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 Quercus 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 de- crease 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 pro- duction efficiency and an increase in the stem to leaf bio- mass ratio was observed in 1989. Temporal changes in canopy leaf area index (LAI) were not statistically sig- nificant. 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: Quercus ilex - Canopy structure - Leaf area index - Water deficit - Mediterranean sclerophylls Introduction The structure of vegetation canopies, as determined by the spat:ial arrangement of 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 ac- companied by reductions in leaf surface area. Studies on the spatial variability in total leaf area index (LAI) in for- ests 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 con- tribute 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 in- tensity 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 avail- ability. 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,