Interactive effects of a non-native invasive grass Microstegium vimineum and herbivore exclusion on experimental tree regeneration under differing forest management.
Invasive plants, herbivores and site management history can play crucial roles in determining plant community composition. The net effects of invasive species on plant communities are well known, but we have a poor understanding of the relative contributions of direct competitive effects of invasive species and their interactions with herbivores and management practices. Understanding interactions among plant invasions, herbivores and management history is critical for predicting and managing long-term ecological effects of invasions on native communities. We investigated the effects of the invasive annual grass Microstegium vimineum, vertebrate herbivory, site management history and their interactions on seedling performance of five native tree species in six forest sites. Half of the sites had been subjected to recent timber harvesting, and half had not been managed for timber for at least 20 years. We evaluated tree seedling survival and biomass after two growing seasons in blocked split-plots where the invasive grass was present or experimentally removed and where vertebrate herbivores were excluded by fencing or allowed onto the plots. Removal of the invasive grass and herbivore exclosures both had positive effects on seedling survival. Survival of all species was significantly increased by removal or by the interaction of removal with exclosure or timber harvest history. There was a significant interaction between exclosure and removal on seedling biomass where seedlings were larger in exclosure treatments with the invasive removed but smaller in exclosure treatments when the invader was present. Mean seedling biomass was 79.6% greater in harvested sites than in non-harvested sites. The significant positive effects of removal and recent timber harvest on the biomass of two Quercus species suggest strong impacts of invasion and management history on this important genus. Synthesis and applications. An invasive grass directly inhibited tree seedling performance but effects depended on herbivore exclusion treatment, timber harvest history and tree species. Our results suggest that forest management strategies should incorporate invasive plant control given that regeneration of desirable tree species can be influenced by plant invasions and their interactions with herbivores and previous management practices.