Tree invasions can lead to changes in the plant community composition and reductions in the number of native species. Acacia dealbata Link (silver wattle) was introduced to Chile as an ornamental. This species originated in Australia and Tasmania has been documented as invader in several ecosystems in the world. In Chile, this species is strongly associated to roads, rivers and human disturbances, performing as an invader in natural and semi-natural ecosystems. However, A. dealbata impacts at community levels have been scarcely studied. In order to determinate the relationship between A. dealbata invasion and the species richness and composition of plants in natural communities of south-central of Chile, we recorded the plant composition in three study conditions: under Acacia canopy, inside an adjacent native forest and in the edges between both conditions. We selected five study sites near to Concepcion, Chile (37 degrees S) and in each condition we surveyed three transects of 20x2 m(2) with a total of 15 transects at the three conditions. Results showed that the total species richness is significantly lower under Acacia canopy compared with the stand edge and native forest. Native species richness is higher within native forest and alien species richness is significantly higher in the stand edge. Plant composition also changes in the presence of A. dealbata. This study demonstrates the positive relationship between the presence of A. dealbata stands and the reduction of plant diversity in the south-central area of Chile.