Spatial concordance at a regional scale in the regeneration process of a circum‐Mediterranean relict (Buxus balearica): connecting seed dispersal to seedling establishment

Abstract

Both microhabitat and temporal scales have significant effects on the regeneration process of plant species. However, the consistency of those patterns at large scales is largely unknown, despite of being essential from both an ecological and a conservation viewpoint. In this study, we examined the spatio‐temporal variation in the process of regeneration through seeds of the relict Mediterranean shrub Buxus balearica. By means of observations and experiments, we quantified the losses at different recruitment phases (seed rain, seed predation, seedling emergence and establishment) both at a small spatial scale (distance and microhabitat effects) and at a temporal scale (four years), and compared them at a regional scale (10 different localities). We additionally studied transition probabilities between phases in two populations and measured the size structure of 18 localities encompassing all the distributional range of the species. The results show that seed shadow follows a leptokurtic pattern which is consistently maintained until the seedling phase (i.e. there is spatial concordance). Despite post‐dispersal factors are spatially uncoupled, they do not remove the gradient established in the seed shadow. Although the importance of different seed predators varied between regions, post‐dispersal seed predation (ca 80%), together with seedling emergence (usually lower than 10%, some years even nil), and seedling mortality during the first summer (ca 70%) were – consistently in all populations – the most critical bottlenecks in the regeneration of the species. Moreover, an absence of seed bank prevents the buffering of dramatic losses during such phases. Watering of sown seeds increased seedling emergence and survival, and number of leaves per seedling, and survival was consistently higher under vegetation coverage. These results suggest that hydric stress notably contributes to limit the regeneration of this once widespread species evolved under a subtropical climate, which nowadays appears in old‐structured populations throughout its distributional area.