Light and ungulate browsing interact in shaping future woody plant diversity through natural regeneration
Ludwig Lettenmaier, Atle Mysterud, Oliver Mitesser, Christian Ammer, Torsten Hothorn, Simone Cesarz, Nico Eisenhauer, Daniel Kraus, Soumen Mallick, Jörg Müller, Kerstin Pierick
Abstract
Abstract Woody species diversity is crucial for the resilience of forests under climate change. The early stages of regeneration, particularly after canopy disturbance, shape the composition of future forest. Light availability, browsing pressure and their interactions should be key drivers of woody species diversity, biomass and density, but are not well understood due to limited experimental setups. We used exclosures in a mixed broadleaf forest with high woody diversity to protect seedlings from browsing by roe deer ( Capreolus capreolus ), the predominant ungulate in Central Europe. In a full‐factorial design, we paired 75 exclosure (fenced) and control (unfenced) plots in both closed canopy stands and experimental gaps. We measured woody regeneration 1 and 4 years after the start of the treatments. Rarefaction‐extrapolation curves revealed that diversity of common (Shannon diversity) woody species was highest in sunny and shaded exclosures (7), compared to sunny (4.5) and shaded (3.5) controls, for species that had outgrown the 130 cm browsing‐susceptible height of roe deer. Stage‐structured matrix models showed that saplings ≤20 cm had the highest probability of reaching >130 cm within 3 years in sunny exclosures (1.51%), followed by sunny controls (0.32%), shaded exclosures (0.27%) and shaded controls (0.07%). Browsing resulted in homogenized woody regeneration, particularly in forest gaps. Increasing light availability did not compensate for diversity loss by browsing. Given the densities of roe deer in the study area—typical for many Central European forests—effective population control or fencing appears essential to maintain high diversity of future forest shaping and forestry relevant woody species, especially in open forests. Synthesis and applications . Our results provide strong evidence that in the current context of increasing tree mortality and subsequent light availability, roe deer act as a keystone species. It is crucial to implement browsing protection measures and control roe deer population before or immediately after canopy disturbance. Otherwise, the rapid growth of a few dominant, browsing‐resistant woody species will significantly reduce the woody plant diversity of future forests.