Herbivore-induced branching increases sapling survival in temperate forest canopy gaps

Tree architectures reflect the main abiotic and biotic selection pressures determining tree growth and survival. Studies have shown that trees growing in herbivore-dominated ecosystems, such as savannas, develop denser, more divaricate 'cage'-like architectures in response to chronic browsing pressure (also known as 'brown world' architectures). In contrast, trees growing under resource-limited conditions develop traits that allow them to better compete for abiotic resources (referred to as 'green world' architectures); for example, light limitation can induce trees to form more pole-like structures that allow them to grow faster in height. In temperate forests, abiotic drivers have traditionally been assumed to be the main factors affecting plant architecture. However, gap dynamics typically characterize old-growth temperate forests, where disturbances such as storms create gaps with temporary high-light availability, but also attracting intense mammalian browsing. Under such conditions, one would expect tree saplings to exhibit high architectural plasticity enabling them to develop a 'cage' in response to high herbivore pressure, but when herbivore pressure declines switch to a more pole-like structure that allows them to grow quickly in height and compete for light. We tested the ability of five dominant European tree species to induce a 'cage' architecture (hereafter referred to as 'cage plasticity') under contrasting light and herbivory regimes in a full-factorial experiment in Bialowieza Forest. We planted 720 young tree saplings in six replicated blocks in which we manipulated ungulate browsing (exclosure vs. control) and light levels (gap vs. closed forest). Two out of the five species (hornbeam Carpinus betulus and lime Tilia cordata) were able to develop a cage when exposed to ungulate herbivores but only under high-light conditions. These two species had the highest survival rates in the herbivory treatment over a 10-year period. Synthesis. Cage plasticity in temperate forest tree species may reflect an overlooked adaptation to mammal herbivory originating in a past when large mammals exerted strong selection pressures in European ecosystems. Our findings challenge the view of closed-canopy forest communities being solely driven by abiotic drivers and call for a re-evaluation of the importance of large herbivores in shaping the functional traits of temperate tree species.