Short- and long-term impacts of Acacia longifolia invasion on the belowground processes of a Mediterranean coastal dune ecosystem

Many coastal dune ecosystems in Portugal are invaded by the leguminous tree Acacia longifolia (Andrews) Willd. This exotic species was first introduced over one hundred years ago in an effort to mitigate dune erosion and loss of coastal landscapes. However, since then A. longifolia has spread to new areas, displacing the native vegetation. These invaded ecosystems contrast with the native dune ecosystems that are typically dominated by herb and shrub communities. This study characterizes belowground changes to the native environment as a result of recent (<10 y) and long-term invasion (>20 y) by A. longifolia by analyzing a range of chemical and microbial parameters. Both invaded areas accumulated higher litter densities with greater N contents and lower C/N ratios than the native areas, which corresponded to lower C/N ratio and to higher potential rates of nitrification in the invaded soils. Long-term occupation by A. longifolia has significantly altered the soil properties with increased levels of organic C, total N and exchangeable cations resulting in higher microbial biomass, basal respiration, and β-glucosaminidase activity. However, basal respiration and microbial biomass were significantly higher within recent invasion sites when calculated relative to soil organic C. The results from this study show that invasions by A. longifolia have altered the original native ecosystem processes and that the impacts are more pronounced within long-term invaded sites. A positive feedback mechanism is apparent for A. longifolia invading these Mediterranean dunes, which can make the restoration of native plant communities increasingly difficult with time elapsed since invasion.