Legacy of top‐down herbivore pressure ricochets back up multiple trophic levels in forest canopies over 30 years

Removal of top‐down control on herbivores can result in a trophic cascade where herbivore pressure on plants results in changes in plant communities. These altered plant communities are hypothesized to exert bottom‐up control on subsequent herbivory via changes in plant quality or productivity. But it remains untested whether top‐down perturbation causes long term changes in plants that ricochet back up the new food chain that depends on them. In a large‐scale, 30‐yr controlled field experiment, we show that 10 yr of top‐down control of an ungulate herbivore (white‐tailed deer, Odocoileus virginianus ) created contrasting forest tree communities exerting bottom‐up effects that ricochet back up 3 trophic levels 20–30 yr later. Higher ungulate densities during stand initiation caused significant reductions in tree species diversity, canopy foliage density, canopy insect density, and bird density in young (ca. 30 yr old) forests. Because recruitment of trees from seedlings to the canopy occurs over a relatively brief period (ca. 10 yr), with membership in the canopy lasting an order of magnitude longer, our results show that even short‐term perturbations in ungulate density may cause centuries‐long disruptions to forest ecosystem structure and function. In documenting this five‐step trophic ricochet, we unite key concepts of trophic theory with the extensive literature on effects of ungulate overabundance. As predators decline and ungulate herbivores increase worldwide, similar impacts may result that persist long after herbivore density becomes effectively managed.