Direct and indirect effects of a plant invasion alter forest succession

Background/Question/Methods

Recent experiments have shown that invasive species can have significant direct effects on native communities, but much less is known about the role of indirect effects such as altered herbivore pressure on native species.  We examined the direct and indirect effects of the invasive grass, Microstegium vimineum (Japanese stiltgrass), on tree seedling recruitment, growth and herbivore damage.  Using twenty plots across seven natural forests (140 plots total) in southern Indiana that were invaded by M. vimineum, we established a factorial experiment where we removed the invader from half of all plots with a grass-specific herbicide and excluded vertebrate herbivores from half of each plot with fencing.  In the fenced and unfenced portion of each plot we planted two seedlings each of five native tree species.  Survival, height, and biomass of seedlings were assessed after two growing seasons. We predicted that tree seedlings would have the greatest success in plots where the invasion was removed and herbivores were excluded. Further, we predicted that the main effect of invasive removal would have a greater impact on seedling success than herbivore exclusion.  

Results/Conclusions

Both invasive removal and herbivore exclusion had direct positive effects on seedling survival and growth.  Removal of M. vimineum increased average seedling survival by 72% while herbivore exclusion increased survival by 35% across all species.  Rates of survival varied by tree species, Prunus serotina (black cherry) experienced the lowest survival (4%) and Quercus shumardii (Shumard’s oak) had the highest survival (30%) across all treatments. Invader removal and herbivore exclusion independently resulted in greater average second year height of seedlings by 128% and 126%, respectively. The additive effects of invasive removal and herbivore exclusion had the greatest impact with a 133% increase in average seedling survival and 417% greater average second year height.  Seedling biomass was similarly affected with the greatest increase in average biomass (189%) when the invader was removed and the seedlings were fenced.  Our results indicate that the additive effects of plant invasion and herbivore damage limit forest succession. The invasion by herbivore exclusion interaction suggests that the invader promotes herbivore damage on seedlings, possibly by providing cover.  Thus, efforts to restore invaded forests need to consider both direct and indirect effects of plant invasion.