Understory species richness and light intensity regulate emergence, establishment and trait responses of two invasive grasses

The role of native species richness in mediating ecosystem resistance to exotic plant invasion is thought to arise from niche filling and resource limitation that influence the competitive ability of an invader. However, there is a lack of empirical evidence examining the role of species richness and light availability during an invasion event. Here, a mesocosm experiment with constructed grassy woodland understory communities were used to examine the effects of native richness and light availability on the invasion success, growth and leaf traits of two invasive grasses, Paspalum dilatatum Poir. and Eragrostis curvula (Shrad.) Nees. Native species richness significantly reduced the emergence, survival and above-ground biomass of both invasive grasses, albeit with interspecific variation in the relative and specific leaf area and intraspecific variation induced by different light and richness treatments. Strong negative relationships were identified between the above-ground biomass of both invasive and native grasses, indicative of competition effects, whilst a positive relationship was observed between E. curvula and the biomass of native shrubs indicative of a facilitation effect. Distance-based linear modelling revealed ecosystem resistance to both invasive species manifested most strongly with the inclusion of multiple native species from differing functional groups, suggesting that complementarity mediated by functional trait diversity is a principal component of biotic resistance to invasion in this system. These results provide novel evidence that species richness confers invasion resistance beyond interspecific, competitive mechanisms and may limit establishment success and trait expression.