Spatial patterns of native Robinia pseudoacacia and invasive Ailanthus altissima and their influence on regeneration, abundance, and diversity of neighboring trees at local and regional scales

AbstractContext. Native early-successional plants and invasive exotic plants often colonize the same disturbed space and have similar functions, making interactions likely. Knowing whether these interactions are facilitative or competitive across different scales will help determine the influence of such species across a forested landscape. Objective. We evaluated the impacts of an early-successional, nitrogen-fixing native (facilitator),Robinia pseudoacacia,and an allelopathic invasive (inhibitor),Ailanthus altissima,on regeneration, basal area, and diversity of forests at local and regional scales. Methods.Locally, all woody stems were mapped in three post-disturbance, mid-successional plots at two sites in Virginia, US. Plots containedA. altissimaorR. pseudoacaciaor both. Target species were evaluated regionally and spatially using USDA Forest Inventory and Analysis data across 31 eastern states, US. Results.Spatial contagion was found betweenA. altissimaandR. pseudoacaciaat both scales. Regeneration ofA. altissimawas much greater than that ofR. pseudoacaciabut native tree regeneration was negatively associated with both target species. Regionally,R. pseudoacaciawas more common thanA. altissima. At both scales,A. altissimawas more likely to form dense stands thanR. pseudoacacia. Locally, plot types did not differ in tree diversity. Regionally,R. pseudoacaciaplots had higher tree diversity than plots without either target species. Conclusions.Robinia pseudoacaciaandA. altissima’s positive association is maintained into mid- and late-successional forests andA. altissimainhibits regeneration locally and regionally.Robinia pseudoacaciashows a positive effect on diversity but only when compared with mature forests at the regional scale.