Effects of herbivory on shrub performance in a managed forest

Abstract

Large herbivores have an outsized influence on ecosystem structure and function but are declining globally. In managed forests, shrub planting is commonly used to improve habitat for large herbivores, but success of this approach has been mixed. Despite the time and cost involved, planting efforts often fail to establish viable shrub communities, underscoring the need to understand factors that govern interactions between large herbivores and planted shrubs. We evaluated whether and to what degree herbivory during winter limited performance of planted shrubs in a forest managed as winter range for North American elk (Cervus canadensis). Using long-term herbivory exclosures (n = 5) and a before-after-control-impact design, we assessed (1) the nature and magnitude of shrub responses to winter herbivory during the subsequent growing season, and (2) relationships between shrub size (i.e., height, volume) and ability to replace biomass lost to herbivores during winter. Consistent with herbivore optimization theory, shrubs responded positively to winter browsing by producing longer shoots and concomitantly more biomass during the subsequent growing season. However, this response was contingent on shrubs surpassing mean minimum size thresholds prior to being exposed to herbivory. These results indicate that identifying such thresholds and allowing planted shrubs to reach them before being exposed to herbivory could help improve the effectiveness of habitat management for large herbivores. For example, the use of low-cost fencing to temporarily protect young shrubs from browsing, or the planting of older, larger shrubs could lead to more consistent success in establishing viable shrub communities in managed forests.