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

Context 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 contained A. altissima or R. pseudoacacia or 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 between A. altissima and R. pseudoacacia at both scales. Regeneration of A. altissima was much greater than that of R. pseudoacacia but native tree regeneration was negatively associated with both target species. Regionally, R. pseudoacacia was more common than A. altissima . At both scales, A. altissima was more likely to form dense stands than R. pseudoacacia . Locally, plot types did not differ in tree diversity. Regionally, R. pseudoacacia plots had higher tree diversity than plots without either target species. Conclusions Robinia pseudoacacia and A. altissima ’s positive association is maintained into mid- and late-successional forests and A. altissima inhibits regeneration locally and regionally. Robinia pseudoacacia shows a positive effect on diversity but only when compared with mature forests at the regional scale.