Plant invasions can alter community structure and species composition, but the conditions that influence community-level responses to invasion remain poorly understood. Most studies of exotic plant invasion are short in duration and involve systems affected by anthropogenic disturbance. In this long-term study, we quantified an invasion by the exotic shrub Rhamnus frangula L. and the response of co-occurring plants in a large, undisturbed wetland. We first sampled the vegetation in 1991 and repeated the sample 15 years later using the same, permanently located sample units (n=165). We used linear regression to examine the effects of the exotic shrub invasion on the species richness and plant cover in the herbaceous plant and shrub strata, and we used ordination to explore the effects of invasion on changes in species composition. 

Results/Conclusions

R. frangula occurred in 45% of sample units in 1991, but frequency increased to 73% by 2006 and, for those sample units containing the exotic shrub, mean R. frangula cover increased by 46%. In 2006, the total number of R. frangula seedlings exceeded by an order of magnitude the number of seedlings of all other tree and shrub species combined. Despite the dramatic increases in the abundance of R. frangula, the invasion elicited little apparent response by the resident plant community. Species richness and plant cover in the herbaceous plant stratum showed no apparent relationship with change in R. frangula cover. Richness of shrub species other than R. frangula also showed no relationship with change in R. frangula cover, but shrub cover decreased as R. frangula cover increased (P<0.01, R²=0.06). Species composition in the herb and shrub strata did change in the 15 years between sample dates, but changes in R. frangula cover had no influence on compositional shifts. These results suggest that plant invasiveness does not strictly determine the effects of invasion, and disturbance history may influence the degree to which resident species accommodate invasive species.