Role of Native Diversity and Successional Processes on Communityinvasibility in Riparian Primary Forest

Abstract

Numerous investigations have elucidated patterns and processes which govern community invasibility but relatively few have examined invasibility in a successional context. Explored here is the potential for biotic resistance to reduce invasibility of riparian successional forests at the landscape scale (̃100ha) and address the following: 1) Does exotic species richness and percent cover change across successional time? 2) What is the relationship between native and non-native diversity and does that relationship change through succession? Vegetation surveys were conducted to quantify plant species richness on four raised terraces (understory reinitiation to multi-age old growth), six lower terraces (stem exclusion to understory reinitiation), and seven active channel margins, mid-channel islands, or abandoned channels (stand initiation). Exotic species richness and cover declined throughout succession and no exotic species were found on landforms greater than 136 years of age. However, although native richness remained constant throughout succession, native assemblages changed markedly. Thus landform diversity patterns in Zoar Valley likely reflect contemporary and/or past states of high community invasibility, which suggests that invasibility either does not change throughout succession, or that the regional species pool of native species adapted to particular successional stages is similar in size. This finding suggests that minimally invaded, closed canopy forests is due to a lack of exotic species in the regional species pools that are well adapted for establishing in forest understories and opposes the view that these communities are intrinsically less invasible. Future studies of community invasibility, in forests and other systems, may be better served by examining the traits and life-history strategies to which a community is susceptible to being invaded by.