Plant functional composition is an important contributor to ecosystem functioning and should be considered in species selection for restoration projects. Invasive plants present a major challenge to restoration efforts as they substantially affect recipient ecosystems and plant community structure. Functional composition in plant communities may act as a biotic barrier to invasion through two mechanisms: 1) the complementary traits of resident species and/or 2) similar trait syndromes of resident species may act to pre-empt resources and prevent the establishment of invasive species. In the present study, we investigated how functional composition acts as a barrier to invasion in prairie plant communities. We hypothesized that 1) high functional diversity and 2) a high degree of functional similarity to invaders in plant communities results in lower invader biomass. We tested these hypotheses with a greenhouse experiment where the success of two exotic plant species common to tallgrass prairies (Bromus inermis and Melilotus officinalis) was evaluated in 24 plant communities of varying functional diversity and mean trait values. We invaded communities, each containing 5 native plant species, with seeds of the exotic species and allowed them to grow for 8 weeks in greenhouse conditions before harvesting above- and below-ground biomass.
The results of our experiment showed that functional diversity had no (M. officinalis treatments, p > 0.05) or a positive effect (B. inermis treatments, p < 0.001) on invader biomass. While these results indicate that complementarity was not driving invasion resistance, species richness and plant density may have been too low in our study to effectively pre-empt resources from being exploited by invaders. Best-fit models of community mean trait values did provide evidence of limiting similarity in our experiment. Invaders had lower biomass in communities exhibiting trait values associated with our invasive species (low leaf thickness, high SLA and plant height). Likewise, correlation analyses based on multi- and univariate trait dissimilarity showed that invader biomass increased with increasing dissimilarity. These data suggest that functional composition impacts community invasion resistance and should be considered in species selection for restoration efforts.