Neighborhood models of the effects of invasive tree species on ecosystem processes

Changes in the composition of a community due to the invasion by exotic plant species can lead to modi. cation of ecosystem function that, in turn, produces feedbacks that drive further changes in community composition. The development of predictive models of this process requires an understanding of the spatial extent of the impacts of the exotic species, particularly during early stages of invasion. The main objective of this study was to characterize the neighborhood dynamics of ecosystem transformations by the invasive tree species Norway maple (Acer platanoides L.) and tree of heaven (Ailanthus altissima Mill.) in temperate forests of the northeastern United States. By using a neighborhood approach in multiple sites, we sought to determine (1) the magnitude and spatial extent of the effects of the two invasive species on soil properties and processes; (2) whether the effects of the invasive species were site dependent; and (3) the differences in the effects of invasive versus native tree species on ecosystem processes. Our results showed that Norway maple and tree of heaven alter the functioning of temperate forest ecosystems even at relatively low densities by increasing cycling rates (i.e., net N mineralization, net nitrification, Ca mineralization) and nutrient availability (i.e., pH, Ca, Mg, K, N). At the neighborhood scale, the spatial extent of the impact of the two species varied strikingly among soil properties. Moreover, the neighborhood effects of the two invasive species were site dependent, with the magnitude of the impact increasing with soil fertility. At the community level, Norway maple and to a lesser extent tree of heaven had stronger effect on soils than any of the dominant native tree species considered. We conclude that the invasion of northeastern forests by Norway maple and tree of heaven is characterized by predictable, neighborhood-specific acceleration of nutrient cycling rates and localized increases in nutrient pools. These ecosystem alterations have enormous potential for the modi. cation of competitive hierarchies in forest communities. In particular, Norway maple and tree of heaven may change relative abundances within the native community to the benefit of native species that are more competitive on nutrient rich sites.