SEED ECOLOGY OF AN INVASIVE ALIEN SPECIES, ACACIA LONGIFOLIA (FABACEAE), IN PORTUGUESE DUNE ECOSYSTEMS

Premise of the study: Worldwide, invasive plants threaten biodiversity, by disrupting habitats and ecosystem processes, and cause major economic losses. Invasiveness in plants is frequently associated with prolific production of seeds that accumulate in the soil. Knowledge of the extent and persistence of invasive seed banks helps explain invasion processes and enables management planning. A study of Acacia longifolia, an invasive species in Portuguese dune ecosystems, provides an informative example. Methods: Seed rain and dispersal (seed traps), the persistence of seeds in the soil (burial), and the extent of seed banks were measured and analyzed. Key results: Seed rain is concentrated under the canopy with about 12 000 seeds . m(-2) falling annually. The number of seeds in the soil declined with time, with only 30% surviving after 75 mo. Losses were lowest at greater depths. Seed germinability was low (<12%), but viability was high (>85%) for surviving seeds. The seed bank under the canopy was approximately 1500 and 500 seeds . m(-2) in long- and recently invaded stands, respectively. Some seeds were found up to 7 m from the edge of stands, indicating that outside agencies facilitate dispersal. Conclusions: Acacia longifolia produces large numbers of seeds, some of which are lost through germination, decay, and granivory. The remainder form vast and persistent seed banks that serve as a source of replenishment and make it difficult to control the invader once it is established. Control costs escalate as the duration of an invasion increases, highlighting the urgency of initiating and persevering with control efforts.