Biotic resistance to invader establishment of a southern Appalachian plant community is determined by environmental conditions

1 Tests of the relationship between resident plant species richness and habitat invasibility have yielded variable results. I investigated the roles of experimental manipulation of understorey species richness and overstorey characteristics in resistance to invader establishment in a floodplain forest in south-western Virginia, USA. 2 I manipulated resident species richness in experimental plots along a flooding gradient, keeping plot densities at their original levels, and quantified the overstorey characteristics of each plot. 3 After manipulating the communities, I transplanted 10 randomly chosen invaders from widespread native and non-native forest species into the experimental plots. Success of an invasion was measured by survival and growth of the invader. 4 Native and non-native invader establishment trends were influenced by different aspects of the biotic community and these relationships depended on the site of invasion. The most significant influence on non-native invader survival in this system of streamside and upper terrace plots was the overstorey composition. Non-native species survival in the flooded plots after 2 years was significantly positively related to proximity to larger trees. However, light levels did not fully explain the overstorey effect and were unrelated to native survivorship. The effects of understorey richness on survivorship depended on the origin of the invaders and the sites they were transplanted into. Additionally, native species growth was significantly affected by understorey plot richness. 5 The direction and strength of interactions with both the overstorey (for non-native invaders) and understorey richness (for natives and non-natives) changed with the site of invasion and associated environmental conditions. Rather than supporting the hypothesis of biotic resistance to non-native invasion, my results suggest that native invaders experienced increased competition with the native understorey plants in the more benign upland habitat and facilitation in the stressful riparian zone.