Reproductive phenology, diaspore production, diaspore removal, and seed-dispersal by birds of five indigenous and two alien (Rubus ulmifolius and Ligustrum lucidum) woody species were studied in a riparian forest relict located in central Argentina (Punta Lara, Buenos Aires Province). Seed germination, recruitment, and seedling traits of these plants were also determined. During the last half century the weeds heavily invaded the forest, presently constituting a serious threat to the survival of this natural community. In the area the fruit-eating bird assemblage is reduced to only seven resident species and a migratory one, and some native plant species have reproductive constraints imposed by scarce fruit production or limited seed dispersal. Although the fruit : flower ratio and the ratio removed fruits : available fruits did not significantly differ between native and alien plants, the contribution of exotic plants to the seed pool were higher than that of indigenous plants (ca. 1700 seeds dispersed per canopy unit - m(2)- vs. 800 seeds for the best dispersed native species). Aliens have advantages due to the seed germination rate (Ligustrum) or seedling competitive ability (Rubus) too. Many native species reach their southernmost distribution at the Punta Lara forest; the latter's current depauperate condition is probably mainly explained by the weeds' intrinsic invasiveness coupled with suboptimal habitat location for native plants.