Diversity and Invasibility of Southern Appalachian Plant Communities

We propose that the relationship between diversity and community invasibility depends on the degree to which community composition is driven by immigration processes. When immigration is enhanced by high propagule pressure or low-intensity disturbance, the relationship between diversity and exotic species invasion should be positive. Only when such immigration processes are limited should competitive interactions lead to a negative correlation between diversity and invasibility. Moreover, competition should be more apparent at smaller scales where individual plants compete directly for space; thus, diversity and invasibility are more likely to be negatively correlated at small spatial scales. We tested these predictions by comparing exotic and native species diversity of vascular plants across five spatial scales in riparian and upland plant communities in the southern Appalachians. We found a positive relationship between species diversity and exotic invasion in riparian areas at large scales (100 m2), which graded into a negative relationship at small scales (0.01 m2). In uplands, there was a slight positive relationship between native and exotic species diversity at both scales of observation. Overall, riparian areas had more exotic and native species than upland areas, and both native and exotic species diversity increased with flood frequency within the riparian zone. Corresponding Editor: T. J. Stohlgren.