How do sapling and adult demography explain beech predominance along environmental gradients in Central European forests?

Abstract

Understanding how species abundances are driven by biotic interactions along environmental gradients is a fundamental question in ecology. For abundances at competitive equilibria in Central European forests, a classical ecological theory formulated by Ellenberg (1963) predicts that beech (Fagus sylvatica L.) outcompetes other tree species within a mesic range of soil pH and water levels, while other species prevail under less favorable conditions. While the theory is generally accepted in forest ecology, only certain aspects of it have been substantiated by empirical evidence. Moreover, the demographic processes driving the turnover from beech to other tree species at the extremes of the soil gradients remain mostly unexplained. To address this, we inversely calibrated a parsimonious forest model (JAB model) with a sapling stage and interacting populations with short time series of observed tree abundances from the German national forest inventory. By modelling how demographic rates vary along pH and soil water gradients, we were able to test the prediction that beech naturally predominates only at favorable soil conditions. Moreover, we tested with simulations how the environmental response of demographic rates explains Fagus changing relative abundance along the soil gradients. Our results largely confirm that Fagus out competes other species in a central environmental range. Environmental change of Fagus relative abundance is primarily explained by environmental variation of its net basal area increment, followed by its competition response at the overstory and at the sapling stage. We found that even though sapling tolerance to shading is the primary mechanism for Fagus predominance, it only plays a secondary role for the environmental variation of its relative abundance. Synthesis: By inverse calibration of a forest population model with demographic rates that respond to the environment, we confirm the predictions of Ellenbergs classical, albeitonlypartially-evidenced, theory on F. sylvaticas predominance in Central European forests. Furthermore, for thefirsttime, we substantiate the theory by elucidating how the environmental variation in species composition is based in demographic processes. This demonstrates that our approach can be utilized to predict distributions of interacting species and to explain the dynamics between species, as influenced by their environment.