Does invasion by an alien plant species affect the soil seed bank?

Questions: How does invasion affect old-field seed bank species richness, composition and density? How consistent are these effects across sites? Does the soil seed bank match vegetation structure in old-fields? Location: Menorca, Balearic Islands, Spain, western Mediterranean basin. Methods: We monitored seed germination in soils from old-fields that were both uninvaded and invaded (legacy effect) by the annual geophyte Oxalis pes-caprae. We also added O. pes-caprae bulbs to uninvaded soils to test O. pes-caprae interference with seedling emergence (competitive effect). We compared species composition in the seed bank with that of the vegetation. Results: Species richness in the seed bank and in the vegetation was not significantly different between invaded and uninvaded areas. Uninvaded areas did not have larger seed banks than invaded areas. More seedlings, especially of geophytes, emerged when O. pes-caprae bulbs were added to the soil. Species similarity between invaded and uninvaded areas was higher in the seed bank (74%) than in the vegetation (49%). Differences in species composition were as important as differences among sites. The degree of species similarity between the seed bank and the vegetation was very low (17%). Conclusions: Despite invasion by O. pes-caprae not affecting species richness, the variation in the seed bank species composition in invaded and uninvaded areas, and the differences between the seed bank and the mature vegetation, highlights that even if the invader could be eradicated the vegetation could not be restored back to the exact composition as found in uninvaded areas.