With kind permission of Springer Science and Business Media
Abstract
Plants across diverse biomes tend to produce smaller leaves and a reduced total leaf area when exposed to drought. For mature trees of a single species, however, the leaf area–water supply relationship is not well understood. We tested the paradigm of leaf area reduction upon drought by a transect study with 14 mature Fagus sylvatica forests along a steep precipitation gradient (970 to 520 mm yr-1) by applying two independent methods of leaf size determination. Contrary to expectation, average leaf size in dry stands (520-550 mm yr-1) was about 40% larger and SLA was higher than in moist stands (910-970 mm yr-1). As a result of increased leaf sizes, leaf area index significantly increased from the high- to the low-precipitation stands. Multiple regression analyses suggested that average leaf size was primarily controlled by temperature, whereas the influence of soil moisture and soil C/N ratio was low.
Summer rainfall of the preceding year was the most significant predictor of total leaf number. We assume that leaf expansion of beech was independent of water supply, because it takes place in May with ample soil water reserves along the entire transect.
In contrast, bud formation, which determines total leaf number, occurs in mid-summer, when droughts are severest. We conclude that leaf expansion and stand leaf area of beech along this precipitation gradient are not a simple function of water availability, but are controlled by several abiotic factors including spring temperature and possibly also nitrogen supply, which both tend to increase towards drier sites, thus overlaying any negative effect of water shortage on leaf development.
Keywords: adult trees, bud formation, drought, European beech, LAI, leaf expansion, leaf population, precipitation gradient
Leaf size and leaf area index in Fagus sylvatica forests Chapter 3
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Introduction
There is a vital debate on how temperate trees may respond to an increased frequency and severity of summer droughts as is predicted in recent climate change scenarios for parts of Central Europe (IPCC 2001, 2007, Rowell & Jones 2006). This question is particularly relevant for economically important tree species such as European beech (Fagus sylvatica L.). This species has a sub-oceanic distribution and exhibits a number of physiological and morphological traits that characterise it as comparatively drought-sensitive (Backes & Leuschner 2000, Granier et al. 2007). Therefore, reduced summer rainfall could threaten beech forests in regions of Central Europe, where this species is actually growing near its drought limit.
Numerous laboratory experiments with herbaceous plants and tree seedlings have shown that leaf area reduction is a common response to soil water shortage (e.g., Fischer & Turner 1978, Begg 1980, Poorter 1989, Lof & Welander 2000, Pedrol et al.
2000, Otieno et al. 2005), thereby reducing the transpiring surface area and avoiding severe decreases in cell water potential and turgor (Hinckley et al. 1981, Kozlowski &
Pallardy 1997). Next to the environmental control on leaf area development, genotypic variation may also interfere. Tree leaf area and stand leaf area index (LAI, the one-sided cumulative surface area of all leaves per unit ground area) are of paramount importance for forest biogeochemical fluxes because radiation interception, productivity, canopy conductance and stand transpiration are all closely linked to LAI (Gholz 1990, Bréda & Granier 1996, Kozlowski & Pallardy 1997, Welander & Ottoson 1997).
If the response of trees and forests to a possibly drier climate is to be predicted, term adaptive responses and highly flexible resource allocation patterns in these long-lived plants must be taken into account. This makes simple extrapolation from laboratory seedling or sapling studies to mature forests difficult if not impossible. Much more realistic results can be expected from large manipulation experiments in the field such as throughfall exclusion experiments, where a reduced precipitation is simulated (e.g., Wullschleger & Hanson 2006). However, due to high costs and restrictions in personnel, most large-scale water manipulation experiments in forests suffer from missing replication and short duration with the consequence that adaptive responses of trees are only rarely covered. Another source of information can be comparative studies in forest stands along precipitation gradients which may provide valuable additional information for understanding long-term tree adaptation to drought if the sites are carefully selected and other environmental factors are kept sufficiently constant.
Studies on leaf area index and leaf morphology changes in mature forest stands of a single tree species along precipitation or soil moisture gradients have only rarely been conducted so far. The existing gradient studies in forests focussing on the leaf area-water supply relationship referred to long gradients and typically included a tree species turnover between the moist and dry ends of the gradient (e.g., Grier & Running 1977, Hinckley et al. 1981, Runyon et al. 1994, Turner 1994, Jose & Gillespie 1997, Cunningham et al. 1999, Reich et al. 1999, Wright et al. 2004). This kind of data may allow general conclusions on how water shortage affects forest leaf area within biomes, but it gives no insight into a tree species’ adaptive potential with respect to leaf area development and leaf morphology.
European beech forms mono-specific stands under a broad range of soil chemical and hydrological conditions, from highly acid to basic soils (Leuschner et al. 2006a), and from low to high rainfall regimes. Hence, this species provides unique opportunities for investigating a tree species’ response to water availability in the field by covering a broad range of soil moisture or rainfall conditions. In this study, we compared the leaf area development of 14 mature beech forests of similar age and structure along a steep precipitation gradient (520 to 970 mm yr-1), while other environmental factors were by far less variable. We aimed at testing the paradigm of a decrease in LAI and mean leaf size with declining rainfall for mature trees of a single species, thereby improving our understanding of long-term adaptive drought responses of temperate trees.