Seed dispersal in time can counteract the effect of gene flow between natural populations of Arabidopsis thaliana

Plants may escape unfavorable environments by dispersing to new sites, or by remaining in an ungerminated state at a given site until environmental conditions become favorable. There is limited evidence regarding the occurrence, interplay and relative importance of dispersal processes in time and space in plant populations. Thirty‐six natural populations of the annual ruderal species Arabidopsis thaliana were monitored over five consecutive years, sampling both seed bank and above‐ground cohorts. We show that immigration rates are considerably higher than previously inferred, averaging 1.7% per population yr–¹. On the other hand, almost one‐third of the individuals in a given above‐ground cohort result from seeds shed 2 or 3 yr back in time in 10 of the studied populations. Populations that disappeared one year were recolonized by regeneration from the seed bank the subsequent year. Thus, dispersal in both time and space is an important contributor to the structuring of genetic variability in natural populations of A. thaliana, where a high dispersal rate in time may partly counteract the homogenizing effects of spatial seed and pollen dispersal.