PS 11-160 - CANCELLED - Spatial Patterns of Exotic Invasive Ailanthus altissima and Native Robinia pseudoacacia and their Association with Forest Tree Diversity at a Local and Regional Scale

Abstract: Invasive plant impacts on plant composition and diversity in forests depend on the functional traits of the invasive plants and how the invasive plants interact with other plants. The outcomes of interactions among native and nonnative plants during early succession determine the community composition at later stages of succession and may vary at local and regional scales. We evaluated the spatial distribution and effects of a functionally similar invasive (Ailanthus altissima or Aa, allelopathic) and native tree (Robinia pseudoacacia or Rp, nitrogen fixer) on tree regeneration and diversity. Locally, the spatial relationships of Aa and Rp with tree regeneration were determined by mapping all trees, saplings, and seedlings in three late-successional plots at two sites in western VA. Regionally, the spatial distribution pattern of Aa and Rp in forested land was evaluated using Forest Inventory and Analysis data, (USDA Forest Service), across 31 eastern states. We found a clumped spatial distribution between Aa and Rp at both local and regional scales. Also at the local scale, tree regeneration of Aa was much greater than that of Rp, but native tree regeneration was negatively associated with both Aa and Rp. At the regional scale, Rp was in 5% of all plots and was more common than Aa. Both locally and regionally, Aa was more likely to form dense stands and to be associated with lower neighborhood diversity than Rp. At the regional scale, plots with Rp had higher diversity than plots without either Rp or Aa; this pattern was not found at the local scale. The allelopathic Aa inhibits tree regeneration at both scales but the nitrogen fixing Rp shows a positive interaction effect only at the regional scale. When the two species cohabit a site, the positive effects of Rp are negated by the antagonistic effects of Aa.