
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I
r . '•'••o__. I
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IN 1997
SELLERS ET AL.: BOREAS IN 1997
28,747
tissue,foliage, and soil respirationrates and temperature measurementsto estimatehalf-hourly ecosystemrespirationrates at six coniferousBOREAS sites.These were compared with nocturnal eddy correlation estimates.Soil surfacerespiration was the largest contributor (48-71%) to the total flux and
adding the downwardCO2 exchangeand the soil respiration measuredby soil chambers.Midsummer photosynthesis ap-
foliar respirationwasa significantcontributor(25-43%) at all the sites.The nocturnaleddycorrelationmeasurements(at u *
curredin earlyJuly,were13 •mol m 2 S-1.Mid-Mayto early Octobernet ecosystem exchange (NEE) was -88 g C m-2
proachedlightsaturationabove1000-1200•mol m-2 s- 1 and decreasedin responseto increasingtemperature and saturation deficit. Maximum middayphotosyntheticrates,which oc-
> 0.25 m s-l) werepoorlycorrelated to estimates obtained (sink). This substantialproductivityis consistentwith the obfrom scalingup chamber measurements;estimatesof ecosystem respirationfrom chamberswere 1.2-1.5 times greater than those from eddy correlation. Gouldenet al. [this issue]used eddy correlation equipment to measure CO2 exchangebetween the atmosphere and the NSA-OBS site for 2 years (March 16, 1994 to February 19, 1996) (Figure 9d). This is the longestcontinuouslyrunning eddy covarianceCO2 flux record of the BOREAS TF teams. Nocturnal CO2 fluxeson windy nightswere used to develop a respiration versus temperature relationship for the forest which was then applied to separate net exchangeinto gross photosynthesisand respiratory fluxes. Gross photosynthesis waslargelya functionof PAR flux and air temperaturewith no apparent effectsdue to high evaporativedemand or soil water content. Under moderate light levels, photosynthesiswas higher under cloudythan under sunnyconditions.A singleset of regressionrelationshipsdescribingthe responseof gross photosynthesis to PAR flux and temperatureand the response of respirationto temperatureaccountedfor 72% of the variation in hourly CO2 exchange.Maximum rates of photosynthesis at the NSA-OBS site were small compared to those measured at a deciduous Harvard forest site [Wofsyet al., 19931. McCaugheyet al. [this issue] report on measurementsof sensibleand latent heat and CO2 fluxestaken above the stand at the NSA-YJP site over 119 days in BOREAS 1994. The PAR and clear-skysolar albedoswere approximately0.054 and ().136, respectively,during this time, and the averageevaporative fraction during this period was about 0.3. Nighttime CO2 fluxcs were estimated by using a regressionrelationship between
fluxes
measured
in neutral
to near-neutral
conditions
servedhigh water table and is in marked contrastwith the loss of carbonover the correspondingperiod measuredat the drier NSA-fen site in the sameyear [Lafieuret al., this issue].Roulet et al. [this issue]report that their beaverpond site in the NSA
wasa strongsource of CO2(183g C m- 2) overthe120growing seasondaysof 1994(section6.2.2),whichsuggests that in some boreal landscapes,beaver pondscould make a significantcontribution to the regional carbon budget. Productivityand carbon allocationwithin an ecosystemare criticallyimportant for determiningsite carbonbudgets.Gower et al. [this issue]analyzedtheir detailed measurementsat the TF and auxiliary sitesto show that abovegroundnet primary productivity(ANPP) for the BOREAS forestsiteswas55-310
g C m-2 yr-• for aspen,blackspruce,andyoungandoldjack pine covers, low values when compared to most temperate sites. Thirty to 40% of the ANPP, as estimated at the BOREAS forest sites,fell to the surfaceas detritus;only 6070% wasretainedas biomassincrement.Black spruceecosys-
temscontainedthe highestcarbonstocks(39-48 kg m-2), aspenwasintermediate (16-18kgm-2), andjackpinewasthe lowest(5-8 kgm-2). Leafareaindices rangedfroma lowvalue of 1.25 for jack pine sitesto 5.6 for black sprucesites.Fine root
NPP estimatedwith minirhizotrons rangedfrom 30 g C m-2
yr-• at SSA-OAto 115g C m-2yr-• atNSA-OBS[Steele etal., 1997]. Root elongationwas highlycorrelatedwith soil temperature (10 cm) for aspen,black spruce,and jack pine sitesat both the NSA and the SSA [Steeleet al., 1997]. Three papers present stand-scalesimulation modeling resultswhichusedTF flux data and supportingmeasurementsfor validation. Kimball et al. [this issue]discussenergy and water balancesimulationsperformed with the BIOME-BGC model.
and soil temperature at 1()- and 75-cm depths.The regression In their model, total ET was calculated to be insensitive to estimatesfor June and July agreedquite well with the modeled ecosystemleaf area index, and canopy conductanceto water values calculated by Ryan et al. [this issue].It was estimated vapor averaged67% of its maximum(unstressed)value during that the standfixed224g C m 2 duringthisperiod. the growingseason,with low light and low temperatures(not LaJteuret al. [this issue]collectededdy covariancemeasure- low humidity)causingthe reductions.This reductionin canopy mcnts of sensibleand latent heat, and CO2 fluxes and related conductancecorrelateswith the stablecarbon isotopework of climatic variablesover a period of 124 days (April 9 to Sep- Flanagan et al. [this issue].Frolking [this issue]assessedthe tember 19) in 1994over the NSA fen site. Albedos(solar and effect of climate anomalieson carbon dynamicsat the NSAPAR) decreaseddramaticallyafter snowmelt;during full leaf, OBS site with a black spruce-mossprocessmodel. Net ecosys--2 solar and PAR hemisphericreflectanceswere 0.18 and 0.055, tem fluxesof carbon(sinks)were calculatedto be 120 g C m respectively.Mean evaporativefractionsincreasedfrom about yr • in 1994and 90 g C m-2 yr-• in 1995.The effectsof 0.5, during the snowmeltperiod, to 0.59 at leaf-out, to 0.83- introducingclimate anomalies,in the form of 2-month periods 0.91 near midsummer, and then decreased to around 0.5 at of warm, wet, cool, or dry conditions, into the model runs senescence.The fen acted as a net sink for CO2 only when varied with the timing of the anomalyand were not anticipated vascularplantswere activelyphotosynthesizing to give a daily from simple relationshipsof grossprocesseswith climate. If meanfluxof -0.81 g C m 2 d-i. Overthe 124-dayperiodthe the mosslayer remained intact to insulate the soil, large diffen lost30.4g C m 2 to the atmosphere. While the authors ferences in air temperature were simulated to have only a point out that there is considerableuncertaintyin the latter minor (5-10%) effecton decomposition. BonanandDavis[this figure, there is a marked contrastbetween this fen and the one issue]assessed the ability of their land surfaceparameterizain the SSA, which was relatively productive(i.e., acted as a tion (NCAR LSM1) to reproducethe fluxes of energy and carbonsink) duringthe June-Augustperiod; seeSuykeret al. carbonas measuredby eddy covarianceat aspenand jack pine [this issue]who report on atmosphericCO2 exchangesmea- sites.Using generic vegetation parameters,their calculations suredat the SSA-fen site from mid-May to early October 1994. generallymatchedthe observednet radiation and eddy covariCanopy photosynthesisat the SSA-fen site was calculatedby ance CO2 and sensibleheat fluxesbut overestimatedthe latent
28,748
SELLERS ET AL.: BOREAS IN 199/
Airborne Eddy Correlation Dobosyet al.
(Intercomparisons)
Soundings/ABLStudies
Kaharabata et al.
(Footprints)
Wilczaket al. (WindProfiler) Kiemleet al. (Airborne Profiles) BarrandBetts(Analysis) Barret al. (Radiosondes)
Desjardins et al. Ogunjemiyo et al. Oncleyet al. MacPherson andBetts
(Footprints) (Vegetation IndexandCO2 Flux) (Regional Transects) (GustEvents)
K. Davis et al. (AirborneProfiles)
rn t I •
Meso eteorolog'ca / Network
Shewchuk /
/
r•• •
_•-• •b> ,Ft '
/,_ ,• •"•5•F,/•/ •//
Models
Sun et al. (Lakes, MesoscaleEffects) Vidale et al. (MesoscaleCirculations) Cooperet al. (LSP Radiation)
Figure 10. Schematicsummarizingpapersinsection6.1.3; lanscape-scalecarbonwater-energydynamics and surface-atmosphere boundarylayer interactions. heat flux and did not capturereal differenceswithin vegetation couplingof the boundarylayer fluxes from surfacefeatures types,for examplebetweenthe jack pine sitesin the NSA and with increasingheight. The differencesin scale between airSSA. craft- and tower-measuredfluxeswere also examinedby Des6.1.3. Landscape-scalecarbon-water-energy dynamics and jardinset al. [this issue],but the large differencein footprint surface-atmosphereboundary layer interactions. Figure 10 sizebetweenthoseplatformsmakesit questionable to compare summarizesthe regionalscalestudiescarried out in BOREAS flux values directly. Among other things, it appearsthat the usingaircraft, soundingdevices,meteorologicalnetworks,and towerfluxfootprints(around104-105m2 in unstable condicoupled land-atmospheremodels.Shewchuk[this issue]de- tions) may have been drier than the aircraft flux footprints scribesthe mesoscalemeteorological network that was de- (around107m2 for a 10-kmpass),asevidenced bydifferences ployedover the BOREAS 1000 x 1000km regionin 1994 and in sensibleand latent heat fluxes.Desjardinset al. [this issue] ran continuouslythrough 1996. Two siteswere located in the compared fluxes measuredby the Twin Otter, from short SSA, one in the NSA with another nearby, and the remaining passesover specificTF sites(SSA-OA, SSA-OBS, SSA-OJP, six were placed at or near airfieldsin the region to allow for NSA-OBS, NSA-OJP, NSA-YJP), with the TF measurements easy deploymentand servicing.The BOREAS years of 1994 themselves(Figures11a and lib). The aircraft fluxeswere on and 1995 showedwarmer air temperaturesthan normal; 1994 was0.7øCabovethe long-termmean. Wilczake! al. [thisissue] (a) (b) present resultsobtained with a 915-MHz profiling radar lo750 © .............. ¸ YJ P NSA cated near SSA-OJP during BOREAS 1994. The device ,7 ..... rOBS J... '• ..... rOBS • 600_• OJP yielded profilesof wind velocity,virtual temperature,and atmosphericboundarylayer (ABL) depth. m, 450 E 450 ..-; • Severalpapersdescribeintercomparisonsbetweenairborne
750 I ©............ ¸OA SSA
• 6øø/_=OJP O
_
_
flux measurements, both aircraft to aircraft and aircraft to
tower.Dobosyet al. [this issue]presentextensivecomparisons amongthe four flux aircraft involvedin BOREAS, all basedon wing-to-wing flights spaced throughout the BOREAS 1994 IFCs. Overall, the smallestcorrelations(most scatter) were between the LongEZ and the Twin Otter, while the Twin Otter-King Air and Electra-King Air pairingshad higher correlations(lessscatter).At least someof the reducedcorrelation for the LongEZ-Twin Otter comparisonis becausethe air-motion equipment and techniquesused on the LongEZ, with its smallsizeand payloadare different in manywaysfrom the other three aircraft.
Significanteffortswere devotedto determiningthe geometry of the surfacesourcezonessampledby the airborne flux systemsand in correlating the airborne fluxes with surface covertypes.An encouragingstudyof aircraftflux footprintsby Kaharabataet al. [this issue]made use of SF6 releasesto compute diffusionprofileswithin the lower boundarylayer; their results indicate that the tower and low-level flux aircraft mea-
surementscan be expectedto yield site-specificestimatesof the surfacefluxes.The work also exploresthe progressivede-
--
3OO
3OO
+
150
150
150
300
450
600
750
0/•
H + LEm W m-2 (Tower)
t • t
150
300
450
600
750
H + LE inW m-2 (Tower)
(c)
(d)
-0.1 ß
ß
d ß ßß ß
o
-0.4
1.8
2.0
2.2
2.4
2.6
2.8
SpectralGreennessIndex
3.0
-0.6
-0 5
-0.4
-0.3
-0.2
-0.1
0
CO2 flux(mgm-2s-1)
Figure 11. Comparison of the sum of sensibleand latent
heat fluxesmeasuredby the Twin Otter (FT) aircraft over shortrunscloseto severalTF towersin (a) NSA and (b) SSA, from Desjardinset al. [this issue]. Comparisonof aircraftmeasured(FT) daytime(c) CO2 fluxeswith spectralgreeness index and (d) latent heat fluxeswith CO2 fluxesas measured over a grid within the SSA; see Ogunjemiyoet al. [this issue].
SELLERS ET AL.' BOREAS IN 1997
average higher than the towers for latent heat, less than the towersfor sensibleheat, and comparableto the towersbut with muchmore scatterfor CO2. The bestcomparisons (both in magnitude and in degree of scatter)wcre for momentum fluxes. Two-dimcnsionalpatternsof surface-ABL couplingare presentedby Ogunjemiyo½tal. [this issue]for multiple grid flights in both studyareas(Figures 11cand 1ld). Strongcorrelations were observedbetweenCO2 flux and spectralgreennessindex, a very encouragingresultfor modelerstryingto estimatelargescale surface fluxes over the boreal zone. These
results concur
with similar findingsfor grasslands[Cihlar et al., 1992], leafscalestudiesin BOREAS [Dang et al., 1997], and theory [Sellers et al., 1992b].At the same time, sensibleheat fluxeswere noticeablydecoupledfrom the atmosphere-surfacetemperature difference (see also Mahrt et al. [this issue], Fining and Blad [ 1992],and Hall et al. [1992]). Oncleyetal. [thisissue]documentedseasonaland spatialflux changesalong a regional transect,from just south of the SSA, up over the NSA, and into the tundra near Churchill, located near the southwestcorner of Hudson Bay. Traces of CO2 fluxes from three of these transects, in BOREAS 1994 IFC-1,
IFC-2, and IFC-3, showchangeswith latitude, season,and land cover. There were also large-amplitudevariations along each transect,superimposedon the other trends. Airborne flux measurementstaken over lakes highlight the complexity that must be addressedin modeling the regional surface energy budget. Oncleyet al. [this issue]estimate that aircraft-level fluxes measured in their regional transectsoriginated from surfaceareas (footprints) that were up to 40% water (lake surfaces).They also estimate that surface heat storageaccountedfor 30% (forests)to 40% (tundra) of the net radiation, with a large portion of the storagebeing assignedto the numerous lakes in the region. The length scalesof the flight-levelfluxespoint to specificforcing mechanismsand to the difficultiesfaced in mergingaircraft and tower flux data. A detailed analysisof flux fieldsover and around severallakesin the SSA by Stln et al. [this issue] indicates that the areaaveragedsensibleheat fluxes in the SSA were 12-24% lower than they would have been if no lakeswere present.A welldefined divergent lake breeze circulation was observed over three lakes in the SSA during the day by the BOREAS flux aircraft with the circulation strength dependent on lake size and wind speed.The modelingstudyof Vidaleet al. [this issue] investigatesthe contribution of mesoscalecirculationsto the large-scalesurface-atmospherefluxesof heat, water, and momentum.They concludethat thesecontributionscan be significant under low-wind conditions and are generated by the differencesbetween the energy balancesassociatedwith the different surfacetypesfound in the region,for example,lakes, dry uplands, and wet forested areas. They argue that these mesoscaleeffects should be parameterized in large-scale atmosphericmodels. Aircraft measurements, radiosondes, surface measurements
28,749
and Betts[this issue]calculatedarea-wideestimatesof entrainment rates from a budget analysis,based on data from the BOREAS radiosondenetwork. They found the entrainment parameter A•e above the boreal forest to be 0.21, which is in closeagreementwith the generallyacceptedvalue of 0.20. The surface available energy, calculated by summing the sensible and latent heat fluxes derived from the budget analysis,was within 4% of the independent estimate provided by the BOREAS meteorologicalnetwork radiation measurementsof Shewchuk[this issue].They estimatedthe mean surfaceevaporative fraction over the forest during the summer to be 0.49, which can be comparedwith the value of 0.71 observedover the grasslandFIFE site [Smithet al., 1992]. Ban' et al. [this issue]compared the surface fluxes derived from the radiosondebudgetanalysiswith the eddy fluxesmeasured by the Twin Otter aircraft for the three growingseason IFCs
of BOREAS
sensible
1994.
and latent
The
sum of the aircraft-measured
fluxes was 25%
less than
the radiation
network estimate of available energy, in contrast to the 4% underestimategiven by the budget analysis.However, the aircraft-derived estimate of the evaporative fraction was 0.52, which agreeswell with the budgetanalysisfigure of 0.49, which implies that the aircraft measurementsmay be systematically and proportionatelyunderestimatingthe turbulentfluxes.The authorspoint out that both methodshavevalue for the studyof landscape-scalefluxes, with the budget analysisproviding a large-area integral view, while the aircraft provide insightinto spatial variations and associationsbetween different surface flux regimesand land cover type. Davis et al. [this issue(a)] describe studies of the convectiveboundary layer based on aircraft
measurements
in BOREAS
1994. Convective
bound-
ary layer divergenceduring the midday period and early afternoon was often observedto dry out the atmosphericboundary layer; this drying may have had feedback effects on the vegetation which could have further reduced evapotranspiration, thus reinforcing the drying of the ABL; see also discussionby Sellera'et al. [1995b].MacPhersonand Betts [this issue]document the structureof the strongABL vorticesseen by aircraft at low levelsin 1994in conditionsof deepABLs and low winds. Cooperet al. [this issue]used the BOREAS meteorological observations,soil and vegetation parameters, and satellitebased
estimates
of
radiation
fluxes
to
drive
a surface-
atmosphereflux exchangemodel for an area coveringmost of the BOREAS domain. According to sensitivityanalysesconducted with their model, root depth, soil moisture, and soil depth are the most important quantitiesin controllingtranspiration fluxes. The radiation balance of the BOREAS region was studiedby a number of BOREAS investigatorsusingsatellite data analyses(section6.4.4). 6.2.
Trace
Gas Fluxes
Carbon exchange(CO2, CH 4, and CO) was the principal focusof the TGB group,largelybecauseof the great interestin the carbon balance of the boreal ecosystembut also because early measurementsshowedthat the fluxes of nitrogen gases were small exceptin very recentlyburned areas. Most of the BOREAS trace gas studieswere conductedwithin the NSA, which can be roughly divided into drained upland sandysoils usually dominated by jack pine and aspen; lesswell drained clay-rich soils dominated by black spruce;and wet peat soils dominated by sphagnumand sedges(Figure 12). TGB mea-
and analyseswere all used to explore exchangesamong the land surface, the boundary layer, and the free atmosphere. Inversion-level(ABL-top) entrainment,the captureof air from the free troposphere into the ABL, is an important factor which affectsmanyof theseexchanges. Kiemleet al. [thisissue] used an airborne laser to remotely senseprofiles of aerosols and water vapor that were used to build 2-D cross-sectional maps of entrainment structures and estimate entrainmentzone water vapor fluxes. They found good agreement with surements were focused at the NSA tower sites, some landdirect eddy-covariance measurementsof the samefluxes.Ban' scapetransects,a chronosequenceof sites disturbedby fire,
28,750
SELLERS ET AL.' BOREAS IN 1997 CH4 and N20 fluxes (SSA-OA) Simpson et al.
Peroxide
Fluxes
Hall and Claiborn
CH4 Chambers Amaral and Knowles (Dryer Sites) Moosaviand Crill (Transects,Water Table Effects) Savage et al. (Transects,Drainage Effects)
Fire Sites Burke et al.
Zepp et al. Beaver
Fen (CH4)
Pond Tower
Rouletet al. (CO2,CH4) Bourbonniereet al. (CO)
LaFleuret al. (CO2) Suykeret al. (CO2,CH4)
Figure 12. Schematicsummarizing papersin Section6.2; tracegasfluxes,principallyCH4 and CO. (CO2 fluxesare coveredin Section6.1).
and a small tower at the NSA beaver pond. The CO2 flux ment program and were able to model CH 4 fluxesusingcormeasurements of the TGB groupare summarized in section6.1.1; relationswith surfaceorganicmatter and the soil temperature at 20 cm in NSA upland soils. Weakly emitting sites were in this sectionwe summarizethe TGB CH 4 and CO research. 6.2.1. Small-scale fluxes with chambers and enclosures. usually dominated by black sprucewith a feather moss or Amaral and Knowles[this issue]found that drier sites,dominatedby aspen,jack pine,or birchwith a vascularplantground coverand a thin (1-5 cm) surfaceorganiclayer,often actedas (a) 250 strongCH 4 sinks(Figure 13a). These siteswere warmer and drier with faster nitrogen cyclingand shorterpath lengthsto CH 4 oxidationlayers.Consistentwith early indicationsin tem•-- 250 perate soils,Amaral and Knowles[this issue]determinedthat • 'el E the location of CH 4 oxidation activitywas near the surfaceof the mineral soil and at the baseof the organiclayer. A signifE 250 icant positiverelationshipbetween CH 4 uptake and surface C:N ratio was also observed.Their microbiologicalwork dem• 250 onstratesthe potential for a near-surfaceorganic layer of drainedsoilsto produceCH 4 underanaerobicconditions.This effect
is obvious
documented
on the macroscale
for well-drained
but has not been well
soils.
Moosaviand Crill [this issue]observedCH 4 effiuxesat all sites along their experiment transect which extended from someof the driestuplandsoilsto inundatedwetlandsnear the NSA beaver pond tower (NSA-BP) and the NSA-OBS site. Fluxes along this transect varied by 4 orders of magnitude; strongCH 4 sourcesrequire a consistent,highwater table, as at the beaver pond mire (Figure 13b). Their work showsthat upland sitesmight be convertedfrom net sinksto net sources by minor changesin water table; also, low soil temperatures strongly inhibited methane flux rates even under saturated conditions.
Bubieret al. [1995] reported a large range in CH 4 fluxes(3 ordersof magnitude)from different plant communitiesin the NSA fen complex,with the largestfluxesfrom open graminoid fens and the smallestfrom frozen peat plateaus and treed peatland areas. Temperature at the average position of the water table was the best predictor of seasonalaverage CH 4 fluxes,and floatingpeat sitesmaintainedhighfluxesduringthe dry late summerperiod in 1994.The correlationsbetweenCH 4 flux and plant communitydistributionmay be usefulfor scaling fluxesfrom the chamberto the landscapeusingremotesensing. Savageet al. [thisissue]conducteda dark chambermeasure-
•
I• Spruce I 250 Aspen• 250
0
50
100
150
200
Days since first measurement
(b) 3000
,
.-. 2500
--
Mire
,•
Pond
•-u2000 E
E 1500 ,= 1000 5OO
0 138
158
178
198
218
238
258
Day of Year, 1994
Figure 13. (a) Methane (CH4) consumptionby vegetation type in the NSA estimatedfrom chambermeasurements, from Amaral and Knowles[this issue];(b) methane productionas measuredover a beaver pond site in the NSA, from Moosavi and Crill [this issue].
SELLERS ET AL.: BOREAS IN 1997 Airborne
28,751
Soil Moisture
..•ecketal. (7-ray)
Hydrological Modeling Nijssen et al.
•
•.•i• •;.•. • ,•,'• ,• InSituSoilMoisture•;• •
•
Cuenca et al. (TDR)
',A'A
.
,•;.• •;• SnowPhysics
•;,• •
RainRadar
Schnur etal
Hardy etal.
• •
Y.••/ Levine and Knox
Figure 14. Schematicsummarizingpapersin section6.3; soil and snowmoistureand runoff.
report lower values from a beaver pond in the SSA. This indicatesthat beaverpondscouldbe significantcontributorsto the boreal carbon cycle with source strengths4 to 5 times larger than the sink strengthsobserved for boreal soils by Hardenet al. [thisissue]or asestimatedfor borealpeatlandsby Gorham [1991]. The surfacewaters of the NSA beaver pond site were also shownto evolveCO, most likely from degradationby incident radiation of dissolvedorganiccarbon [Bourbonniere et al., this issue].This illustratessome of the difficultiesof estimatinga regionalflux of CO to the atmosphere. Simpsonet al. [this issue]measuredCH 4 and N20 fluxes part byBubieret al. [thisissue](section6.3.1). Firedisturbance isanimportant fac•0rin theborealforest. above the SSA-OA stand using a tunable diode laser. Small Burkeet al. [thisissue]measuredfluxesof CO2 and CH 4 across emissionsof both gaseswere observedfor the measurement 16,1994),around1.9-2.5ngm-2 a chronosequence of burnedsiteson both clay (typicalof less period(April16-September well-drained black spruce dominated sites) and sandy soils s-1 for N20 and 21-28 mg m-2 s-1 for CH4. The authors measurements with some(below (typicalof well-drainedjack pine dominatedsites).All of the correlatedthe above-canopy siteswere net sinksof atmosphericmethanewith medianfluxes canopy)chambermeasurementsto concludethat CH 4 emisrangingfrom -0.3 to -1.4 mgC m-2 d-1. Medianfluxesof sion from scatteredanoxicpatchesin the aspenforest overcarbondioxidefrom the forestfloor to the atmosphereranged whelmed CH 4 uptake by the larger drier areas. This result between1 and2 g C m-2 d-1. Bothecosystem characteristicsshould be comparedwith the observationsof Amaral and (e.g.,soilandvegetationtype) andburninghistory(time since Knowles[this issue], describedabove, who found the drier burn and fire intensity)appearto have someeffect on atmo- areas to act as strongCH 4 sinks;however,they did not comsphericmethaneconsumption and carbondioxideemissionby ment on the signof the net CH 4 flux on intermediate(stand these forest soils.Their resultssuggestthat soil CO2 effluxes level) scales,as Simpsonet al. [thisissue]have done. Understandingthe sourcesand sinksof hydrogenperoxide from uplandblack sprucestandsmay not be immediatelyimpactedby fire. By 2 yearspostfirethere appearsto be a signif- (H202) and organicperoxides(ROOH) is importantfor unicant reduction in soil CO2 flux, while soil respiration rates derstandingatmosphericchemistryin generaland acid depositionin particular.Hall and Claiborn[thisissue]usedconcenrecoverto preburn levelsby 7 yearspostburn. from the tower of the SSA-OJP Carbon monoxide fluxes were slightly higher in more re- tration gradientmeasurements centlyburned areas[Zeppet al., this issue].However, CO flux site to infer depositionvelocitiesfor thesecompounds.They wasshownto be affectedby thermogenicand photogenicpro- report daytimeand nighttimedepositionvelocitiesfor hydrogenperoxide of 5 cms- • and1 cms- • respectively, withmuch ductionaswell as plant consumptionand production. 6.2.2. Stand and plot-level trace gas dynamicswith towers. lower ratesfor organicperoxides.The highestdepositionrates The boreal forest is in a constantstate of regenerationdue to occurred from 1100 to 1500 LT, indicating that deposition continuousdisturbanceby fire and beavers,insects,and to a rates are primarily turbulencelimited. These rates are similar lesserextent man. Roulet et al. [this issue]made continuous to values obtained over deciduous forests in the southern measurementsof fluxesover a beaver pond in the NSA using United States. a flux gradient method on a 1.25-m tower. It was discovered 6.3. Soil and Snow Moisture and Runoff that the beaverpond was a strongand continuoussourceof Soil moisture and snow measurementsin BOREAS ranged bothCO2 andCH4;678g m-2 GO2 (183g C m-2) and11.3g m-2 CH4 (8.3g C m-2) for the 120-day measurement period. from point measurementsin the vicinity of the TF sitesto Subsequent surveysof otherbeaverpondsin andnear the NSA airborne measurementsover the TF sites,alongshort transects for surface emissionsand near-surface CO2 concentrations within the studyareas,and alongan extendedtransectbetween showedsimilar results,althoughStaeblerand Edwards[1997] the studyareas(Figure 14). Traditional catchmenthydrologi-
sphagnummossground cover and a thick organiclayer of 20-50 cm; thesesitestended to be colder and wetter with long pathsto the zone of CH 4 oxidation.Unlike other studies,the effects of H20 on CH 4 uptake could not be observedin BOREAS becauseof limited variability in the soils over the seasonat mostof the uplandBOREAS sites.However,Savage et al. [this issue] observedthat a single heavy precipitation eventin early springchangedmanyof the jack pine sitesfrom CH 4 sinksto sourcesfor a short period of time. The problem of scalingthe chambermeasurements of theseborealwetlands to larger regionsusingremote sensinghas been addressedin
28,752
SELLERS ET AL.: BOREAS IN 1997
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Plate 2. Use of regionalremotesensing to monitor(a) landcoverandfire disturbance fromSteyaert et al. [thisissue];(b) regionalFPAR fromCihlaret al. [thisissue];and(c) soilsfreeze-thaw statusfromWayet al. [thisissue](imagebrightness proportional to radarreturnrendering darkareasasfrozensitesandlightareas as thawed).
SELLERS ET AL.' BOREAS IN 1997
(a)
28,753
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Figure 15. (a) Absolutevalueof total head(or soilwaterpotential)asa functionof depthmeasuredusing time-domainreflectrometryat a tube at the NSA-OJP site,from Cuencaet al. [thisissue];(b) comparisonof modeledandobserved(snowsurvey)snowdepthsfor a plot at the SSA-OJPsite;seeHardyet al. [thisissue]. cal studieswere performedin both the SSA and the NSA; also, km x 5 km surroundingthe tower sites.The model simulated in the SSA a precipitation radar was deployed during the redistribution of soil moisture in the saturated zone due to BOREAS 1994. A number of supportingmodeling studies local topographiceffects.Model estimatesof latent, sensible, were made of soil moisturedynamics,snowphysics,and catch- and groundheat fluxes,and net radiationwere comparedwith tower observations.The model-predictedheat fluxes agreed ment hydrology. 6.3.1. Point measurements and modeling of soil moisture reasonablywell with those observedfor the NSA-OBS, SSAdynamics. Soil moisturemeasurementswere collectedvia in ORS, and the SSA-OA sitesafter the model had been modified situ time domain reflectrometry(TDR) observationsin the to incorporatemossmoisturestorageeffectsand temperature vicinity of the tower sites [Cuencaet al., this issue](Figure feedbacksto canopyresistance.A slighttime lag in the diurnal 15a).The typicalmeasurement designusedinvolvedtransects cyclesof latent and sensibleheat was attributedto deficiencies of five sitesspaced5-20 m apart. These transectsprovideda in the ground heat flux representation. 6.3.2. Stand-level soil and snow moisture dynamics. Soil unique opportunityto studythe space-timevariabilityof soil moistureprofilesover footprintsrepresentativeof the imme- moisturetransectswere monitoredvia an airborne gammaray diate area surroundingthe tower sites.In addition to the ex- sensorfor selecteddates from August 1993 to the fall of 1994 pected evolution of the depth profiles through the season, [Pecket al., this issue].Comparisonsof the airborne observathese measurementsystemsshow that strong soil moisture tions with TDR probes at the SSA-OA site showedthat the consistently underestimated(by a factor variationsin the near-surfacelevel (depthsto 20 cm) occurred aircraftmeasurements 2) the near-surfacemeasurements takenby TDR. even at spatialscalesof lessthan the transectlengths.These approaching variationswere especiallypronouncedearly in the seasonand However,the flightline wassomedistance(almost1 km) from followingprecipitationevents.Cuencaet al. [this issue]also the tower, so spatialvariabilitymay explainat leastpart of the estimatedevaporativefluxes in the vicinity of the NSA-OJP difference.In the vicinityof the SSA-OJPsite,the aircraft and and SSA-OJPtowersfrom changesin soil moistureprofiles. in situ measurementsagreed more closely.Especiallyduring Comparisonswith tower-measuredfluxes indicatedthat the the dry periods,spatialvariabilityof soil moisturein this area evapotranspiration rates estimatedfrom soil moistureobser- wasquite low. Attempts to comparein situ and aircraft surface vationswere about two thirdsof thosemeasuredby eddy corre- moisture observations at the SSA-OBS site were abandoned lation.Reasonsfor the discrepancies are not clear at thispoint. due to the periodic presenceof surfacewater, and moisture Nijssenet al. [thisissue]describethe testingof a distributed storagein the mossand peat layer which are not measuredby hydrologicalmodel, which will ultimatelybe applied to sub- the TDR techniques.However, gravimetricsurfacesoil and catchments in the SSA and NSA, and to smaller areas of 5 moss/peatmoisture transectscollected for calibration of the
28,754
SELLERS ET AL.: BOREAS IN 1997
of the studyregionswith respectto the boreal aircraft measurementsshowedthat large spatialdifferencesin resentativeness soil and other surface moisture (e.g., moss) contentsexist forestbiome as a whole (Figure 16 and Plate 2). In supportof thesegoals,RSS investigatorsacquiredmeawithin the "footprint" of the SSA-OBS site, which must be taken into account when relating the SSA-OBS tower flux surementsof reflectance,emittance,and backscatteringproperties of forest canopies and background,and atmospheric measurementsto ground data. Daviset al. [thisissue(b)] performeda sensitivityanalysisto scatteringfrom the leaf to the regional scale. In parallel, a examinethe effectsof coniferheight and canopydensityon the significantamount of ancillaryvalidation data on forest stand timing and rates of snowablation.The analysisassumedradi- and soil biophysicalpropertieswas acquiredin situ (Table 4). ation to be the first-order
cause of snowmelt
consistent with
These
combined
data sets are a valuable
resource
for devel-
the results of Hardy et al. [this issue]. Forest maps of tree oping and testing new algorithmsfor mapping terrestrial bioheightand canopydensitycombinedwith genericconifercan- physicalparameters;specifically,they will prepare the way for opy propertiesprovidedboundaryconditionsfor the calcula- the next generation of Earth-observingsatellite instruments tions,which showedthat canopydensityand tree height are of such as MODIS, MISR, and RADARSAT. The data will be more or less equal importancein controllingsnow ablation producedon CD ROM by the BOREAS Information System rates. The calculatedtime seriesof snow depth showedclose (BORIS) andeventuallyarchivedat the Oak RidgeData Analagreementwith surveydata from jack pine sites,but measure- ysisand Archiving Center (DAAC). The RSS investigatorsalso engaged in algorithm developments showedslower melting than the model calculationsin ment and testingand producedmultiyear site-level,studyarea black sprucestands. level, and regional-scalemaps of radiation and biophysical Hardy et al. [thisissue]comparedmodelpredictionsof snow parameters.A numberof land coverparametermaps(Plate 2) grain growth,compaction,and melting,with measurements of have been producedas part of BOREAS to provide inputsfor snowdepth at the SSA-OJP site (Figure 15b). They adjusted ecosystemor climate models and to evaluate the expected meteorologicalmeasurementsfrom Shewchuk[this issue]to performanceof future spacebornesensors.The articlessummimic conditionsat the forest floor and over a nearby open marized below represent a significant cross section of area. Levine and Knox [this issue]presentresultsfrom a soil BOREAS remote sensingscience activities and range across and snowphysicssimulationmodel that includesa description the entire multiscaledesignof BOREAS. of the effectsof the overlyingvegetationcanopy.The simula6.4.1. Ground and aircraft measurements of biophysical tions were run for the SSA-OBS, SSA-OJP, and NSA-OJP
and optical characteristics and understory and canopy reflec-
sites. The results showed good fits to measured snowpack tance. Remote sensingalgorithm development,as with prothicknessand soil temperatureprofilesand alsopredictedthe cessmodel development,involvesa scalingstrategyfrom the extendedfrozenperiodin the NSA and the formationof an ice leaf level, where electromagneticproperties can be carefully lens at the SSA-OBS site. measured, to the pixel level (meters to kilometers) where 6.3.3. Landscape-scaleprecipitation and soil moisture dy- ground-basedmeasures are impractical and remote sensing namics. An importantcauseof spatialvariabilityin soilmois- algorithmsmust function autonomously.Translatingdata and ture, and hence in surfacemoisture and energy fluxes,is the knowledgeamong these scalesinvolvesa mix of measurement space-timevariabilityof precipitation.Schnuret al. [thisissue] and modeling in the form of radiative transfer (RT) models operateda truck-mountedC-band precipitationradar, located that can accuratelycompute reflectanceas a function of canto the south of the SSA from May to September1994. Com- opy structure,optical and biophysicalcharacteristics,etc. The parisonsof radar-derived precipitation fields with estimates validatedRT modelsare then usedfor algorithmdevelopment from a gagenetwork showedthat in termsof area averages,the to infer canopycharacteristics over larger areasgivensatellite radar estimated slightly higher precipitation accumulations. reflectance values. The algorithms take the form of either Simulationsperformed by usinga spatiallydistributedhydrol- simplefunctionalrelationsbetweenvegetationindicesand bioogy model showedthat spatial averagesas well as local esti- physicalcharacteristicsdevelopedby usingRT simulationsor matesof surfaceenergyfluxeswere quite sensitiveto the useof more sophisticateddirect inversionapproaches.The BOREAS radar-basedprecipitationimagesascomparedwith a gage-only experimentwas designedin the contextof this generalscaling product. However, when the effectsof spatialresolutionwere and algorithm developmentstrategy,and the remote sensing isolated(by usingonly the radar precipitationproduct at dif- papersin this specialissuereflect this approach. An important unresolvedissuegoinginto BOREAS was the ferent spatial resolutions), the sensitivityof area-averaged (over the 40 x 50 km southern modeling subarea) energy accuratein situ characterizationof canopybiophysicalparamfluxes to the precipitation product was generally less than eters over plots large enough to validate processmodels and about 10%. However, local differences were much more sen- remote sensingalgorithms. BOREAS staff and PIs designed and implemented an approach which combined destructive sitive to the spatial resolution of the precipitation product. sampling,allometry,and opticalmethods(hemisphericalpho6.4. Remote Sensing Science tography,photosyntheticallyactive radiation (PAR) sensors, Remote sensing served three critical roles in BOREAS. multibandphotography)to gencrategroundestimatesof canFirst, it provided local- to regional-scaleland cover data and opy architecture, leaf area index (LAI), biomass density solar radiation forcingswhich allow us to investigatethe pro- (BMD), and the fraction of PAR absorbedby the vegetation cessesgoverningforest-atmosphereinteractionson timescales canopy (FPAR) over 30 x 30 m plots and 100-m transects ranging from minutes, with the GOES satellite, to decades, within the BOREAS tower and auxiliarysites(Figure 2). Sevwith the Landsat seriesof satellites.Second,ground, aircraft, eral activitiesreported here deal with groundmeasurementsof and satellite data (Table 4) were used to develop and test these biophysicalcharacteristics. ecosystem-atmosphere processmodels.Finally,remote sensing Chenet al. [thisissue]useddirect and indirecttechniquesto imageswere processedto define the overall context and rep- estimateLAI for the BOREAS tower and auxiliarysites.They
SELLERS ET AL.: BOREAS IN 1997
28,755
GOES
AVHRR, LANDSAT, SPOT
SSM/I, ERS-1, Radarsat
Optical Studies Hall et al. (Classificationand Biomass;Landsat) Steyaertet al., (Classificationand Fire; AVHRR) Li et al. (Fire Detection;AVHRR) Cihlar et al. (Parameter maps; AVHRR) Mahrt et al. (Ts)
Czajkowski etal.(Ts)
"'
Markham etal.(Aerosols)
Radar
Studies
Way et al. (Freeze, Thaw; ERS-1)
Radiation
Fluxes
Gu and Smith (S, PAR; GOES) Kaminskyand Dubayah (Rn) Li et al. (APAR; AVHRR)
Airborne Optical RSS Radar Ranson et al. (Classification,biomass;RD)
Russellet al. (Images; RC) Bicheronet al. (Images; RC) Miller and O'Neill (Aerosols; RP)
,-d••• Loechel etal. (Radiances, RH) Passive Microwave
••-
Chang et al. (Snow; RT)
Models Ni et al. (RT Model)
Goel etal.(RTModel) Privetteet al. (PARABOLA, RT Models)
Local Scale Optical Studies Chenet al. (LAI) Fournieret al. (LAI, Architecture)
'•'•ll•l?,••'-'-:;' !• Middletonet Kucharik etal. (LAI, Architecture) I i; •':;;'*%•" t• r•'• al. (Leaf Spectra)
Milleret al. (GroundReflectance) Bubieret al. (MossReflectance)
Figure 16. Schematic summarizing papersin section6.4; remotesensingscience.(RC, RD, RP, RH, RT refer to aircraft platforms;seeTable 2).
compareresultsfrom their indirectoptical techniquewith those from a similar manufactured
instrument
and estimates
derived from direct destructivesamples.They found that the manufactured
LAI instrument underestimated
the destructive
of canopyelementsand crownform, data whichprovidecritical inputsfor their and other RT models.With their model they are able to computeand validatearchitecturalvariables suchasgapsizeasa functionof viewzenith.Their datasetswill
amongstandstructure, sampleestimates of LAI becauseitsalgorithmassumed a spa- be usedto explorethe relationships tially randomdistributionof leavesto computeLAI asa func- light regimes,canopyreflectance,and ecologicalfunctionin tion of canopygapfraction.Chenet al. [thisissue]developeda general. Kuchariket al. [this issue]utilized a two-bandvisibleand clumpingindexto correctfor thiserror andusedit to estimate camera,the multithe LAI of severalBOREAS forestsites.Chenet al. [thisissue] near-infrared(NIR) digitalcharge-coupled of found that LAI rangedfrom I to 4 for jack pine and aspen bandvegetationimager(MVI), to map the nonrandomness standsand from I to 6 for blackspruce.They alsoreport errors canopygaps,i.e., the spatial"clumping"of leaves.As menof the order of 25%, a magnitudeconsistentwith direct de- tionedabove,the spatialdistributionof leavesis a key paramstructivesamplingtechniques.The difficultieswith ground eter for opticalLAI-measuringdeviceswhichrelate gap fraccharacterization of biophysical variablesmustbe kept in mind tion to LA!; the distribution may also be critical to canopyCO2assimilation modelsandRT modwhen usingthem to evaluatesatellite-based remote sensing parameterizing els relatingcanopyreflectanceto biophysical parameters.Asalgorithms. wereimagedfrom belowwith Fournieret al. [thisissue]combinehemispherical photogra- pen andbalsampoplarcanopies phy, standmapping,destructivesamplingand modelingto the MVI to generatean NDVI image, which permitted the characterizethe architectureof vegetated boreal landscape identification of sunlit and shaded foliage, discrimination elements.They employ a variety of destructiveand optical amonglive foliage,twigsandbranches,and sky-clouddiscrimof techniquesto characterizethe three-dimensional distribution ination.Analysesof the imageryshowedthat an assumption
28,756
SELLERS ET AL.: BOREAS IN 1997
spatially random foliage leads to underestimatesof LAI in aspenby about 45%, which then leads to underestimatesin CO2 assimilationrates by about 39%. In supportof this,Dang at al. [this issue]show that the relationshipbetween canopy photosyntheticcapacityand spectralvegetationindex can vary considerablydependingon the scalingalgorithm used. An important and sometimesneglectedcomponentof forest remote sensingstudiesis the bidirectionalreflectanceof understoryvegetation.Millar at al. [this issue]describea multiteam effort to measure nadir understoryreflectancefor the tower
flux sites. Shaded
and sunlit reflectance
measurements
were made for black spruceand jack pine sitesnear solarnoon throughout the year, capturing the variability due to species and seasonalphenologybetween and within the tower flux sites.Temporal-spectralvariationswere observedwhich correlated with changesin vegetationtype and phenology.The data were shownto be usefulfor improvingregressionrelationships between leaf area index and vegetation indices. Multiband optical propertiesof mosseswere found to be a critical input for understandingthe relationshipbetween canopy reflectanceand biophysicalproperties.In addition to the leaf opticalpropertiesmeasuredby Middlatonat al. [thisissue], discussedin section6.1.1, Bubiar at al. [this issue]conducted high spectralresolutionmeasurementsof mossground cover on the BOREAS wetlandssites.They found that high spectral
The advantagesof multiangle observationsfor forest-type classificationare also addressedby Bicharonat al. [this issue] who describean analysisof bidirectional reflectance data acquired from the airbornePOLDER (Polarizationand Directionality of Earth Reflectances)instrument.The instrument wasflown aboardthe NASA C-130 aircraft over fivevegetated areas representativeof BOREAS land cover. Using multiple spectralbandsand three view directionsgreatly improved the discrimination
of forest cover.
6.4.2. Radiative transfer models and algorithm development. The most direct approach to using RT models in in-
ferring biophysicalcharacteristics of the land coveris through mathematicalinversionof the RT model given a set of reflectancemeasures.However, this approachis often limited by the number of parametersin the RT model, which in many cases exceedsthe number of remote sensingmeasurementsnecessaryfor inversion.To circumventthis problem, sensitivityanalyses often reveal which parameters can be held fixed with minimum impact to the inversion,thereby reducingthe number of bands required for inference. BOREAS investigatorsand staff acquired validation data setswhich are ideal for evaluating different direct inversion techniques.Privattaat al. [this issue] describesome of the above-canopymeasurementsacquired in BOREAS with the PARABOLA instrument,a three-bandradiometer capableof resolution reflectance data could be used to discriminate the rapidly acquiring reflectance measurementsover the entire brown and sphagnummossesassociatedwith bogs, fens, and viewing hemisphere.PARABOLA was used to collect BRDF water-saturated conifer stands from the feather mosses assodata setsat the BOREAS SSA aspen,black spruce,and jack ciated with more productivestandsgrowingon mineral soils. pine flux tower sites(SSA-OA, SSA-OBS, SSA-OJP) usinga SinceHardan at al. [this issue]report that drainageclassis a 100-m-long tram-mounted system elevated several meters strongdeterminantof carbonstoragerate, the resultsof Bubiar abovethesecanopies.The observedBRDFs were usedto evalet al. [this issue]provide a basisfor designingremote sensing uate the performancesof 10 different RT modelsin predicting algorithms that can map these important classesover large albedo and nadir reflectanceover nine land cover typesrangareas. ing from grasslandsat the FIFE site to the forest data setsof Moving upscalefrom ground to aircraft,Loachalat al. [this BOREAS. Their findings provide an important lesson for issue]report on a seriesof helicopter-basedreflectancemealarge-arearemote sensingalgorithms.They showthat simpler surements acquired over the BOREAS tower and auxiliary sitesthroughoutthe 1994 growingseason.They comparedred RT models using fewer parameters may be both faster and and NIR reflectance measurementsto ground measuresof more robust for inversion than complex RT models, even overstoryLAI. Their resultsshowthat reflectancein individual thoughthe simplermodelsdo not representthe BRDF aswell. bands as well as common vegetation indices based on these They attribute this to the fact that with simpler models,more bands are poorly correlated with LAI, confirmingthe results accurate inversionsare possiblewith sparseremote sensing data sets. obtainedby Hall at al. [1995] over similarspeciesin the SupeGoal at al. [thisissue]demonstratea computer-based simurior National Forest in Minnesota.Hall at al. [1995] analyzed lation approach for investigating the utility of reflectancethe difficultiesfound by Loachalat al. [thisissue]and proposed and evaluated alternative approachesthat demonstrateskill based remote sensingalgorithms and vegetation indices to for those situationswhere vegetationindicesperform poorly. infer canopybiophysicalcharacteristics.They use a sophistiHall at al. [this issue]successfully applied thesetechniquesto cated computergraphics-basedmodel to simulatethe relationshipbetweenred and NIR reflectanceand canopybiophysical black sprucecoversin BOREAS (section6.4.2). characteristics at variousview and illumination angles,includBidirectional reflectanceimages similar to those to be obtained from the MISR sensor to be flown aboard EOS AM-1 ing the "hot spot"where the view and solarilluminationangles were acquired over the BOREAS SSA and NSA sitesfrom the are identical. They varied the magnitudesof the biophysical airborneadvancedsolid-statearrayspectroradiometer (ASAS) characteristicsused as parametersin the canopyreflectance mountedon the NASA C-130. Russallat al. [thisissue]report model (crowndimensions,crownspacing,leaf size,etc.) over on these spectral and bidirectional reflectancedata acquired realisticrangesto simulatethe covarianceamongtheseparamover the BOREAS SSA tower sites.They found that bidirec- etersand reflectance.This permitsthem to investigatehow the tional reflectancedistributionfunction(BRDF) measurements relationshipbetweenreflectanceand eachof the canopymodel at 26ø backscatterin the solar principal plane were best for parameterssuchas leaf sizeis affectedby knownvariationsin discriminationamongspecies.They alsoconfirma large num- the remainderof the canopymodelparameters.They show,for ber of previous studies showingview angle to significantly example,that for deciduouscanopies,the ratio of NIR reflecaffect NDVI and its relationship to biophysicalproperties. tance for a givenview angle to hotspotreflectanceat a high Their measurementsshow that view angle effects are much Sun zenith angle is the best index for estimatingleaf size since more important for conifer standsthan for aspen. it minimizesthe effectof variabilityin the other modelparam-
SELLERS ET AL.: BOREAS IN 1997
eters. For conifers they show that leaf size is difficult to estimate exceptfor sparselyspacedcanopies. Ni et al. [this issue]further developgeometricaloptical canopy reflectance models to include light interception effects within tree crownsfrom the nonrandomspatialdishibutionsof leaf material typicalof needleleafcanopies[seeChenet al., this issue;Fournieret al. this issue;Kuchariket al., this issue].For a specifiedcanopyleaf area index,leaf clumpingincreasesgap probabilitywithin a tree crown,increasinglight transmissionto the canopyfloor; however, this is partially offset by reduced light transmissivitythrough the clumps.The primary motivation for the work of Ni et al. [this issue]was to improve the radiative computations for snowmelt models, an important problem for properly parameterizingalbedo in weather forecastand climate models[seeBettsand Ball, this issue],as well as in initiatingphotosynthesis in carbonmodels[seeFrolkinget al., 1996].Consistentwith this motivationthey test their model by comparing its estimates of downwelling solar flux through the canopy profile to measured values. These comparisons showtheir modified model to agreewell with measuredvalues; in addition, they show that the vertical distribution of radiation is quite different than would be predictedby Beer's law, pointing out the importanceof proper representationsof tree morphologyto both photosynthesisand snowmeltmodels. 6,4.3. Landscape-scaleland cover and biophysical characteristics algorithms. A number of BOREAS investigations developedand producedmultiyear site-level,studyarea level, and regional-scalemaps of radiation and biophysicalparameters. This parameter set was defined by joint efforts between the BOREAS modelinggroup and the RSS investigators,identifying those land cover classesand radiation parametersconsideredto be essentialas inputsto the processmodelsand also spectrallyseparableusing remote sensingmultispectral,multidate reflectancemeasurements(see Figure 17). Hall el al. [this issue]developeda physicallybasedapproach to integratedland cover classificationand biophysicalparameter estimationusing LandsatThematic Mapper (TM) data. They utilized geometric RT models to compute the expected spectralsignaturesof the land coverclassesas a function of the distribution of biophysicalparameterswithin each class.The algorithm does not depend on unrealisticstatisticalassumptions, such as multiw•riatc normal distributions,to represent classsignatures.Further, it can compute the signaturesas a function of changing view and illumination conditions and other w•riablesthat give rise to global variations in the signatures, thus rendering the signaturesmore invariant to changing global conditions. Using this algorithm, they classify the BOREAS SSA and estimatethe proportionsof the land cover classesused in modeling. In addition, they estimate biomass density(BMD) for black sprucein the studyarea. BMD is an importantparameterfor ecosystemmodelssinceaboveground carbon is related to AN}'}' and litter fall [Gower et al., this issue].BMD is usuallyrelated to maintenancerespiration,and in conifer wetlands, BMD may be related to below-ground drainage and age class. As Harden et al. [this issue] show, drainage and age classcorrespondto varying annual carbon storage rates. The Hall et al. [this issue] map for wetland conifers in the SSA showsBMD to be bimodally distributed
28,757
BOREAS studyregion at 1 km resolutionusingan algorithm developedby Lovelandet al. [1991] (Plate 2a). The algorithm utilized monthlycompositesof normalizeddifferencevegetation index(NDVI) observations acquiredby the advancedvery high resolutionradiometers(AVHRR) carriedon the NOAA meteorologicalsatellites.The resultingland coverproductwas evaluatedby comparisonto the Hall et al. [thisissue]30-m TM land cover product to investigatethe effects of scale on land coveridentification.The Steyaert et al. [thisissue]map showed that wetland conifer dominatesthe BOREAS study region. Wetland conifer is an important landscapeelement in carbon sequestration[Goweret al., this issue;Hardenet al., this issue]. Steyaertet al. [this issue]also discoveredanother result key to understandingthe regionalcarbonflux; the boreal ecosystemis more fire prone than previouslybelieved,with nearly 30% of the area having been cleared by fire in the last 25 years. Ransonet al. [thisissue]report on the resultsof usingSIR-C, the X-band syntheticaperture radar carried aboard the NASA DC-8, and LandsatTM to classifythe BOREAS SSA and map abovegroundwoody biomass.Plot-level measurementsof leaf, branch,bole size,and angledistributionswere usedto develop relationshipsbetweenbackscattercoefficientsin variousbands, then the relationshipswere applied to imagery covering the -2 SSA. Pixel-levelBMD accuracieswere estimatedat 1.6 kg m
overa rangeof biomass from0 to 15 kgm 2. Thistechnique could be a valuable mappingtechniquefor the boreal ecosystem which is often obscuredby cloudsor smoke. Liet al. [this issue(b)] usedthe 3.0-/•m channelof AVHRR single-date images to detect fire events during 1994 in the BOREAS region. This work complementsthe historical firescarmapsof St•yaertet al. [this issue].The Li et al. [this issue (b)] 1994fire map showsthat 99 firesoccurredin the BOREAS region during the summer of 1994, consuming about 20,000
km•, or about2% of thetotalBOREASregion.Their accuracy assessmentshowed that they detected 87% of the grounddetectedfires and, in addition, identified many fires missedby conventionalmethods.Compositeddata were not effective in fire detection, which limits the usefulnessof this technique when smokefrom fire and cloudsobscurethe region. Cihlar et al. [this issue]developedand applied an AVHRR preproccssingalgorithm to the NOAA 11 1994 growingseason AVHRR record to compute 10-daycompositesof surface reficctance
in bands 1 and 2 and emitrance
in band 4 based on
data with improvedcalibration,registration,and correctionfor atmosphericand angular reflectanceeffects. The composited data were
also screened
for cloud
and snow contamination
using the multidate algorithm developedby Cihlar et al. [this issue]. Clear-sky values in bands 1 and 2 were corrected for fixed and assumed densities of molecular, aerosols, and water
vapor absorption and scattering. Band 4 was corrected for atmosphericwater vapor effects to generate surface temperatures usingsplit-windowtechniques[Collet al., 1994]. Where no clear-skypixelsexistedin a 10-dayperiod during the growing season,replacementpixelsfor bands1 and 2 were substituted from adjacentcompositedperiodsusinglinear interpolation. Because the composited data set consists of observationsat multiple view angles,Cihlar et al. [this issue] also apply land-cover-dependentcorrectionsto bands 1 and 2 withthelowermode(1-6 kgm •) consisting of poorlydrained for view-anglevariations in surface reflectance.The preproareassuchas bogsand fens and the upper mode (8 to 10 kg cessed,composited data set was then evaluated by using TM m 2) consisting of moderately to betterdrainedareaswith a data and further processedusingalgorithmsdevelopedat the slightlylower water table. CanadianCentre for Remote Sensing(CCRS) to producereSteyaertet al. [this issue] mapped the 1000 x 1000 km gionalmapsof seasonalmultidate NDV1, surfacetemperature,
28,758
SELLERS ET AL.: BOREAS IN 1997
LAI, greenFPAR, PAR, albedo,and APAR (absorbedPAR) over the 1000 x 1000 km BOREAS studyregion. These parameter maps are being used in various processstudiesbut need further validation and comparisonwith other products (see also Plate 2b). Two papersdeal with the combineduse of satellitethermal and optical data to infer certain thermodynamicpropertiesof the vegetatedland surfaceand environmentalvariablesin the atmosphericboundarylayer. Mahrt et al. [this issue]examine the use of radiometrictemperatureto infer aerodynamictemperature, a key variable in sensibleheat transfer between the surface and the atmosphere.In essence,their work confirms similar research done in other ecosystems[e.g., l/ining and Blad, 1992;Hall et al., 1992], in that radiometrictemperature cannot be used to accuratelyinfer aerodynamictemperature for sensibleheat computation when using Monin-Obukhov similaritytheory. In the boreal environmentthe warm sphagnum mossbackgroundcontributessignificantlyto radiometric temperaturebut insignificantly to sensibleheat transfer(aerodynamictemperature).They show,however,that the difference in thesetwo temperaturesdecreaseswith red reflectance correspondingto densercanopiesand, subsequently, low visible backgroundfraction.Czajkowskiet al. [thisissue]usedthe AVHRR surfaceNDVI versusradiometrictemperaturein an attempt to infer surfaceair temperature.They found that most of the air temperaturevaluesinferred in thiswayfell within _+5 K of ground-measuredtemperatures,but there were observed differencesas large as _+15K. They concludethat there is a consistentbias of 3.2 K causedby errors in the split-window estimatesof surface temperaturesand subpixelclouds and standingwater.The main advantageof thisapproachwouldbe more explicit spatial information on surfaceair-temperature distribution;however, the frequent presenceof clouds and smokein the regionlimits opportunitiesfor applyingthistech-
menting model predictionswith direct measuresof the water statewill be a valuableadjunctto landscape-scale carbonflux
prediction.From ground measures,Way et al. [this issue] showedthat transitionsin soiland stemthawingrelate directly to the stateof soil respirationand canopyphotosynthesis, respectively.Using ERS-1 satellite radar images,they showed that shiftsin backscatteringcrosssectioncorrelatewell to soil thaw and possiblyto canopythaw as two independenttransitions (Plate 2c). 6.4.4. Radiationand atmosphericeffects. Five papersfocuson radiativecomponentsof the surfaceenergybalanceand
the effectsof atmosphericaerosolson downwellingand upwelling radiation retrieved from satellite observations.The net radiation R,• absorbedby the surfacethrough radiative exchangewith the atmosphereis a criticalparameterin carbon, water, and energycycling.To infer R,•, the componentsof the surface shortwaveand longwave radiation balance must be quantified.
Rn = S{1 - a} + Lwd- Lwu,
(1)
whereS isthe downwelling shortwave radiationenergyflux(W
m-2), a is the albedo,andL,•d andL,•u aretheincidentand emittedlongwave radiationat the surface(W m-2), respectively. In addition to S the downwellingphotosynthetically activeradiationflux (PAR) is alsoa key variablein the surface energybalanceof vegetativesurfacessinceit regulatesphotosynthesisand the stomatalcontrol of evapotranspiration. Gu and Smith [this issue] describe the modification and evaluationof a publishedalgorithmfor inferring S and PAR fluxesfrom GOES 7 imagery.Their mapswere generatedfor the entire BOREAS studyregion at 30-min intervalsand at 1 km resolution.Their modifiedalgorithmincorporatesa radiative transfer model to estimate aerosol transmittance
and re-
flectanceand accountsfor columnwatervapor.Satelliteresults nique. were compared with measurementsacquired in situ from The final two papers in this sectiondeal with the remote ground-basedradiometersand agreedto within about 2 to 7% sensingof snowand soil temperatures,two importantvariables for S and PAR fluxesrespectively,with a relativeprecisionof that other BOREAS studieshave shown stronglyinfluence around20% over clear, cloudy,and smokydays. interannualvariationsin carbon uptake by wetland conifers Kaminskyand Dubayah[thisissue]explorethe relationship [e.g., Frolkinget al., 1996]. Timing of snowmeltis critical in betweenR,• and surfaceshortwavefluxesusing1 year of 15determining the course of surface albedo, subsequentsoil min surface flux data from nine BOREAS surface stations to thawing,and the initiation of photosynthesis by wetlandconi- determinehow effectivelyR,• couldbe inferred from a knowlfer. Changet al. [thisissue]usebrightnesstemperaturediffer- edgeof the shortwavecomponentalone.Their analysisshows ences in the 18- and 37-GHz bands from microwave multiband that if the term S( 1 - a) in (1) wasknownprecisely,thenR,• radiometers
flown aboard the Twin Otter
to determine
snow
could be estimated with an rms error of from 18 to 36 W m -2.
water equivalent(SWE) and lake ice thickness.They evaluate two algorithmsthat differ primarilyin the parameterizationof meansnowgrainsize.In nonforestedareasthe two algorithm's SWE estimatesdiffer by from 4 to 10 mm from groundmeasuresfor a snowpackof about 40 mm SWE. In forestedareas the differencesare largerwith estimatedSWE valuesgenerally lower than ground observations.Overall, differencesin ob-
A multifrequencyradiometer, the TIROS N vertical sounder
ing SWE estimatesfor forest cover.
AVHRR
Completingthe pictureof the beginningof activephotosynthesis,Wayet al. [thisissue]exploitthe sensitivity of microwave backscatteringcrosssectionto dielectric differencesbetween frozenandliquid moistureto map soilandcanopyfreeze/thaw boundariesin the borealspringtime.Currently,soilandcanopy temperaturesare modeled as a function of soil and canopy thermalproperties,moisture,and air temperature.Thesevariablesare not easilyobtainedat the landscapescale;thusaug-
photosynthetically active radiation absorbedby the surface, APARsfc, a key variable in both surfaceenergybalanceand photosynthesis calculations.They comparetheir estimatesto
(TOVS), was used to obtain atmospherictemperatureand water vapor profiles over the FIFE site for input to atmosphericradiativetransfermodelsto estimateL,• d [Breonet al., 1990]. The satellite-derivedestimateswere compared to ground measuresof L,• d. Breon et al. [1990] estimatedthe error on a pixellevelto be about20 W m-2, underbothclear servcd and inferred SWE in the SSA with its denser forests are and cloudyconditions. larger than for the NSA, indicatingthe importanceof correctLiet al. [this issue (a)] apply an algorithm to process red and NIR
reflectance data to infer the amount of
flux tower measurements
and determine
their estimates to
havea biasof about10 W m-2. TheymapAPARs• for the 1000 x 1000 km BOREAS region and showthat surfacevariationsin cloudiness andsurfacealbedocausesignificant spatial variation in APAR,•c.
SELLERS ET AL.: BOREAS IN 1997
Miller and O'Neill [this issue] used the compact airborne spcctrographicimager (CASI) instrument to measure shortwave upwelling and downwellingirradiance at multiple altitudes
to examine
the effects
of airborne
smoke
from
forest
fires on downwelling radiation. They report on an episode where smoke from wildfires
reduced
surface insolation
at noon
from 800 to 680 W m 2 and PAR from 320 to 250 W m -2
They found also that smoke from distant fires, consistingprimarily of sulfate aerosols,had much higher single-scattering albedos
and lower
attenuation
coefficients
than aerosols
from
nearby fires from which the soot had not settled out. Markham et al. [thisissue]alsoverified the important impact of forest fire on atmosphericopacityfrom analysisof aerosol optical depth measurementsacquired by a network of solar radiometersscatteredthroughoutthe BOREAS region. They alsoreport on water vapor columnabundanceestimates.Their results show a dramatic interannual variation in atmospheric opacitydue to smoke.In 1994 and 1995 the numerousfires in
28,759
Of thesefactors,experiencewas probablythe mostsignificant in terms of augmentingefficiency. BOREAS is a large enterprise and therefore inevitably servesas a test case for what "big science"(i.e., large-scale, interdisciplinary,coordinated)projectscan and cannot do. So far, the indicationsare that asin FIFE and similarprojects,the sum will be much greater than the parts in terms of scientific dividends.A data set is being constructedwhich will link our understandingof small-scalebiologicalprocessesto zonal climate variations and the global carbon cycle. The multiscale designof the experimentwill permit rigoroustestingof the scaleintegrationtechniquesthat will be employedin the process. Most of the papers in this specialissuereport on data acquisitions,data analyses,or the resultsof preliminarymodeling studies.On the basisof previousexperience(see,for example, Sellersand Hall [1992] and Hall and Sellers[1995]), another couple of years will see integrated studiescarried out using multiple data sets from all the contributing disciplines in BOREAS. These follow-on studies are expected to directly addressthe goalsof BOREAS and are discussedin section7.5. However, even at this early stage in the analysisphase of the project, some important scientificfindingshave emerged. The papersin this specialissueand the synopses of chapter 6 are organized by scientific subdisciplineand spatial scale.
the BOREAS region reportedby Liet al. [this issue(b)] and Steyaertet al. [this issue]resultedin a much wider variation in aerosolopticaldepthsacrossthe regionthan in 1996where few fires were detected. Atmospheric optical depth varied widely over the area from as low as 0.02 (at 500 nm) under clear-sky conditionsto as much as 4.5 (at 500 nm) becauseof smoke. The scientific review in this final section reverts to the structure Data from Markham et al.'s [thisissue]measurementsare used outlined in the introductionand in Figure 1, namely, physical widely in atmosphericcorrection algorithms to correct satelclimatesystem,carbonand biogeochemistry, and ecology,with an lite-measured radiance to surface reflectance. Such corrections additionaldiscussion of progressmadein remotesensingscience. are critical for use in physicallybasedclassificationand biophysicalparameter retrieval algorithmsof the typesdeveloped 7.1. Physical Climate System by Hall el al. [this issue]. BOREAS results have already had a direct impact on climate modelingand numericalweather prediction(Figure 17). Observationsfrom the mesoscalemeteorologicalnetwork and 7. Scientific Summary and Future Research tower flux sites have provided new insight into the radiation Directions and energy budgets of the forest and how these affect the Planning for BOREAS was initiated in 1990. Starting in regional climate. 1993, the scicncc team and staff set in place a significant infrastructure
in the
Canadian
wilderness
and
mounted
10
major intensiveficld campaigns(IFCs): one in late 1993,five in 1994,and four in 1996.Thc campaignswere complementedby a monitoring program of data acquisitionwhich spannedthe period 1993-1996. Each IFC, and the BOREAS experimentas a whole,wasconductedaccordingto detailedexperimentplans which translatedscientificgoalsinto operationalrealities. 1994 saw the most intcnsc and wide-rangingactivitiesin the field, includingover 350 airborne scienceflights; 1995 was a year of analysis,assessment, and planning;and 1996 sawthe redeployment of many field teams to fill critical gaps in the 1994 data set.
When snow free, the BOREAS
coniferous sites were ob-
served to have the lowest growing season albedos for any vegetated surface that we know of, about 8%. The winter albedo
of the forest
was measured
to be much lower
than the
value used by a number of AGCMs. Most NWP models treat snow albedo effects over the forest in the same way as for grass-coveredor agriculturalsurfaceswhere the vegetationcan be completely covered by snow to give a very high surface reflectance.In fact, forestvegetationusuallyprojectsabovethe snow,and even bare deciduoustrees can present a very dark surface to the solar flux, which is also incident at a glancing anglefor much of the winter (Figure 7a) [Bettsand Ball, this issue].Shortwaveradiation is therefore efficientlyintercepted by the forest and largely convertedinto sensibleheat flux and outgoing longwave radiation during the winter. Omission of this effect in the European NWP model, which carried a winter albedo of around 0.80 as opposedto a value of around 0.25 as observedin the field, resulted in the systematicunderestimation of near-surfacewinter air temperaturesby up to 15øC during the BOREAS 1996 winter field campaign(Figure 7b). Since then, the model has incorporated more representative winter albedovalueswhich hasresultedin great improvements in the predictionof the near-surfaceair temperature,reduced lower tropospherictemperaturebiases,and improvedforecast
In terms of the personneland resourcesinvolved,BOREAS is roughly3 times the sizeof the FIFE experiment[Sellerset al., 1992; Hall and Sellers,1995] and was also conductedover a much larger, sparselypopulated area. Operations often involved coordinated activitiesby over 200 people and up to 10 aircraft at a time over the two studyareas.The BOREAS data set is estimatedto be roughly5 timesthe sizeof the FIFE data set and is considerablymore complex.Likewise,the sciencein BOREAS spans a wider range of disciplinesthan in FIFE. However, the managementand staff head count for BOREAS is almostthe same as it was for FIFE, which was possibledue to the experienceof many of the veteran staffersand scientists scores over the North Pacific and North Atlantic. As of Deinvolved,improved procedures,and the use of better technol- cember 1996, the European NWP model included these ogyfor communications,data collection,and data compilation. changesin its operational forecasts,and there are plans to use
28,760
SELLERSET AL.: BOREASIN 1997 AtmosphericGCMs
Regional/Mesoscale ß Deep, Dry, Boundary Layers ß Reduced
Cloudiness
and
Precipitation
Global
Satellite/airborne
RSS
Surface, Upper A•r Meteorology
ß Surface condition ß Radiation ß Land cover
Radiation
Mesoscale
Regional
Local
Scale
Heat,H20,"[:fluxes ,•
ß Low Evaporative Fractions
ß Very Low Summer Albedos ß Low Winter
Albedos
Local Scale
H20J/'
Plot Scale
/'1• //'
Canopy physiology
ad•abon budget
Low Canopy Conductances Due To:
ß Frozen Roots in Spring
Plot Scale
ß Low PhotosyntheticRates in Summer
Figure17. Schematic summarizing gainsin thephysical climatesystem science areadueto BOREAS;see section 7.1.
satellitedata to track snow-relatedalbedodynamicsduringthe springand fall.
to 3000 m), and reducedcloudiness.In the EuropeanNWP model, the systematic overestimation of the latent heat flux
The low evapotranspiration ratesobservedat the BOREAS forest sites,particularlythe coniferouscovers,are not representedcorrectlyin mostatmospheric models.In the middleof
over the boreal forest resulted in overpredictionof precipitation and cloudinesswithin the region during the BOREAS
thegrowingseason, the depressed evapotranspiration ratesare
1994 growingseason[Sellerset al., 1995b].Again, BOREAS resultshavehad an immediateimpacton NWP modeling,and
due to low stomatal conductances associated with the low it is expectedthat the BOREAS datawill be extensively used in climatemodelsin photosynthetic ratesof thesespecies, whichare adaptedto to improvelandsurfaceparameterizations nutrient-poorenvironments. In the springthe veryhighsensi- the near future [Sellerxet al., 1997]. ble heat fluxes observed over the forest seem to be due to late
thawingof the soil, so the root systems remainfrozen and canopytranspirationis cut off. In wetlandareasthe forest canopyinterceptsand processes almostall of the available energy,so the wet soil or moss-covered surfaceplaysonly a minor role in the surfaceenergybalance.Interceptionand partitioningof mostof the radiationfluxby the forestcanopy thusdelaysthe warmingandthawingof the underlyingsoil,so the bulk of net radiation is shunted into sensible heat flux. Airborne flux measurements in BOREAS have also shown
how the thermal inertia of the lakesgreatlyreducestheir contribution to the surface heat fluxes. All these effects combine
to make the boreal forest,which includeslarge proportionsof lakes,wetlands,and fens, a surprisinglystrongsourceof sensible heat and a weak source of latent heat, comparedwith
temperategrassland sites[Sellers and Hall, 1992]and tropical forests[Shuttleworth et al., 1984a,b]. This situationoften resultsin the generationof a very dry warm lower troposphere with a deepandturbulentatmospheric boundarylayeroverthe forest during the growingseason,more typical of a lowerlatitude arid zone than would be expectedfor a high-latitude biome suppliedwith plentiful water. These phenomenawere directlyapparentto flux aircraftcrewsin BOREAS 1994who, in makingthe transitfrom the agriculturalregionnear Prince Albert northward into the forest, often encounteredgreatly increasedturbulence,deeper boundarylayers(sometimesup
The flux aircraftprovideda mesoscale pictureof the surfaceatmospherefluxeswhich complementsthat providedby the tower flux measurements.In general, aircraft-to-aircraftand aircraft-to-towercomparisons were very encouragingand reinforcethe perceptionthatthe regiongivesoff muchlesslatent heatthanpreviously thought.As in otherfieldexperiments, the flux aircraft seemto systematically underestimatethe surface fluxes,most likely becauseof problemswith capturingvery
longwavelength turbulentstructures; seeDe,sjardim • e! al. [this issue]and paperscited by Selle•:¾ and tIall [1992]. AircraftmeasuredCO: andwatervaporfluxeswere foundto be closely relatedto spectralvegetationindices[Ogtmjemiyo et al., this issue],which concurswith previousfindingsfor grasslands [Cihlaret al., 1992]andwith theory[Selle•:• e! al., 1992b].This latest result for boreal forest holds considerablepromise for future modelingwork. Lakes were seen to act as important
playersin the regionalenergybalance,and more attention needsto be paid to their role in the regionalclimate.It seems that the lakes act as large storesfor heat and give off little sensibleheat flux duringthe growingseason.They may alsobe associatedwith generatingmesoscalecirculationsduring periods of weak synopticflow. A number of BOREAS investigationsdeal with the problemsof modelingsnowinterceptionand snowmeltin the forest; the resultsof these will doubtlessimprove the realism of winter-springtransitionsas modeledin AGCMs. Also, satcl-
SELLERS ET AL.: BOREAS IN 1997
28,761
Regional Estimates
Regional Carbon Budget
RegionalCO2, RSS
Vegetation
From Aircraft
ß Correlationof AirborneCO2
Classification
and Vegetation Properties
Fluxes With RSS Data
From Satellite
ß Classification, Fire Extent
Data
ß Interannual Variability Flux Measurements
Drivers of Photosynthesis and Respiration
Integrated Models
ß PAR
ß Temperature
CO2
Flux Tower Scale Integrations
ß Water
Balance
ß Ecosystem State
ß Carbon SequestrationRates for Different Cover Types ß Weather and Interannual Variations in Carbon Fluxes
ß Scaling-Up Studies
Process Studies
Canopy physiology Soil/Moss/Stem Respiration Trace Gas Exchanges Litterfall
Models
Chambers, etc.
RootSystemDynamics
Carbon Cycle Components ß Low PhotosyntheticRates
ß Canopy Profilesof PAR, N, Photosynthesis,Conductance
ß Controlson Soil, CO2, CH4 Emissions
Slow Carbon Cycle
Figure 18. Schematicsummarizinggainsin the carboncyclesciencearea due to BOREAS; seesection7.2.
lite-borne radar has been shownto provide usefulinformation about these transitionswhich are important for both climate and carbon models. Correct representationof soil moisture and surfacehydrologicalprocessesin AGCMs and regional hydrologicalmodelsis obviouslycrucial, and it has been observedthat small variationsin the hydrologyof this biome can have significanteffectson the carbonand trace gas budgets. The BOREAS hydrologicalstudieswere well integratedwith tower flux measurementsand have been used as an independent checkon the growingseasonlatent heat fluxesas well as furthering our understandingof soil moisture controls on evapotranspiration. The radiation budget of the forest can be estimatedwith good accuracyfrom satelliteobservationsprocessedby algorithmswhichwere refined usingBOREAS data. However, the
been used to partially validate global data setsgeneratedfor globalmodels[Meesonet al., 1995;Sellerset al., 1996b]. 7.2.
Carbon and Biogeochemistry
The BOREAS field program has provided us with a comprehensivedata set covering almost all componentsof the plant and soil physiologicalcontrols related to the energy, water, and carboncyclesin the region (Figure 18). BOREAS resultshave alreadyshedconsiderablelight on someaspectsof the missingglobal carbonproblem. Could the boreal ecosystem with its low productivitiesbe sequesteringa significant
portionof the 1-3 Gt C yr-• whichis unaccounted for in the annual global carbon budget? The deciduousand coniferousforests of the boreal ecosys-
temoccupy12-20millionkm2, depending on thedatasources
used[seeWhittaker,1975;DeFriesand Townshend, 1994;Sellers shown to be more difficult than first thought; the radiometer et al., 1996b].Dependingon whichestimateof the borealforest intercomparison effort of HodgesandSmith[thisissue]showed area we use, the globalboreal forest need only sequesterbethat a commonlyused instrument systematicallyunderesti- tween50 and80 g C m-2 yr-1 to explaina 1 Gt C yr- • global mates the net radiation flux as its transparent dome cover sink,and this doesnot includecarbonuptake by high northern deterioratesover time (see also Smith et al. [1992]). Surface latitude tundra, which has been implicatedas a carbonsink by measurements have shown that smoke and associated aerosols Myneni et al. [1997]. What do the BOREAS plot-level carbonflux measurements are importantfactorsin reducingthe incidentsolarflux during the growingseason.The large-scalevegetationclassification of canopyand soilsindicatewith respectto theseflux magniproductsgeneratedfrom satellitedata will alsobe essentialto tudes? Measured annual NEE values at the BOREAS tower correctlydefine the contributionsof different surfacetypesto sitesrangefrom 130g C m-2 yr-• (a moderatesink)in the the regionalfluxesof heat, water, momentum,as well as car- southernold aspen(SSA-OA) site[Blacket al., 1996]to a weak bon, discussed below.The BOREAS albedo data have already carbonsourceof 50 g C m-2 yr-1 at the northernold black in situ measurement
of net radiation
fluxes over the forest was
28,762
SELLERS ET AL.: BOREAS IN 1997
to 50g C m-2 yr- • overthelastcentury. Modelsimulations for
The trace gas studiesin BOREAS showedthat drier sites (aspen,jack pine) with fast nitrogencyclingin the soilscould act as strongsinksfor CH4, while many of the wetter siteswere strongsources,particularlythe beaver ponds and fens. Some
conifer wetlandsfor the years 1968 to 1989 yield a calculated
intermediate
spruce(NSA-OBS) site for 1996 [Gouldenet al., this issue].A numberof chambermeasurements[Trumboreand Harden,this issue]showthat the boreal wetland peatshave accumulatedup
sites switched from source to sink status based on
variationin carbonfluxfroma loss(source) of 50g C m-2 yr- • variationsin the water table. Surprisingly,chamber measureto a gain(sink)of -140 g C m-2yr-• [Frolking etal.,1996].His ments made at fen siteswere found to scaleup well to match 1994modelvalueof -51 g C m-2yr -• for theNSA-OBSsite eddy correlation measurementsof CH 4. CO fluxes from a agreeswell with the measuredvalue of Gouldenet al. [this issue]. Thus although the boreal forest is characterizedby low
aboveground productivity of between 50 and100g C m-2 yr- • [e.g.,Goweret al., this issue],equal or greaterannualrates of fine root productioncombinedwith long carbonturnoverrates of 500 years [Trumboreand Harden, this issue] in the cold anaerobic soils of the boreal forest wetlands appear to be sufficientto explain a significantsink of global carbon when extrapolatedover the large area occupiedby the circumpolar boreal ecosystem. It shouldbe emphasizedthat at this early stagein the analysis,inferencesof boreal carbon flux at the global scalefrom the BOREAS sitescan only serveto showthat we cannotrule out the boreal ecosystemas an important global carbon sink. Clearly, the dynamicsand strength of carbon sequestration differ by land cover type, climate, soils,topography,and many other factors that are more aptly captured using satellite extrapolations combined with ecosystemprocessmodels. For example,small chambermeasurementshighlightedthe large spatialvariabilityin soilandmosscarboneffluxeswhichappear to be highly dependent on soil moisture content and litter quality. Beaver pondswere observedto be very strongsources of CO2. The observedlags between carbon uptake by photosynthesis,which is confinedto the short growingseason,and carbonlossby respiration,whichcontinuesthroughoutmostof the year, emphasizethe need to correctlymodel the different controls on carbon sinks and sourcesin large scale models. Cihlar et al. [thisissue]usedsatellitedata analysesto showthat fires burned over 3% of the BOREAS region during 1994. Steyaertet al. [this issue] show that nearly one third of the BOREAS studyarea hasbeenburnedwithin the last 25 years. Carbon
contributions
from
fire disturbance
must be factored
into global estimates.A more reliable estimateof the magnitude of the global sinkwill be made after scalingstudiesin the BOREAS follow-on program has been completed. By partially elucidating the mechanismsunderlying atmosphere-biospherecarbon exchange,BOREAS tower flux and chamber measurementsare also leading to a better understandingof the magnitude,trends,and interannualvariationin
range of fire siteswere measured,but more work needsto be done before an estimateof regional CO fluxescan be made. The meteorologicaltrendsfor 1961-1990 analyzedby Chapman and Walsh[1993] showthat air temperatureincreasesare most rapid over high-latitudecontinentalinteriors,with documented temperature trends as high as 1.25øC per decade. These
observed
trends are also consistent
with AGCM
simu-
lations showing high-latitude seasonal air temperature increaseswith increasingatmosphericCO2 concentrationsand inferencesaboutthe extensionof the boreal growingseasonby about six days over the last 15 years (see also Keelinget al. [1996] and Myneni et al. [1997]). If the increasedwinter and springtemperaturesare coupledwith relatively cool summers, as it would appear, global warming could initially lead to increasedproductivity,hence increasedcarbon sequestrationat these latitudes. However, BOREAS soil carbon flux data show
that longer warmer growingseasonscould also lead to more rapid loss of older deeper soil carbon stores which would partially offset gainsfrom productivityincrease. The biometric studiesshowedthat the abovegroundnet primary productivity(ANPP) of the boreal forest is relatively small and the abovegroundcarbon stocksare low compared with temperate and tropical ecosystems.However, large amounts of carbon are stored in wetlands, and these stores are
sensitiveto climate change.With regard to the annual carbon balance,we are presentedwith a picture of an ecosystemoperatingvery closeto the marginsof carbonprofit and loss,with growingseasonlength and summertimetemperaturesperhaps being the deciding factors. Sequestrationof carbon by any ecosystemis a nonequilibrium process.In the longer term, changesin fire frequency,climate regime and vegetationsuccessionpatterns, and warming and drying of boreal soilswith associatedincreasesin respirationcould greatly alter or even reverse
the
"boreal
carbon
sink"
scenario
described
above.
Follow-on analyses of ecological data collected during BOREAS will provide valuable insight into the mechanisms driving future changes. 7.3.
Ecology
The future courseof carbon dynamicsin the boreal zone is
globalcarbonflux.For example,measurements andmodeling thought to depend on variationsin the physicalclimate in the have shownthat interannual variation in the timing of snowmelt is a major factor in determining annual carbon uptake rate. BOREAS measurementshave shownthat carbonuptake does not begin until the snowmelts and the root zone thaws. Other measurementsshowthat carbon respirationlossis not appreciablebelow about 17øC;thus immediatelyafter snowmelt and soil thaw, cool air temperaturesare conduciveto rapid carbon uptake. However, as the soil warms up and air temperaturesrise above 17øC,soil respirationis enhancedand the net ecosystemcarbon lossincreasesrapidly. These observations are supported by BOREAS modeling studieswhich indicate that years with early springsand cool, wet summers are associated with strongerannualcarbonuptakerates [Frolking et al., 1996].
near term and on ecologicalresponsesin the midterm and long term. Will warming drive the boreal ecosystemto a higher productivity state, populated by plant communitiesthat can sequesterhigherlevelsof carbonthan the presentborealcommunities, or will a new low-productivity equilibrium be achievedlimited by the paucity of the boreal soils?Paleoclimate studiesrevealthat this ecosystem migratedto muchmore southerlylatitudesduring the last glaciation,then retreated to its present northerly position as the ice receded. Will the anticipatedrapid warming induce ecologicalinstabilities,resulting in rapid carbonlossfrom the great reservesof carbonin the largely anaerobicconifer wetlands before other plant forms caneither fix the carbonin their livingbiomassor act asagents in stabilizingthe soil? To understandthese longer-term dy-
SELLERS ET AL.: BOREAS IN 1997
28,763
Satellite
RSS
AVHRR,
ES
LANDSAT, SPOT
Regional/Mesoscale
SSM/I, (DMSP),
ERS-1
Biophysical Parameters VegetationClassification Fire Extent, Frequency Regional
Freeze/Thaw
,•
i
....i
ß
!
-•
Scale
Models
Intercomparisons Airborne Optical and Microwave
Mesoscale/Local
(Activeand Passive)RSS
Scale
:•:•'..' ........ •:•.'.*.'•:•i• •.:....
BiophysicalParameters VegetationClassification MultiangleTechniques
Study Area
Scale
RT Models
,• • In Situ/Plot
In-Situ Ra. diometr[c and
Scale
ß BiophysicalParameters (LAI, FPAR....) ß Reflectances, emittances
Local
Scale
ß In Situ Verification
Figure19. Schematic summarizing gainsin ecology andremotesensing science dueto BOREAS;see sections 7.3 and 7.4.
effecton the namics,a numberof questionsmustbe addressed. Where is inganddryingclimate,couldhavean enormous the carbonbeingscquestered: in the canopy,litter or below carbonand water budgetsof the region.This speculationis groundbyfinerootturnover? Whatis the relativeimportance supportedby the tower flux and flux aircraftmeasurements of the variousboreallandcovertypesin thisprocess? How will carriedout at largerspatialscales,both of whichreconfirmed andphotosynthesis the source/sink strengthdependon climate,particularlygrow- the tightlinksbetweenevapotranspiration ingseason length,andwill respiration beginto dominateover during the growingseason.The flux aircraft observations carbonupphotosynthesis in response to globalwarming;renderingthe showedthat mesoscalepatternsof photosynthetic ecosystem a net sourcerather than a net sink of carbon'? takewerecloselyrelatedto spectralvegetationindexinformaWouldincreasing firefrequency leadto increased or decreased tion, which can also be used to estimate large-scalecanopy The highesttranspirationand carbonuptake carbonsinkstrengths overall?DatacollectedduringBOREAS conductances. rates were observed over the deciduous stands near the southwill be usedto addressthesequestions. by the The ecophysiological studiesin BOREASclarifiedmanyof ern edgeof the biome,whichwere alsocharacterized the linksbetweenvegetationtype and stateand the carbon, highestspectralvegetationindexvalues.The towerfluxmea-
energyandwaterbudgets of the region,whilebiometric and surementsshoweda wide variation in carbon fluxesamong the
remotesensing studieshavelaid the groundwork for carrying differentcovertypesstudiedin BOREAS, but mostof them out regionalsurveys of ecosystem stateusingremotesensing seemed to be weak sinks on an annual basis.Given that up to data(Figure19).The ecophysiological workhasbeencovered 3% of the boreal forest is consumedby natural fire eachyear, for an equilibrium borealcarbonbudalready,so thisdiscussion is limitedto a reviewof progress it is entirelyreasonable of weaksinkscoveringmostof the regioncounmadein understanding ecological processes on relativelylong getto consist teredby catastrophic fire eventswithinrelativelysmallareas, timescalesand over large spatialscales. The leaf-scaleandcanopy-scale measurements conducted in their recoveringto a strongersinkas newgrowthdevelops.It BOREASpresentuswitha consistent pictureof an ecosystem should also be borne in mind that in a warming and drying wouldchangealongwith physiological operating at a relatively lowlevel:thedeciduous canopies were climate,fire frequency Observedto have much lower photosynthetic capacitiesthan
theirtemperatecounterparts, andtheconiferous species were observedtOhaveevenlowercapacities, abouthalf thoseof the
and specieschanges. 7.4. Remote SensingScience
Remote sensing research and development during deciduous sl•ecies. Clearly,large-scale replacement of the cothe nifersbydeciduous species, whichislikelyto occurin a warm- BOREAS producedresultsthat are key to investigating
28,764
SELLERS ET AL.' BOREAS IN 1997
soil and bole thaw, also critical in initiating photosynthesis in the models.Remote sensingalsoshowedthat the role of fire in Landcover Type Canopy Physiology the boreal ecosystemis evenmore importantthan was known Soil Processes Fire Frequency BiophysicalAttributes Scaling Methods previously. Steyaertet al. [thisissue]usedAVHRR visibleand LAI, Biomass Surface-ABL Models Albedo near-infrared data to show that about one third of the study Canopy Conductance/Photosynthesis Estimates regionhasundergonefire over the last 25 years,a much larger proportionthan previouslybelieved;Cihlar et al. [this issue], usingAVHRR data showedthat 3% of the ecosystemburned during 1994 alone. Importantly,both of theseefforts showed Improved Carbon-Energy-Water Regional Fields for that AVHRR data canbe usedto reliablymap the major land Submodels for Global Models Global Models covertypes,making it possiblefor a careful assessment of the Radiative Transfer AlbedoSurfaceConductance, zo global boreal biome. Myneni et al. [1997] showed how the Momentum Transfer Carbon Sink/SourceStrength Heat, Water Exchange Land Use/Land Cover Change 15-yearAVHRR record can be used to monitor the response Carbon Cycle of the biome to climate warming. Finally, remote sensingwas usedduring BOREAS to developseasonallyvarying,regionalscaleparametermapsfor all the major drivingvariablesgoverningcarbon/waterand energyprocesses(Figure 16). These Global Change maps should prove to be invaluablein scalingmodels and • Validation/Forcing Validation/Forcing Model .,. hypotheses from the plot level to the region. , AGCM ! Satellite Retrievals Meteorological i!!!!! Process
Satellite Algorithms
Studies
! ,
!
Analyses
,'
t
,
,
,
7.5. I --Freeze/Thaw Radiation Fluxes
i
Surface Model II!!li -Snow Cover
Calculated Time-Series Fields
Comparisons
PhysicalClimate System Parameters
Isotopic Analyses GlobalTracerModels
: ,
,
,,
Carbon
Fluxes
Figure 20. Schematic showing how different elements of BOREAS sciencecouldcombineto improvethe performance and realism of global change models; see section 7.5. The resultsof the BOREAS processstudieshaveled to better land surfaceparameterizationsfor AGCMs and alsoimprovedcarbon cyclesubmodels;see two boxesin top left of the figure. The remote sensingscienceand ecologicalresearchwill lead to better classificationschemesand more accuratespecification of biophysicalparametersover the entire boreal zone;seetwo boxes in top right of figure. The combinationof improved models,globalparameterfields,and large-scalevalidationdata sets will result in greatly enhanced simulationsof physical climatesystemfields(temperature,precipitation,etc.) and carbon cycleflows for the biome. These spatiallyresolvedfields can be area integratedand comparedto zonal-scaleinferences aboutthe borealcarboncycleprovidedby atmosphericisotopic analysesand CO2 tracer models;see bottom half of figure. In this way we hope to bridge the gap that currentlyexistsbetween our understandingof small-scalephysiological processes and global-scaleinferencesabout the carbon cycle.
role of the boreal ecosystemin global change(Figure 19). Considerableprogresswas made castingremote sensingalgorithmsinto a more physicallybasedframework[e.g.,Hall et al., this issue],progressthat shouldultimately result in more robust global classificationand biophysicalparametermapping techniques.The improvedradiativetransfermodelsdeveloped during BOREAS should also provide more reliable components in numericalsnowmeltmodels,a key to forecastingthe effects of climate warming on the timing of snowmelt,which we saw was crucial to determining interannual variation and longer-termtrendsin carbonsequestration.In addition, Wayet al. [thisissue]demonstratedthat radar can be usedto monitor
Future
Research
Directions
Over the next few years,BOREAS data setswill be usedto improve many of the critical processsubmodelsneeded for land-atmospheremodels and carbon balance calculations.In particular, the BOREAS follow-on program will encompass effortsto improveradiativetransfermodels,leaf- and canopyscale photosynthesis-conductance models, canopy, root and soil respirationmodels,soil moistureand surfacehydrological schemes,and surface-atmosphere turbulent transfer models. The integrated nature of the BOREAS data setswill permit rigoroustesting of the performanceof these processmodels, togetherand in isolationfrom eachother, over the diurnal and seasonalcycles(Figure 20). With respectto weather and climatemodels,a criticalshortterm task is to applythe BOREAS data set to further improve numerical weather forecasting by improving land surfaceatmospheretransfer codes.Tests of coupledphotosynthesisconductancemodels against BOREAS data have indicated that theseschemesare robustandwill lead to improvedsurface evapotranspirationestimatesfor the AGCMs. Incorporation of better albedovalueshasalreadyimprovedthe performance of a leading operationalforecastmodel; further performance improvementsare expectedwhen satellite-basedestimatesof albedo are incorporated,permitting these modelsto respond seasonallyto the effects of snow and vegetation phenology. BOREAS provided new data on the effects of smoke and associatedatmosphericaerosolson the surfaceradiation balance.Theseeffectswere more widespreadand severeover the biome than was previously thought (see also Sellerset al. [1995b]).GCM radiationcodesmayneedto includethiseffect. A final area of improvementin weather and climate forecasting shouldresult from incorporatingimprovedparameterizations of boundary layer dynamicsutilizing BOREAS radiosonde and aircraft
measurements.
In carbonmodelsthe long-term trends and interannualvariability in carbondynamicsfor the biome appear to be driven primarily by the timing of snowmeltand the summerair temperatures. Incorporation of improved snowmelt models into regional and global carbonbalancemodelsshouldhelp us to understandthesedynamicsand thusbetter simulatethe annual atmosphericCO2 signal. In terms of growingseasoncarbon fluxesthe photosynthesis modelsseem to be realistic,but it is essentialto improve the respirationmodels not only for the
SELLERS ET AL.: BOREAS
abovegroundcomponentsbut also for the roots, litter, and soils.Finally, the effectsof fire and disturbanceare important to understand.Scalingstudiesinvolvingpoint models and remote sensingwill be key to gagingthe effectsof disturbanceat the regional scale. BOREAS has catalyzedseveraladvancesin remote sensing algorithmdevelopmentwhichnowenableusto monitorboreal vegetationby type and stateand to track changesthat may be due to fire, direct human activity,or climate change.Algorithm developments due to FIFE and BOREAS havealreadyled to the productionof AVHRR-derived globalvegetationmapsfor the years 1987 and 1988 [seeMeesonet al., 1995;Sellerset al., 1996a,b, 1997;Randall et al., 1996]. These pilot productswill be expandedto cover the 15-year AVHRR record and will allow usto developtime seriesfieldsof land cover,biophysical parameters,phenology,and snowcover.All thesecanbe com-
pared with the physicalclimate record and to seasonaland interannual variations in atmospheric CO2 concentration. AVHRR data will also be used to monitor changesin the fire disturbanceregime over the same period. The radiometric qualityof the AVHRR data serieswill haveto be enhancedto meet thesetasks;this requiresthe developmentof techniques for improvinglong-term calibration and atmosphericcorrection of the data. The MODIS/MISR
and other sensors to be
launched aboard the EOS-AM platform in mid-1998 should provide significantadditionalcapabilityfor monitoringland vegetation and should be merged with the A VHbtbt data streamto producea seamlessmonitoringdata set into the 21st century.Finally, the useof radar satellitessuchas ERS-1 and JERS-1 will be used to monitor the interannual variability in the freeze-thawboundaryin the boreal ecosystem,shownto be a keyfactorin the interannualvariabilityof the carbonflux.To take advantageof the different attributesof optical and radar sensors,further remote sensingresearchand developmentis required;in particular,data fusionalgorithms,whichcombine optical and microwavesensorsas well as other data suchas topographicdata, couldbc developedto providericher information
about the biomc.
Enhancementof land-atmosphereprocessmodelsand largescaleparameter quantificationusingsatellite data would be considerable
achievcmcnts
for BOREAS.
Success in these two
areas is almostcertain,basedon the early resultsand work in progress.However,the real prize for BOREAS would be to incorporatethe improvedprocessmodelsand remote sensing data setswithin large-scaleenergy-water-carbon modelsto calculate surface-atmosphere fluxes of these quantitiesfor the biome over the period of record of the earth-observingsatellites, sayfrom 1980to the presentday (see the bottom half of Figure 20). For this calculationthe surfacestate shouldbe constrainedby satellitedata,while the atmosphericconditions are specifiedfrom meteorologicalanalysesor via direct couplingwith an AGCM. Incorporatingthe role of fire in a carbon cyclemodel representsa real challenge,particularlywhen it comesto predictingchangesin fire frequency,areal extent,and intensityas a resultof climatechange;the satelliteand meteorologicaldata recordsof the last 25 years are essentialresourcesfor addressingthisproblem.Initially, it is very unlikely that theseintegrarivemodelswill correctlysimulatethe value or even the signof the net carbonflux for the biome, but they shouldbe capableof reproducinginterannualvariationsin the flux, superimposedon a relatively invariant bias. The time series of calculated
interannual
variations
in the net carbon
flux can then be comparedwith equivalentnumbersinferred
IN 1997
28,765
from global tracer and isotopicanalysesto shedlight on which processesare responsiblefor perturbationsin the terrestrial carbonbudgetand where, geographicallyand biologically,they operate. In this way we hope that BOREAS will help us to bridge the huge gap that currentlyexistsbetweenglobal-scale inferences about the changingterrestrial carbon budget and our local-scaleunderstandingof the controllingecophysiological processes.When this is done, we can see our way toward constructingusefulpredictivemodelsthat can anticipatefuture interactionsbetweenthe globalphysicalclimate systemand the carbon cycle. Notation
ABL atmosphericboundarylayer. AES AtmosphericEnvironment Services. AFM airborne fluxesand meteorology. AGCM atmosphericgeneral circulationmodel. ANPP above-groundnet primary productivity. APAR absorbedphotosyntheticactive radiation. AR entrainment parameter. ASAS advancedsolid-statearray spectroradiometer. AVHRR advancedvery high resolution radiometer.
BAHC
biosphericaspectsof the hydrological cycle. BMD biomassdensity. BOREAS Boreal Ecosystem-Atmosphere Study. BORIS BOREAS Information System. BP beaver pond. BRDF
bidirectional
reflectance
distribution
function.
CCRS DAAC ECMWF
EOS AM-1
ET FIFE FM FPAR GCM GCTE GEE GOES
Canada Centre for Remote Sensing. Data Analysisand Archiving Center. European Center for Medium-Range Weather Forecasting. Earth Observation System-AM-1 platform. evapotranspiration. First ISLSCP Field Experiment. frequencymodulation. fraction of PAR absorbedby the canopy. general circulation models. global changeand terrestrial ecosystems. grossecosystemexchange. GeostationaryOperational Environmental
Satellite.
GPS
Global PositioningSystem. HAPEX-Sahel HydrologicalAtmospheric Pilot Experiment-Sahel. HYD hydrology. 1FC intensivefield campaign. IGAC International Global Atmospheric Chemistry. IGBP International GeosphereBiosphere Program. ISLSCP
International
Satellite
Land Surface
Climatology Project. LA1
leaf area index.
LSP land surfaceparameterizations. MISR multiangle imaging spectroradiometer.
28,766
SELLERS ET AL.: BOREAS IN 1997
MODIS
moderate resolutionimaging spectrometer.
MVI NASA NCAR
multiband vegetationimager. National Aeronautics and Space
NIR
NMHC NOAA NSERC
normalized differencevegetationindex. net ecosystemexchange. near infrared.
nonmethanehydrocarbon. National Oceanographicand AtmosphericAdministration. Natural Sciencesand Engineering Council.
northern study area. numericalweather prediction. old aspen. old black spruce. old jack pine. photosyntheticallyactiveradiation. portable aparatusfor rapid acquisitionof observations
of land and
atmosphere. Polarization and Directionality of Earth Reflectances.
SART SIR-C SSA SWE TDR TE TF
Fitzsimmons
of Prince
Albert
National
Park
also
Darrel Williams, branch head of 923, NASA/GSFC, is also thanked for
R n surfacenet radiation. RSS remote sensingscience. RT
man and Michael
National Center for Atmospheric
bidirectional
POLDER
Sandra Schussel, D'Arcy Snell, Sarah Steele, John Stewart, Karl Spence,David Terroux, Gillian Traynor, and JasonVogel. Dave Dal-
provided much needed help to BOREAS. All are warmly thanked.
Research
NSA NWP OA OBS OJP PAR PARABOLA
Halliwell, John Martin, Joe Niederleitner, John Norman, Paul Rich,
Administration. Research.
NDVI NEE
thony Young, Barrie Atkinson, Mary Dalman, Tom Gower, David
radiative
transfer.
surface-atmosphereradiative transfer. shuttle imagingradir. southern study area. snowwater equivalent. time domain reflectrometry. terrestrial ecology. tower flux.
TGB TIROS-N TM TOVS VHF WCRP-GEWEX
trace gasbiogeochemistry. multi-frequencyradiometer. thematic mapper. multi-frequencyvertical sounder. very high frequency. World Climate ResearchProgramGlobal Energy and Water Cycle Experiment. YA young aspen. YJP youngjack pine.
maintaininghis tolerance,goodhumor, and goodsensethroughoutthe entire project.The researchaircraftcrewsand managementperformed their tasks with unflagging professionalismand patience. Special thanksgo to GeorgeAlger, Chris Jenison,and Richard Rose (C-130); Gary Shelton(ER-2); Willie Dykes,Charlie Walthall, CharlesSmith, Jeff Sigrist,Ed Melson, Pete Bradfield,and John Schaefer(helicopter); Larry Hill, LawrenceGray, Ted Senese(chieftain);ChrisScofield and Michele Vogt (DC-8); CharlesLivingstoneand Brian Spicer(CV580); Tom Carrol and Rob Posten (aerocommander);Paul SpyersDuran, Henry Boynton,andJerryTejcek (Electra);JohnAitken, Brian Bertrand, Murray Morgan, John Croll, and Charles Taylor (Twin Otter); George Bershinskyand Ernest Gasaway(Kingair); and Ed Dumas and Robert McMillen (for LongEZ). While these (generally unsung)heroesare specificallymentioned,we are fully aware that a tremendousjob was done by many other people under frequently trying conditions.In addition, the crews of the Saskatoon,Prince Albert, and Thompson Flight Service Stations and the staff of ECMWF and NMC who providedBOREAS with invaluableweather predictionproductsduring the field campaignsare warmly thanked. BOREAS benefited immenselyfrom a seriesof independentexternal reviewsconductedby the ScienceAdvisoryGroup (SAG): Mike Unsworth(chairman),Pat Matson, Tzvi Gal-Chen, Joe Landsberg,Jim Ritchie. BOREAS is deeply indebted to thesepeople. BOREAS contributes to both the U.S. and the Canadian Global Change Research Programs.For the United Statesthe effort is beingled by the National Aeronauticsand SpaceAdministrationMission to Planet Earth, with participationfrom the National Oceanic and AtmosphericAdministration, the National ScienceFoundation, the U.S. GeologicalSurvey, the U.S. Forest Service, and the Environmental Protection Agency. ParticipatingCanadian agenciesinclude the Canada Centre for Remote Sensing,Environment Canada, Natural Sciencesand Engineering Research Council, Agriculture and Agri-Food Canada, National ResearchCouncil,Heritage Canada(Parks),CanadianForestService, the Institute for Spaceand Terrestrial Science,and the Royal Society of Canada.The BOREAS project was overseenby programmanagers from the participatingagencieswho made up the BOREAS Coordinating Committee (BCC). The membershipof the BCC is Micheal Allen, Richard Asselin, Carmen Charette, Michael Coughlan, Bruce Hicks, Hank Margolis, Gordon Miller, Jarvis Moyers, Leo SaynWittgenstein,Mac Sinclair,Lowell Smith, Robert Stewart,John Stone, Jeffrey Watson, and Diane Wickland. BOREAS is an element of the International Satellite Land Surface Climatology Project (ISLSCP) whichis part of the World Climate ResearchProgram-GlobalEnergy and Water Cycle Experiment (WCRP-GEWEX). BOREAS is also contributing to three International Geosphere Biosphere Program (IGBP) core projects;BiosphericAspectsof the HydrologicalCycle (BAHC), Global Change and Terrestrial Ecosystem(GCTE), and International Global AtmosphericChemistry(IGAC). References
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(Received September 10, 1997; revisedNovember 17, 1997; acceptedNovember 17, 1997.)
