AVIAN DISPERSAL AND REPRODUCTIVE SUCCESS OF 2 SPECIES OF LIGUSTRUM (OLEACEAE) IN A SUBTROPICAL FOREST RELICT IN ARGENTINA

Flowering, fruit production, and seed dispersal by birds of two exotic plants (Ligustrum lucidum Ait. and Ligustrum sinense Lour.) were studied from 1986 to 1988 in a riparian forest relict located in central Argentina. L. lucidum has heavily invaded this forest whereas L. sinense has been less successful. This fact is not clearly explained either by L. sinense's lower fruit/flower ratio (14% vs. 21%) or by its lower seed production. Eleven native bird species living permanently in the ama eat fruits of both introduced plants. Of these, only three species are important for effective seed dispersal, while two others destroy the seeds. The population of the main seed predator increases considerably during the availability period of ripe Ligustrum fruits. Dispersal mode varies according to different species (isolated seeds or clumps of seeds). Migrant fruit-eating birds are not involved in seed dispersal of these plants. Ligustrum lucidum and L. sinense are perfectly naturalized in the area. Avian seed dispersal is apparently not essential for Ligustrum perpetuation although such dispersal was probably instrumental in colonization. There were no significant differences in germination between bird-dispersed seeds and those that had not passed through the birds' digestive tracts.