Banking on the past: seed banks as a reservoir for rare and native species in restored vernal pools

Abstract

Soil seed banks serve as reservoirs for future plant communities, and when diverse and abundant can buffer vegetation communities against environmental fluctuations. Sparse seed banks, however, may lead to future declines of already rare species. Seed banks in wetland communities are often robust and can persist over long time periods making wetlands model systems forstudyingthe spatialand temporal links between above- and belowground communities.Usingcollectedsoilsandgerminationtrials,weassessedspeciesdiversityanddensityintheseedbanksof restored, ephemeral wetlands (vernal pools) in California's Central Valley, USA. Using long-term vegetation surveys, we compared the community structure of seed banks to that of aboveground vegetation and assessed the temporal links between below- and aboveground communities. We also compared the proportional abundances of different cover classes as well as the abundance of native plants in seed banks to aboveground communities. The proportional abun- dances of both rare and native species were greater in seed bank samples than in aboveground samples, yet the seed bank had lower species richness than aboveground vegetation. However, the seed bank had greater levels of differen- tiation among pools (beta diversity;b) than aboveground samples. Additionally,the seed bank was more similar to the earlier(2003-06)abovegroundcommunitythanthemorerecent (2007-10)abovegroundcommunity.Thecorrelation ofspeciescompositioninthecurrentseedbanktoanearlierabovegroundcommunitysuggeststhatseedbanksexhibit storage effects while aboveground species composition in this system is not driven by seed bank composition, but is perhapsduetoenvironmentalfiltering.Weconcludethattheseedbankofthesepoolsisneitherpronetothesametem- poral rates of invasion as the aboveground community, nor is seed abundance presently a limiting factor in the above- ground frequency of native species or a promoting factor in plant invasions of these restored habitats.