Can a seed bank maintain the genetic variation in the above ground plant population?

There are indications that a persistent seed bank can protect small and isolated plant populations from local extinction. Genetic mechanisms contributing to this phenomenon are the increase of local effective population size - and hence the decrease of genetic drift - through a reservoir of persistent seeds, and the accumulation of intergenerational genetic diversity in the seed bank. To find evidence for these mechanisms, we conducted two formal meta-analyses. First, we analyzed 42 published habitat fragmentation studies and investigated whether the degree of genetic differentiation between fragmented plant populations was mediated by seed longevity. Second, we reviewed 13 published studies reporting the genetic diversity of both the seed bank and the above ground plants, aiming at comparing genetic diversity contained in the seed bank with the above ground vegetation. We conclude that a persistent seed bank may indeed mitigate the consequences of habitat fragmentation and protect a species from genetic drift and population genetic differentiation. We found no evidence, however, of high levels of genetic diversity accumulating in the soil seed bank. If genetic differences are present between the standing crop and the seed bank, they are very likely the result of local selection acting either directly or indirectly as a filter on the alleles present in the seed bank. We finally suggest that 1) the role of the seed bank should not be neglected in habitat fragmentation studies and 2) it is not very fruitful to continue comparing seed bank genetic diversity with above ground plant genetic diversity, unless this is performed under different selection regimes.