Causes and consequences of exotic plant invasions in eastern deciduous forests
Invasive exotic plants are widespread in eastern deciduous forests in the USA where they threaten to reduce native biodiversity and alter ecosystem processes. Understanding factors that determine the distribution of plant invaders and the effects of invasions on native communities is important for predicting invasions and prioritizing species for research and management. Evaluating the response of native communities to invasive plant removal can aid in developing restoration strategies and quantifying impacts of invasions. Using observational and experimental field studies, I evaluated the effects of habitat features on the distribution of plant invasions, the impacts of invasions on native species, and the effectiveness of different restoration methods in invaded systems. At forested sites throughout central and southern Indiana, invasive exotic shrubs occurred at higher densities along roads than in interior forest and in young and mid-successional forest than mature forest. Experimental plantings of exotic shrub seedlings revealed that seedling growth was promoted by the environmental conditions near roads and in younger forests. However, seedling survival was not significantly inhibited in interior or mature forests over three growing seasons, demonstrating that the conditions in those locations do not exclude invasions. Introductions of the annual grass Microstegium vimineum (Japanese stiltgrass) in an experimental field study showed that after two years, invasions reduced native herbaceous plant productivity (by 64%) and diversity (by 38%). Microstegium abundance was also negatively correlated with native plant productivity and diversity. An additional field experiment conducted at eight forested sites demonstrated that multiple methods, including hand weeding and herbicides can be used to eradicate Microstegium invasions, but that the removal method determines the recovery of native plant productivity and diversity. Furthermore, increases in plant productivity and diversity following Microstegium removal again showed that invasions were having significant negative effects on native plant communities. In total, this work has shown that plant invasions are predicted by particular landscape features and have significant negative consequences for the productivity and diversity of native plant communities in eastern forests. However, plant invasions can be eradicated and invaded communities can be restored by using approaches developed here.