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BATTLE OF THE GIANTS: PRIMARY AND SECONDARY INVASIONS BY LARGE HERBACEOUS SPECIES

Alterations in the structure of the vegetation and of the soil seed bank associated with primary invasions by alien species could create novel conditions that facilitate secondary invasions by other alien or weedy species. Here, we present a unique dataset collected from a site experiencing a process of secondary invasion by Fallopia japonica after having long been invaded by Gunnera tinctoria. These are among the largest herbaceous plant species in the world and among the worst invaders in Ireland. The main objectives of this study were 1) to test for differences in the soil seed bank associated with these invaders; 2) to discuss the potential role of changes in the soil seed bank associated with G. tinctoria invasions in promoting secondary invasions by F. japonica; and 3) to discuss the potential role of niche and fitness differences in determining the outcomes of competition between these species. The differences in the characteristics of the seed bank invaded by these species were evaluated by collecting samples in comparable, adjacent areas invaded by G. tinctoria and by F. japonica. Soil samples were collected at to two points in time (May and October), from three depth categories (0� 5cm; 5� 10cm; 10� 15cm). The seedling emergence approach was used to assess the structure of the soil seed bank. Fallopia japonica showed a capacity to alter substantially the structure of invaded soil seed bank within a short period of time. Significant changes in the composition, richness and abundance of the seed bank were observed, in both the transient and the more persistent component of the seed bank. A capacity for a rapid displacement (few years only) of long-term stands of G. tinctoria suggest a higher competitive ability for F. japonica and/or an availability of empty niches not occupied by G. tinctoria. Our findings show that the formation of a persistent soil seed bank by an invasive species is not a sufficient condition for preventing its displacement by another invader. This study represents an important starting point for evaluating the long-term implications of plant invasions on invaded ecosystems and on the potential causes of secondary invasions.