American Journal of Science

[American Journal of Science, Vol. 309, April, 2009, P. 271–290, DOI 10.2475/04.2009.01]

American Journal of Science APRIL 2009

BIOLOGICAL ENERGY IN LANDSCAPE EVOLUTION JONATHAN D. PHILLIPS Southern Landscape Systems Research Program, Department of Geography, University of Kentucky, Lexington, Kentucky 40506-0027: [email protected] ABSTRACT. Traditional conceptual models of landscape evolution view topography as an outcome of endogenic forces (uplift) working against exogenic forces of denudation. The energy considerations of these concepts have focused on the conversion of the potential energy of landscape relief to kinetic energy. The concept of the biosphere as a planetary membrane for capturing and converting solar energy, coupled with the critical geomorphic role of biota, call for a consideration of biotic contributions to geomorphic work. A review of estimates of global rates of kinetic energy of denudation and uplift, and net primary production (NPP) indicates that the energy density of NPP is, on average, three to seven orders of magnitude greater than the others. If even a tiny fraction of NPP is geologically significant, then the biological subsidy to the energy of landscape evolution must be considered on a par with that of geophysical and geochemical phenomena. A case study in eastern Kentucky shows that even if only 0.1 percent of NPP is geomorphically significant, it still far exceeds the energy inputs from uplift and conversion of potential to kinetic energy by denudation. This is unlikely to be unique, though the relative importance of biological and geophysical processes must obviously vary with climate, tectonic setting, and other factors. Results indicate that, particularly where biological activity is significant, geomorphic work performed by biota may be greater than that associated with endogenic processes and with the kinetic energy of denudation. introduction

Landscape evolution is typically conceived, at least since Gilbert (1877) and Penck (1924), as the outcome of the interplay of endogenic forces such as tectonic uplift and the exogenic forces of denudation. Exogenic forces are usually assessed in terms of the rate of mass removal over time in denudation. With respect to energetics, this is typically approached in terms of the potential energy represented by mass above base level and the kinetic energy released by the flux of this mass to or toward base level. The uplift vs. denudation and potential-to-kinetic energy based frameworks are incomplete, even for the dominantly denudational portions of the earth surface. Significant amounts of geomorphic work occur which do not necessarily result in denudation. When denudation does occur, the potential-to-kinetic framework ignores the tremendous amount of energy required to convert solid rock into transportable material. The importance of climate-driven processes is widely recognized, as is the fact that all such processes are directly or indirectly driven by solar energy inputs. However, the extension of solar radiation energy balances directly to geomorphic work is poorly understood. Conversions of potential to kinetic energy via gravity-driven mass transfers (including fluvial and glacial processes) involves only mass fluxes or mass storage above a base level. However, a broader consideration of geomorphic work recognizes that, for example, surface runoff which transports sediment may subsequently infiltrate and perform further geomorphic work via weathering and solute transport. These sorts of energy cascades are particularly relevant to biologically-driven processes. Some of the

271

272

Jonathan D. Phillips—Biological

solar energy captured by a tree, for instance, may be used in root and trunk growth which physically displaces surficial materials, and in the formation of organic complexes which are involved in chemical weathering. Upon death, the tree or parts thereof add mass to the surface, the decomposition of which may also facilitate weathering processes. The goal of this paper is to take a step—albeit preliminary and incomplete— toward a more complete understanding of the energetics of landscape evolution. The focus is on the biological energy subsidy to landscape evolution rather than on the mechanics of biogeomorphic processes. Specifically, the general global magnitude of geomorphic work attributable to biological processes compared to tectonic uplift and conversion of potential to kinetic energy by denudation will be compared. The relative importance of uplift, denudational mass removal, and biological energy for a specific region of Kentucky (USA) will also be assessed. The overall conceptual framework for this work arises from two basic premises: (1) The global biosphere is a membrane for capturing and converting solar energy (Vernadsky, 1926; Smil, 1991, 2002; Lapenis, 2002). (2) Organisms not only have crucial influences on geomorphic processes, but are also important in directly performing geomorphic work and effecting landscape change (for example, Johnson, 1990, 2002; Butler, 1995; Paton and others, 1995; Gabet and others, 2003; Johnson and others, 2005; Meysman and others, 2006). Geomorphologists and ecologists alike are increasingly demonstrating not just strong interrelationships, but extremely tight coupling between ecological and geomorphic systems, and the coevolution of landforms, ecosystems, and soils (for example, Schwartzman and Volk, 1991; Drever, 1994; Johnson, 1994; Lucas, 2001; Hay and others, 2002; Stallins and Parker, 2003; Coblentz and Riiters, 2004; Corenblit and others, 2007, 2008). A more comprehensive consideration of the biological subsidy to landscape evolution is therefore timely. Considerations of landscape evolution have traditionally focussed on mass fluxes in the form of removal (denudation and erosion) and deposition. In this sense geomorphological impacts of biological processes may appear to be minor relative to energy capture by biota. Sediment transport attributable to pocket gopher burrowing at two California sites, for example, is only about 1 percent of reported burrowing energy (Yoo and others, 2005). Most of the burrowing energy “is thus used for shearing, mixing, and elevating soils rather than for generating net downslope transport” (Yoo and others, 2005: 918). That study reports that net sediment transport by gophers uses only a tiny fraction (⬃0.001%) of the net primary productivity of these grasslands. But biological productivity also adds mass, results in displacement and mixing, directly or indirectly drives chemical weathering, and facilitates hydrological fluxes (for example by increasing both micro- and macroporosity of soil, and by root penetration of bedrock fractures) critical to geomorphic work. A significant amount of geomorphological work does not necessarily involve additions or removals from or to, but transformations and translocations within a geomorphic unit. To the extent weathering is a necessary precursor of mass removal, the energy necessary to turn solid rock into transportable material may far exceed that required for entrainment and transport, as discussed later. Depending on the spatial scale of interest, geomorphic units can be defined at a variety of scales from particles to planets, though in the context of landscape evolution study scales from soil or weathering profiles to large drainage basins are most common. A conceptual model of soils developed by Simonson (1959, 1978) holds that soils are a function of additions and removals of mass and energy, and of translocations and transformations to, within, and from soil profiles. Schaetzl and Anderson (2006: 321)

energy in landscape evolution

273

consider the model to be part of a broader class of mass balance models, which views the nature of soils as the outcome of the balance and character of simultaneously operating additions, removals, transformations, and translocations. This viewpoint can be broadened to geomorphic units in general, so that processes such as regolith mixing, vertical translocation of solids and solutes, and in situ weathering are taken into account, even when no mass losses or additions are involved. Further, this perspective considers not just mass additions from geological processes as traditionally conceived, but also those resulting from conversion of solar energy to biomass. Biological and Geomorphological Energetics A full review of biological influences on geomorphological processes is beyond the scope of this paper, but table 1 lists the major categories of biotic (direct biological effects) or biologically-mediated (indirect effects) processes which contribute to geomorphic change, along with a selection of supporting literature. Solar energy captured by plants is used to synthesize plant biomass (GPP), in photorespiration (a process distinct from normal metabolic respiration), and in the reduction of nitrate and sulphate. As a global average, about half of GPP is fixed as biomass and the remainder expended in respiration (Woodward, 2007). Respiration results in release of H2O and CO2, and is important in helping to drive some chemical weathering processes, as well as providing resources for other organisms. The GPP

Table 1

Major categories of geomorphological effects directly or indirectly attributable to biological productivity. A selective list of supporting references is given for three general classes of effects (biomechanical, biochemical, and hydrological). The references listed are only a fraction of the relevant literature.

Types of geomorphic influence •Microbial weathering. •Weathering by lichens and other macroscopic organisms. •Dissolution and solute transport by organic acids. •Chelation and mineral uptake by vegetation. •Mass additions of organic matter and wastes. •Mass displacement by root growth. •Faunalturbation by digging, tunneling, mounding, and burrowing. •Floralturbation by tree uprooting. •Formation and subsequent infilling of cavities via root and stump rot or combustion. •Facilitation of moisture flux via root channels, burrows, and porosity. •Concentration of moisture flux via stems, roots, channels, and burrows. Supporting references Bioturbation and biomechanical effects: Balek, 2002; Butler, 1995; Gabet and others, 2003; Hole, 1981: Johnson, 1990, 2002; Johnson and others, 2005; Leigh, 1998; Lutz and Griswold, 1939; Meysman and others, 2006; Peacock and Fant, 2002; Phillips and Marion, 2006; Scatena and Lugo, 1995; Schaetzl and others, 1990. Weathering and biochemical effects: Boyle and Voight, 1973; Bull and Laverty, 1982; Drever, 1994; Egerton-Warburton and others, 2003; Hinsinger and others, 2001; Jones, 1998; Kelly and others, 1998; Lucas, 2001; Schwartzman and Volk, 1991; Sterflinger, 2000; Chen and others, 2001; Taborosi, 2002; Turkington and Paradise, 2005; Viles, 1984, 2008; Warshscheid and Braams, 2000. Hydrologic effects: Bond and others, 2002; Bowden and others, 2001; Burch and others, 1987; Burt and Swank, 1992; Byrnes and Kardos, 1963; Cheng and others, 2002; Elsenbeer, 2001; Gyssels and Poesen, 2003; Herwitz, 1993; Landeweert and others, 2001; Noguchi and others, 1999; Sidle and others, 2001.

274


fixed as biomass (that is, net primary productivity or NPP) may perform direct geomorphic work by displacing soil and rock, and indirect work by, for instance, facilitating moisture fluxes. Some of the plant biomass is added to the surface or near-surface as litter and soil organic matter. The decomposition of this organic matter plays a critical direct role in some weathering processes, such as oxidation and reduction. Both living and dead organic material interact with meteoric, soil, and ground water to produce organic acids which are important in weathering, solutional denudation, and translocation. The bacteria and other microbial populations affiliated with plants and soil organic matter often play crucial roles in weathering and other biogeochemical transformations. Plant biomass that is passed up the food chain may result in further geomorphic work via faunalturbation and other faunal impacts (see Butler, 1995). Figure 1 summarizes the direct and indirect geomorphic work associated with the capture and transformations of solar inputs by the biosphere. Biota may offset geomorphic change in some respects by utilizing water that would otherwise run off, and by increasing the resistance of the ground surface via vegetation and litter cover and the role of biota and biogenic substances in creating stable soil aggregates. This is well illustrated by, for instance, the inverse relationship between vegetation cover and sediment yields. However, even though transpiration reduces runoff, this plant water use contributes directly to biochemical cycling and other energy subsidies to geomorphic systems. Further, the protective effects of organisms

Fig. 1. Geomorphic work associated with solar energy capture and transformations by biota. Direct effects are underlined; indirect effects in italics. Phenomena which may slow some geomorphic processes shown with ⫺negative sign.


275

with respect to surface erosion are associated with inputs of organic matter, facilitation of infiltration and subsurface moisture flux, and other effects, which result in mass inputs or otherwise contribute to geomorphic work. Biological effects on landscape evolution are obviously quite variable geographically (as are tectonics, mechanical weathering and erosion, and other geophysical processes). However, even in environments with essentially no vegetation cover, biological effects are present and significant, as illustrated by, for example, the effects of biological crusts in drylands (Viles, 2008) and bioturbation in Antarctica (Cannone and others, 2008). geomorphological energetics

A global energy balance approach to geomorphology was developed by Devlin (2003), and a comprehensive survey of earth surface system energetics by Smil (1991, 2008). Devlin (2003) divides the major energy sources into solar (Esolar), radiogenic heat from radioactive decay within the earth (Egeothermal), and rotational energy (Erotational). Potential energy is stored as landscape relief (Epotential), and energy is also stored in the biosphere (Ebiota) and as heat in the solid earth (Eheat). The energy associated with atmospheric processes and the hydrologic cycle is computed based on latent heat exchange (evapotranspiration) and is represented by Devlin (2003) as Ehydologic. The energy balance is then E received ⫽ E lost ⫹ ⌬E stored ⫽ 共E solar ⫹ E geothermal ⫹ E rotational 兲 ⫽ 共E radiated ⫹ E reflected 兲 ⫹ ⌬共E hydrologic ⫹ E biota ⫹ E potential ⫹ E heat 兲

(1)

The units are total energy flux for Earth (J sec⫺1). Devlin (2003) reduces eq. (1) by assuming that Eheat, Ebiota are in steady-state and not significantly changing, and that Erotational is small relative to potential and geothermal (endogenic) energy: 共E solar ⫹ E geothermal 兲 ⫽ 共E radiated ⫹ E reflected 兲 ⫹ ⌬共E hydrologic ⫹ E potential 兲

(2)

Burbank and Anderson (2001) point out that only a fraction of the energy driving tectonic processes is actually expressed in surficial movements, suggesting that an additional term reflecting dissipation of geothermal energy could be added. In this study the assumptions regarding steady-state Eheat and the insignificance of Erotational are accepted. Steady-state Ebiota does occur, particularly at shorter time scales. Steadystate is not assumed for this study, however, both due to the explicit interest in biological processes in this study, and due to the observation that ecosystems may develop toward maximum rates of energy capture and conversion (for example, Budyko, 1986; Jorgensen, 1997; Eagleson, 2002; Lapenis, 2002; Reynolds, 2002; Phillips, 2008). The energy loss and Ehydrologic terms may be approached, as a first approximation; by assuming that solar radiation inputs and the radiation balance are a function of surface area and a parameter that reflects latitude and local/regional climate. This approach is conceptually similar to Volobuyev’s (1964, 1974) energy balance of soil formation. The quantity of energy participating in pedogenesis (Eped) is represented as E ped ⫽ R e ⫺共1/mK兲

(3)

Where R is the net radiation balance at the surface, m is a factor reflecting biological productivity, and K is the ratio of mean annual precipitation and evaporation (Volobuyev, 1964, 1974). Runge (1973) independently developed a superficially conceptually similar approach, which has come to be called the “energy model” of soils, though his model does not deal directly with energy fluxes. Rasmussen and others (2005; Rasmussen and Tabor, 2007) recently (and apparently independently) updated Vo-

276


lobuyev’s line of attack by developing an index of energy inputs to pedogenesis from solar radiation and precipitation, based on mean temperature and precipitation values. Some of the endogenic energy in the landscape is stored as the potential energy of landscape relief, PE ⫽ m g h

(4)

where m is the mass, g the gravity constant, and h the height above base level, which may be sea level or a more local base level, depending on the unit of analysis. PE can be converted to kinetic energy by transferring mass down-gradient by gravity-driven processes: KE ⫽ 0.5 m V 2

(5)

The biosphere is capable of capturing, storing, and converting solar energy, which would otherwise be reflected or dissipated as heat. Vernadsky (1926) viewed the biosphere as a planetary membrane for capturing and processing solar radiation energy. Organisms, particularly vegetation, may also serve to maximize the rates of moisture cycling (Eagleson, 2002; Lapenis, 2002). With respect to soils, at least, energy from solar radiation in the form of vegetation productivity and precipitation are the dominant source of energy inputs (Rasmussen and others, 2005). Soil carbon content reflects the interaction between the rate of net primary productivity (NPP) and energy and nutrient demands of the local ecosystem, so that at steady state an assessment of NPP and precipitation could be used to estimate soil properties based on energy inputs (Rasmussen and others, 2005). However, no assumption of steady state is necessary to support an argument that ecosystem NPP reflects the solar energy transferred to the earth surface environment by biological processes. This is, in fact, an underestimate of the biological subsidy to geomorphic processes, as respiration (NPP is essentially gross primary productivity minus respiration) is indirectly involved in geomorphic processes (such as providing CO2 for weathering). Work on energetics of soil formation considers essentially all NPP to represent pedologically relevant energy inputs (Volobuyev, 1964, 1974; Rasmussen and others, 2005; Rasmussen and Tabor, 2007). This argument is more difficult to make for landscape evolution, since not all soil biological processes are geomorphically relevant. However, NPP is a reasonable index of the potential biological subsidy to landscape evolution for the purposes of highlighting the potential importance of biotic energy inputs in geology, and hopefully instigating further research. global rates of uplift, denudation, and npp

The standard units of force, energy, and power, respectively are typically Newtons (N; kg m sec⫺2); Joules (J; N m); and watts (W; J sec⫺1). Denudation and uplift rates are reported in various units, many of which have dimensions of LT⫺1, such as mm yr⫺1 or m ma⫺1. Ecological productivity is typically reported in dimensions of ML⫺2T⫺1, most often g or kg m⫺2 yr⫺1. Uplift and denudation can be converted to similar units by assuming a constant crustal density (here taken as 2.65 Mg m⫺3). Primary productivity is converted to energy units using a conversion factor of 1 g ⫽ 17.1 kJ, as suggested by Smil (2008). This is at the conservative end of the range; factors of up to 1 g ⫽ 22 kJ are commonly used in the ecology and soil science literature. The storage of potential energy by uplift per square meter of surface area (this does not account for the energy driving tectonic processes) can be estimated based on equation (1): PE uplift ⫽ ␳ g U ⫺3

(6)

where rock density (␳) is assumed to be 2650 kg m and U is uplift in meters. With an uplift rate expressed in m yr⫺1, the rate of PE storage in units of kJ m⫺2 yr⫺1 is given by multiplying 25.97 by the uplift rate. A similar approach can be used to estimate the power density of denudation.

277


Again, this does not account for the various energy expenditures of disaggregating and transporting rock. The potential-to-kinetic energy conversion associated with the mass transfers required to level the continents to sea level, based on a mean continental elevation of 874 m, is 22,698 kJ m⫺2. Using tensile strength as a crude indicator of the energy required to disaggregate or decompose rock, the necessary energy inputs can be estimated. The standard unit for tensile strength, the MPa, is equivalent to 1 MJ m⫺3. Typical ranges of tensile strength for common rock types reported by Costa and Baker (1981), Yatsu (1988) and Selby (1993) are in the range of 1 to 25 MPa, implying an energy input of 1000 to 25,000 kJ m⫺2 for each meter of rock weathered. Even if a mean rock thickness above sea level of only 500 m is assumed (to account for soil, regolith, deltas, and other sedimentary basins), 5.00 X 105 to 1.25 X 107 kJ m⫺2 of energy would be required to weather this rock— one to three orders of magnitude greater than the kinetic energy associated with downward movement. Global Rates of Denudation and Uplift Estimates or compilations of mean global denudation rates are shown in table 2. The power density of denudation spans five orders of magnitude, from 10⫺2 to 103 J m⫺2 yr⫺1. These compilations and estimates are focused on relatively long time periods

Table 2

Estimates of global denudation rates. Derived units based on assumed crustal density of 2.65 t m⫺3 Source

Original units

J m-2 yr-1

Summerfield and Hulton, 1994: compilation of 6 studies of global river transport of solids to the oceans.

31.8 to 145.4 mm ka-1

0.826 – 3.776

Summerfield and Hulton, 1994: compilation of 5 studies of global river transport of solids to the oceans, assuming 40% of total solute load is non-denudational

4.8 to 5.9 mm ka-1

0.125 – 0.153

Summerfield and Hulton, 1994: estimated total denudation rates of World’s 35 largest drainage basins

3 to 677 mm ka-1

Milliman and Syvitski, 1992: sediment transport of rivers With drainage area >250,000 km2

0.001 to 0.341 mm yr-1

0.026 – 8.856

Holmes, 1965: estimated mean global denudation

1 m per 30 ka

0.866

Crickmay, 1976: estimated mean global denudation

1 ft/8760 to 1 ft/3810 yr

0.973 – 2.236

Ollier and Pain, 1996: average denudation rates

50 to 500 mm ka-1

1.299 – 12.985

Saunders and Young, 1983: compilation of erosion and Surface lowering rates

1 to 500 cm ka-1

0.260 – 129.850

Von Blanckenburg, 2005: compilation of 7 studies of cosmogenic-nuclide denudation measurements1

3 to 100 mm ka-1

0.078 – 2.597

Bloom, 1998: average denudation rates

10 to 100 cm ka-1

2.597 – 25.970

1

Adjusted for different density estimate used in original source

0.078– 17.582

278


and broad areas, recognizing that negative (accumulation), zero, and extremely high rates of denudation may occur locally in space and time. Several estimates of global uplift rates are shown in table 3. These are generally based on crustal uplift rates, with isostatically-driven uplift not necessarily distinguished from tectonic. These are biased toward tectonically active areas; many portions of the earth at any time are tectonically stable (or subsiding). The typical range is taken as 0 to 260 J m⫺2 yr⫺1, with the latter applying to areas of very high tectonic activity such as the Himalayas and the Southern Alps, New Zealand. Solar Input and Biological Productivity The mean power density of solar radiation at the top of the atmosphere is about 1370 W m⫺2. Considering the actual interception at the ground surface, and reflection within the atmosphere, total radiation absorbed by the atmosphere and the ground is no more than 240 W m⫺2. According to Smil (1991: 15), “this flux is the actual energizer of the Earth.” This accounts for fluid motions and transfers and biological productivity. Of this, mean absorption by the Earth surface is estimated at 170 W m⫺2; 180 for terrestrial surfaces (Smil 1991), which varies according to latitude and other factors. The latter figure is equivalent to 5.68 X 106 kJ m⫺2 yr⫺1. Maximum energy fixation due to photosynthesis is 50 to 65 g m⫺2 d⫺1 (18 to 24 kg ⫺2 m yr⫺1) (Smil, 2008). Gross primary productivity (GPP) includes all fixed phytomass, before reduction by respiration to net primary productivity (NPP). Despite the fact that NPP ignores respiration and consumption other than autotrophic respiration, it has become the standard measure of productivity in ecology, biogeography, and biological energetics, largely because of its relationships to biomass and carbon fixation. Table 4 shows global or large-area measurements and estimates of terrestrial NPP, which range from about 1 to 40 MJ m⫺2 yr⫺1. Again, substantially higher and lower NPP occurs locally. Note that the units in table 4 (MJ) are 106 times greater than those in tables 2 and 3. Table 5 shows a comparison of a typical range of estimated global rates of denudation, uplift, and NPP. The global estimates show that NPP vastly exceeds energy inputs from uplift and kinetic energy of denudation, by three to five orders of magnitude. This raises the question of what portion of NPP is geomorphologically relevant. To date, biogeomorphological studies have focused on a single or a small number of biotically-driven processes. However, even these often show a highly significant biological energy subsidy. In studies of biomechanical effects of trees (uprooting, displacement, and creation of stump holes) in Arkansas forests, Phillips

Table 3

Estimates of global uplift rates. J m⫺2 yr⫺1 units represent rate of storage of potential energy, based on assumed crustal density of 2.65 t m⫺3 Source

Original Units

J m-2yr-1

Summerfield, 1991: compilation of 6 studies from various locations of surface or crustal uplift

300 to 10000 m ma-1

Ollier and Pain, 1996: average global uplift rate

1000 mm ka-1

Bloom, 1998: Typical uplift in tectonically active areas of the Pacific Rim

1 to 5m ka-1

25.970129.850

Burbank and Anderson, 2001; typical vertical deformation rates

< 1 mm yr-1