Comment Rubus alceifolius, une espèce exotique envahissante, pourrait-elle progressivement coloniser la totalité d'une forêt tropicale humide ?

Abstract

Few quantitative studies confirm or refute actual theories on the capacity of exotic species to invade and colonize new environments. To evaluate these theories, we tested some biological traits of an invasive bramble (Rubus alceifolius Poiret), known for the importance of its sexual reproduction on lowlands, at La Réunion (Indian Ocean). We studied R. alceifolius seed distribution in one of the last tropical lowland rainforests of the island. Seed count was greater under R. alceifolius patches (> 10 000 seed/m2) than in understoreys not colonized by the bramble (approximately 3000 seed/m2). In R. alceifolius patches, localized within gaps, our results showed that seed-rain and seed-bank before and after the fruit period decreased significantly from the center to the edge of the gap. This pattern was certainly due to the variation in luminosity, which decreased from the center of the gap to the edge. We conclude that seed dispersal of this species within the forest is mainly via running water. A number of seeds (10%–20%) were able to wait for a new opening in which to germinate. A small proportion of these seeds remained viable in the soil for many years. Monospecific R. alceifolius patches can be considered nurseries and are the main source for colonization of new areas within the forest. Our quantitative results confirm that copious sexual reproduction, a large soil seedbank, and a high germination rate are biological traits that greatly contribute to the invasiveness of this plant. They also provide important information for forestry services establishing adapted control plans.Key words: seedbank, dissemination, introduced plant, biological invasion, life history traits, viability.