Community shifts across canopy composition: Threshold responses of forest floor vegetation indicate overstory change in temperate beech forests

Introducing new and more tree species to species-poor forest canopies is seen as a key measure of silviculture to adapt temperate forests to climate change by distributing the risk of climate-related mortality and disturbance events across multiple tree species. In Central Europe, this strategy meets forests that would naturally be dominated by European beech ( Fagus sylvatica ), which excludes most of its competitors by shade-casting. Analyzing a dataset of native and non-native alternative tree species that are candidates to partly replace beech in managed forests, we posed the question how this strategy would affect the species richness and species composition of forest floor vegetation. We selected beech stands in southwestern Germany with canopy fractions of <10%, 25%, 50%, 75%, and > 90% of other tree species, including Douglas fir ( Pseudotsuga menziesii ), silver fir ( Abies alba ), sessile oak ( Quercus petraea ), red oak ( Quercus rubra ), and mixtures of several native broadleaves. Where appropriate, we differentiated between stands on acidic and base-rich soils. We found increases in species richness with increasing canopy fractions of Douglas fir, silver fir, sessile oak only on base-rich soil, and mixed broadleaves compared to the naturally species-poor beech forests. These increases were consistent with increased light availability with reduced share of beech in the canopy and to better nutrient availability from non-beech litter on base-rich soil. Species richness did not change with the introduction of red oak and sessile oak on acidic soil. Species composition also changed, with several species characteristic of closed beech forests showing early declines along some canopy gradients of other tree species, whereas more generalist, nitrophilic, or light-demanding species increased mainly at higher admixture levels. Our results suggest that partial displacement of beech by other tree species causes significant changes in the ground vegetation. These threshold responses indicate that even low admixture levels can alter characteristic beech forest vegetation and should be considered when balancing climate-adaptation and conservation objectives.