Mechanisms of resistance of Mediterranean annual communities to invasion by Conyza bonariensis:: effects of native functional composition

Recent studies have shown that a high species or functional group richness may not always lead to a greater resistance of plant communities to invasion, whereas species and/or functional group composition can more reliably predict invasion resistance. The aim of this study was to understand the mechanisms through which functional group composition can influence the resistance of Mediterranean annual communities to invasion by the exotic Conyza bonariensis. To analyse the effects of functional composition on the performance of individuals introduced as seedlings we first examined the relationships between the demographic and vegetative parameters of C. bonariensis and the biomass achieved by each functional group (grasses, legumes and Asteraceae rosettes) in synthetic communities. As a further step to approach the mechanisms involved in community resistance to invasion, we included in the analyses measurements of functional variables taken within the synthetic communities. In agreement with earlier results and theory suggesting that high nutrient availability can favour invasions, an abundant legume biomass in communities increased the final biomass and net fecundity of C. bonariensis, due to positive effects on soil nitrate concentration. Survival and establishment of C. bonariensis were mainly favoured by a high biomass of Asteraceae. Additional results from measurements of herbivory suggested that C. bonariensis survival wasn't related to abiotic conditions but may be owed to a protection against herbivores in plots with abundant Asteraceae. Establishment was on the other hand likely to be hindered by the effects of abundant grass and legume foliage on light quality, and therefore easier within an Asteraceae canopy. We conclude that invasion of Mediterranean old fields by species with biologies similar to C. bonariensis could be limited by favouring communities dominated by annual grasses.