Abstract of PhD Thesis

Impacts of three invasive species on soil seed bank communities

Margherita Gioria (2007) - University College Dublin

Despite increasing appreciation of the ecological and evolutionary importance of soil seed banks, understanding of the impacts of invasive species on this dynamic component of plant communities has been neglected. In this study, the effects of three major invaders, Gunnera tinctoria, Fallopia japonica and Heracleum mantegazzianum, were investigated comparing invaded and uninvaded communities at different spatial scale: site, plot (4 m−2) and soil depth (0-5, 5-10, 10-15 cm). Samples were collected at two points in time, in May and October. Invasion profoundly affected the structure of invaded seed bank communities. Gunnera tinctoria formed a large persistent seed bank (sensu Thompson et al. 1997), with an average value of 60,428 ± 47,131 (mean SD) seedlings m−2. Heracleum mantegazzianum formed a large transient seed bank (9,762 ± 390 seedlings m−2) and was found exclusively in samples collected in October, in the top soil layer. No seedlings of Fallopia spp. were recorded at the study sites. Diversity (measured by traditional diversity indices and taxonomic indices), at species, genus, and family level, was significantly lower in invaded seed bank communities. The numerical abundance of grasses and forbs was significantly lower in invaded areas, although the magnitude of the impact was species-specific and site-dependent. Dominance (measured by the Pielou and Simpson indices, and dominance-diversity curves) was significantly higher in invaded communities, and was independent of the reproductive strategy of the invaders. Species characterised by a persistent seed bank, and/or by effective dispersal mechanisms, were still found in the invaded seed bank. Conversely, species forming only a transient seed bank virtually disappeared. Results showed that Fallopia japonica has a potential to alter seed bank communities within a short time scale (3-10 years), despite not forming a seed bank. Low seedling recruitment from the seed bank was also observed for Gunnera tinctoria at the study sites. This observation was supported by findings based on laboratory experiments, which showed significant effects of light, temperature, seed age, stratification, and their interactions on seed germination for this species.