metre of the canopy. The possible .... approximately 2.4 years (Lled6 1990). Growth occurs ... a leaf area meter Li-3000 (LiCor Inc.) and an IBAS image analyzer.
Trees (1994) 8:254-261
9 Springer-Verlag 1994
Canopy structure within a Quercus ilex forested watershed: variations due to location, phenological development, and water availability Anna SalaJ*, Santiago Sabat61, Carlos Gracia 1, John D. Tenhunen2 1Department of Ecology,University of Barcelona, Diagonal 645, E-08028 Barcelona, Spain 2 Department of Plant Ecology,Bayreuth Institute of Terrestrial EcosystemResearch, University of Bayreuth, Bayreuth, Germany Received: 12 May 1993/Accepted: 12 September 1993
Abstract. Spatial and temporal changes in canopy structure were studied in 1988 and 1989 in a Mediterranean Q u e r c u s ilex forest in north-eastern Spain. Due to differences in precipitation patterns the 1989 growing season was drier than the 1988 growing season. Sampling was conducted in parallel at two sites which represent endpoints along a slope gradient within a watershed (ridge top at 975 m, and valley bottom at 700 m). At both sites, similar inter-annual changes in canopy structure were observed in response to differences in water availability. Samples harvested in the upper 50 cm of the canopy during 1989 exhibited a decrease in both average leaf size and the ratio of young to old leaf and stem biomass relative to samples obtained in 1988. At the whole canopy level, a decrease in leaf production efficiency and an increase in the stem to leaf biomass ratio was observed in 1989. Temporal changes in canopy leaf area index (LAI) were not statistically significant. Average LAI values of Q. ilex at the two sites were not significantly different despite differences in tree stature and density (4.6 m 2 m -2 at the ridge top, and 5.3 m 2 m -2 at the valley bottom). Vertical distribution of leaves and stems within the canopy was very similar at the two locations, with more than 60% of the total LAI in the uppermost metre of the canopy. The possible significance of such an LAI distribution on the canopy carbon budget is discussed. Key words: Q u e r c u s ilex - Canopy structure - Leaf area index - Water deficit - Mediterranean sclerophylls
Introduction The structure of vegetation canopies, as determined by the spat:ial arrangement o f its elements, is the integrated result
* Present address: Department of Biological Sciences, University of Nevada-Las Vegas, Las Vegas, NV-89154, USA Correspondence to: A. Sala
of selection in response to a variety of environmental conditions and competitive interactions. The systematic variation in the physical and biological characteristics of foliage with canopy height documented in different forest and shrubland ecosystems (Eckardt et al. 1978; Caldwell et al. 1986; Hutchinson et al. 1986; Hollinger 1989; Parker et al. 1989) suggests that canopy architecture is optimized for light interception and to maximize carbon gain under the particular habitat conditions experienced by the plant. Because cell growth is very sensitive to water deficits (Hsiao 1973), reductions in available water may be accompanied by reductions in leaf surface area. Studies on the spatial variability in total leaf area index (LAI) in forests have shown LAI to decrease as site water availability decreases (Grier and Running 1977; Waring et al. 1978; Gholz 1982; Gholz et al. 1990) and as the resistance to water flow through the canopy simultaneously increases (Specht and Specht 1989). In addition to the seasonal LAI variation encountered in forests (Vose and Swank 1990), year to year changes in water availability may also contribute to inter-annual LAI variation. Thus, LAI estimates based on destructive measurements carried out over short periods of time can be strongly biased. When accumulated rainfall during the previous fall and winter is low and soils are not completely recharged, water deficits may occur during late spring and become severe during the summer months. In addition to the reduction of leaf gas exchange via stomatal mechanisms observed in Mediterranean sclerophylls (Tenhunen et al. 1987a, b; Rhizopoulou and Mitrakos 1990), whole plant reduction in exposed leaf area has been cited as a response to the intensity and duration of the drought period at different sites (Poole and Miller 1981; Rambal and Leterme 1987). However, short term intra- or inter-annual variations in LAI have seldom been documented, and very little quantitative information exists about canopy structural changes of Mediterranean sclerophylls in response to water availability. The investigations described here are part of a more general study of canopy water use and the hydrological balance of a Mediterranean watershed (l'Avic, Catalonia,
255
200] E
L'Avic 1988-1989 ] 1'88-552 mm '89=534 rnm |
1 5O
Table 1. Characteristics of the two sampling sites RIDGE Elevation (m)
loo
Soil depth (cm)a
"E
Tree height (m)
5o
VALLEY
975
700
47
86
3-6
8-12
9314 43
0
3491 0 3273 1527 473
5.7
4.9
Tree density (Stems ha-l) b
Quercus ilex Quercus pyrenaica Arbutus unedo Phillyrea media Viburnum tinus G (GJ m -2 yr-l) c Ea Eo-I a T (~
J M MJ
S NJ M M J S N
1988
1989
Fig. 1. Upper panel: Monthly precipitation measured at l'Avic watershed during 1988 and 1989. The annual totals are also indicated. Lower panel: Seasonal course of the pre-dawn xylem water potential (PWP) measured in terminal shoots of the upper crown of Quercus ilex trees at the ridge top and valley bottom sites, n ranges from 3 to 6. Vertical bars indicate standard error of the mean (from Sala 1992). - ( 3 - ridge; - Q - valley
S p a i n ) d o m i n a t e d b y Quercus ilex. T h e u l t i m a t e g o a l is to u n d e r s t a n d c o n t r o l s o n s e a s o n a l a n d s p a t i a l p a t t e r n s in can o p y w a t e r u s e ( S a t a 1992), to u n d e r s t a n d t h e s u b s e q u e n t effect of these patterns on the hydrology of the entire wat e r s h e d (Pifiol et al. 1991), a n d to d e v e l o p a p r e d i c t i v e m o d e l o f w a t e r u s e t h a t will a l l o w e x a m i n a t i o n o f c a t c h m e n t w a t e r b a l a n c e w i t h r e s p e c t to a v a r i e t y o f s c e n a r i o s r e l a t e d to l o n g e r - t e r m c l i m a t e c h a n g e . D i s t i n c t e n v i r o n m e n t a l g r a d i e n t s e x i s t at l ' A v i c , w i t h g r e a t e r soil d e p t h f o r w a t e r s t o r a g e a n d a r e d u c e d r a d i a t i o n l o a d at v a l l e y b o t t o m sites as c o m p a r e d to t h e u p p e r slopes. A l o n g this g r a d i e n t , t h e r e are o b v i o u s c h a n g e s in tree h e i g h t , d e n s i t y a n d t r u n k d i a m e t e r . A p r i m a r y g o a l o f t h e p r e s e n t s t u d y w a s to d e t e r m i n e w h e t h e r c a n o p y L A I at the l o w e s t v a l l e y b o t t o m sites w a s g r e a t e r t h a n t h a t o n t h e u p p e r slopes. W e w e r e also i n t e r e s t e d in d e t e r m i n i n g w h e t h e r t o n g - t e r m differe n c e s in site w a t e r a v a i l a b i l i t y m i g h t b e r e s p o n s i b l e for s i t e - s p e c i f i c v a r i a t i o n o v e r t i m e i n c a n o p y structure. O u r t h i r d o b j e c t i v e w a s to o b t a i n a n a v e r a g e d e s c r i p t i o n o f f o r e s t c a n o p y s t r u c t u r e at t h e s e t w o sites f o r m o d e l l i n g purposes.
Materials and methods
Study area. L'Avic watershed (41 ~ 15' N-I ~ 00' E) is located in the Prades Mountains, Catalonia, NE Spain, 30 km from the Mediterranean coast. The watershed occupies 51.6 ha, ranging from 680 to 1007 m in elevation. The average slope is 25.8 ~ facing N-NW. Q. ilex L. (holm oak) forms essentially pure stands throughout the study area. Soils tend to be deeper in the lower parts of the watershed (Pifiol 1990). The climate is typically Mediterranean, with an annual mean temperature of 13.8~ and annual mean precipitation (mostly as rain) of 658 mm. A dry period extends from mid-June to mid-September (period 1957-1988), but there is a significant variability in both the
0 29
< 0.95 12.8
13.9
from Pifiol (1990) b from Lied6 (1990) c from Sala (1992) G, global shortware radiation incident above the canopy Ea, potential evapotranspiration Eo, actual evapotranspiration a
amount and seasonal distribution of precipitation. Total annual precipitation at l'Avic during 1988 and 1989 was very similar (Fig. 1). However, the seasonal patterns differed substantially and, consequently, the summer of 1989 was considerably drier than the summer of 1988. Two sampling sites were located in l'Avic watershed at the ends of a gradient in elevation, microclimate and forest structure: (1) at the bottom of the valley (700 m altitude; valley bottom site), and (2) on the upper open slopes (975 m altitude; ridge top site). Table 1 lists some characteristics of the two sampling sites. Additional information on the altitudinal heterogeneity in vegetation distribution is provided by Lled6 (1990). The forest has not been disturbed since 1950 and structure is assumed to have equilibrated with environmental differences between the two sites. Bellot and Escarr6 (1992) did not find significant differences in total precipitation between two sampling stations located at 720 m and 960 m within l'Avic watershed. However Pifiol (1990) found apparently greater soil water availability at the valley bottom than at the ridge top study sites. The average rate of foliage turnover in Q. ilex at l'Avic is approximately 2.4 years (Lled6 1990). Growth occurs during spring (May to June) and, occasionally, in early autumn (October). Periodic measurements L'Avic forest from 1982 to 1988 indicated that 84% of the mean total litterfall occurred between April and August (Bellot et al. 1992).
Methods. Sampling of canopy elements was conducted in parallel at the ridge top and valley bottom sites during mid-summer 1988 (late July and early August), late fall 1988 (late November and early December), early spring 1989 (early April, before the new growth period), mid-summer 1989 (August), and late fall 1989 (late November). Access to the canopy to sample foliage and branches was obtained using a carefully placed 11 m tall aluminum ladder. During each sampling period, three randomly located square columns (0.5 x 0.5 m 2) were placed at the ridge top and valley bottom sites using an extendible mast traversing from the ground to the top of the canopy. The columns were delineated with nylon strings stretched from a metallic frame (0.5 m on a side) mounted at the top of the mast, to one 10 cm above the forest floor. All branches less than 2 cm in diameter (with or without leaves) included in the column were collected at 0.5 m intervals from the top to the bottom of the canopy and stored in plastic bags. The length and diameter (measured at both extremes) of branches larger than 2 cm in diameter were recorded in the field. Samples were transported to the laboratory and kept in a dark freezer (-1 ~C) until further measurements were performed.
256 Table 2. Total canopy leaf area index (LAI, m2m-2) and projected area of senescent leaves (Sen. m2m-2) of Q. ilex measured during 1988 and 1989 at the ridge top and valley bottom sites of l'Avic watershed. Data are the mean of three vertical profiles (columns 0.5 x 0.5 m on a side) SITE Ridge
Summer 1988
Fall 1988
Spring 1989
Summer 1989
Fall 1989
4.66 (0.23) 0.007 (0.007)
3.58 (0.50) 0.006 (0.005)
5.62 (1.41) 0.006 (0.004)
3.86 (0.93) 0.006 (0.003)
5.23 (1.26) 0.011 (0.007)
5.04 (1.01) Sen. 0.002 (0.002)
4.98 (1.34) 0.004 (0.004)
5.08 (0.15) 0.024 (0.007)
5.86 (0.15) 0.18" (0.058)
5.44 (0.53) O.OlO (0.008)
LAI Sen.
Valley LAI
SE in parentheses * Significant differences (P
