Testing the trait-based community framework: Can limiting similarity increase invasion resistance in restored grassland?

Background/Question/Methods

It has been recently proposed that a trait-based community assembly framework can guide restoration efforts to assemble plant communities that are resistant to invasion.  One specific hypothesis is that community resistance to invasion can be strengthened by selecting native species that are similar to invasive species in resource use, thus increasing competition.  The theory of limiting similarity posits that there is a finite limit to the similarity between the resource-use of co-existing species and predicts that successful invaders will differ functionally from species already present in the community and that successful restoration approaches would be ones that build native communities with traits similar to potential invaders.  While several studies have documented limiting similarity in invaded systems, there have been few attempts to utilize trait data from invasive and native species for the purpose of directing ecological restoration in invaded systems.  Following the theory of limiting similarity, I predicted that invasive species will have lower fitness when grown in the presence of native species with similar timing and pattern of resource use.  This hypothesis was tested with native and invasive species from serpentine grassland in California.

Results/Conclusions

I collected functional trait data from over 50 species in the serpentine grassland, including timing of germination, leaf nitrogen content, leaf mass per unit area, photosynthetic capacity, root depth, root to shoot biomass ratio, and specific root length.   I then used multivariate analyses to identify native species that were similar in water and nutrient use to the dominant invasive species, ryegrass (Festuca perennis).  Five functionally similar and five functionally distinct native species were grown in competition with ryegrass and in monoculture.  Prior to flowering (peak biomass), I measured total above-ground biomass of each species and final resource availability (soil nitrogen, soil moisture and light availability), R*, within mixed-pots and monocultures.  The data partially support the idea that functional traits can be used to predict competitive outcomes and, consequently, invasion resistance.