Climate response of above- and belowground productivity and allocation in European beech

Abstract

European beech (Fagus sylvatica L.) is by far the most abundant tree species of Central Europe’s natural forest vegetation and one of the economically most important trees in Germany. Although this late-successional tree species shows a wide physiological tolerance toward a broad range of climatic and edaphic growing conditions, beech is known to be relatively drought sensitive when compared to other broad-leaved temperate forest trees. As man-made climate change is likely to impair the climatic water balance during the growing season and to increase the frequency of severe drought events over large parts of Central Europe, the future role of beech in forestry is under current debate. Thus, this study aimed at investigating the influence of hydrologic and climatic conditions on the productivity and vitality of beech and to identify basic mechanisms of a long-term adaption to water deficits in this drought-susceptible tree species. For this purpose, above- and belowground biomass production was studied in 12 mature beech forest stands along a natural rainfall gradient (543-816 mm y-1) on uniform sandy geological substrate in the Northern German Lowlands. To consider the potential effect of soil water storage on the precipitation-response, this study was carried out in paired beech stands with similar climatic conditions but growing on sites with contrasting soil texture (sandy vs. sandy-loamy). Influences of water availability and climatic variations on growth patterns were investigated by monitoring (i) total above- and belowground biomass production, (ii) carbon partitioning and resource allocation dynamics, and (iii) morphology of the water absorbing and transpiring surfaces in response to climatic alterations. Quite unexpectedly, differences of annual (or growing season) water availability across the climatic gradient were found to exert only little influence on the productivity of mature beech stands, as neither aboveground NPP (NPPa) nor total NPP showed a decrease towards the dry end of the transect. However, NPPa and stem wood production were found to be primarily controlled by hydrologic conditions in the current peak growing period (June and July). Along the precipitation gradient, a pronounced continuous shift in the patterns of biomass partitioning emerged, resulting in higher fine root production and decreased aboveground:belowground productivity at the drier sites. Unlike aboveground biomass components, fine root production in mature beech was shown to react highly responsive toward changes in water availability. In line with the optimal partitioning theory, this allocational behaviour could be observed in response to decreasing precipitation but also with decreasing soil water storage capacity. Such allocational adaption processes to water shortage were complemented by morphological plasticity (increased surface:biomass ratio) and adjustment of the positioning (increasing concentration of fine roots in the organic layers) of the fine root system. Leaf morphological alterations, however, were not found as a part of long-term adaption to changing hydrologic regimes in mature beech. Fructification was found to be a second important factor for allocational shifts in beech, as a large C allocation toward fruit growth strongly impaired aboveground vegetative growth, especially stem wood increment. For a high sink strength for C and N, fruit growth caused decreases in the weight and size of single leaves and thereby reduced a production of leaf biomass and LAI. Besides decreases in leaf area, pronounced foliar N depletion upon masting assumably lowered the canopy C assimilation in the mast and even in the post-mast year. Observed patterns in the fructification response to solar radiation suggest that floral induction in beech is a threshold controlled reaction to enhanced levels of canopy carbon gain in early summer (June-July) preceding the mast year. By these findings, the temporal pattern of a masting response to climatic cues in beech appears to be feedback-controlled by plant-internal N dynamics and, with continuing high N deposition loads, from this mechanism may arise additional burden for future vegetative growth in beech. A high degree of allocational plasticity in mature trees certainly constitutes an integral part of long-term adaptability of Fagus sylvatica to a wide range of hydrologic regimes which may also support resistance and resilience to single severe drought events.