What we don't seed: the role of long-lived seed banks as hidden legacies of invasive plants

Acacia dealbata is an invasive south-eastern Australian tree that produces a persistent soil seed bank. In order to characterize the seed bank in plots invaded by A. dealbata and to understand its implications for management and re-invasion risk, density, germinability, and viability of the seed bank were evaluated in five sites in central Portugal. Soil samples were collected in A. dealbata invaded plots and adjacent areas and screened for seeds, which were then quantified and germinated at 25 A degrees C. A subset of seeds was first exposed to 60 A degrees C to assess the effect of high soil temperature on dormancy breaking. Variables influencing differences between sites were explored with generalized linear mixed models with a Poisson distribution. Inside A. dealbata invaded plots the seed bank averaged 4608 seeds/m(2), reaching up to 62,747 seeds/m(2); in adjacent areas, up to 14 m from the plots, only 9 seeds/m(2) were found. Seed bank densities were mostly influenced by stoniness, number of fires in the last 10 years, and density of trees and roots. Almost 90% of seeds were viable, but only 8.6% germinated without treatment. Nearly 70% of seeds exposed to 60 A degrees C germinated without any physical stimulation, suggesting that high soil temperatures can effectively break seed dormancy. The high density and viability of A. dealbata's seed bank and its ability to disperse seeds far from the parent plants contribute to the species' invasive success. These features combined with heat events that can overcome seed dormancy need to be considered in the management of this species.