The role of native diversity and successional processes on community invasibility in riparian primary forest

Background/Question/Methods

Elucidating patterns of establishment and spread of invasive species along successional gradients may provide useful insights into understanding the drivers of community invasibility. While a relatively small number of studies have examined community invasibility through the first ~100 years of secondary succession, we are unaware of any studies examining community invisibility through 300+ years of primary succession.  This study explores the potential for biotic resistance to reduce invasibility of riparian successional forests at the landscape scale by addressing the following questions: 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? 3) Are particular plant traits associated with specific successional stages?  Richness of all native and exotic species was measured 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). 

Results/Conclusions

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 the minimally invaded statuses of closed canopy forests may be due to a lack of exotic species in the regional species pool 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.