Impact of an exotic N2-fixing Acacia on composition and N status of a native Mediterranean community

Nitrogen fixing plant species are among the most invasive species worldwide. However, field studies directly estimating the differential effect of native versus invasive exotic N2-fixing plants on plant communities are lacking. The exotic leguminous shrub Acacia longifolia invades coastal dunes across Portugal often co-existing with the native N2-fixer Stauracanthus spectabilis. Moreover, this co-existence with native species is possible due to a relatively low cover of Acacia species particularly in the south of Portugal where drought is intense. In this study we compare the impact of two different legume species (native and exotic) on the nitrogen status of a protected Mediterranean dune system. We document how presence of the exotic invader A. longifolia impacted community structure, soil properties and the foliar nitrogen concentrations and growth rates of native species. A. longifolia invaded areas had decreased biodiversity, fewer seedlings and altered soil properties (e.g., increased soil organic matter, NO3− and NH4+). A. longifolia presence was also associated with significant increases in foliar nitrogen content and δ15N of the endemic shrub Corema album while presence of the native legume Stauracanthus spectabilis had no discernible impact on C. album. Furthermore, a strong correlation between increased foliar nitrogen and enhanced growth rates in C. album indicate a facilitative effect of A. longifolia on this native shrub. We posit that the combination of nitrogen fixation, a rapid growth rate and production of a thick litter layer result in a very substantial impact of the invader in invaded ecosystems.