Stochastic and deterministic processes regulate spatio-temporal variation in seed bank diversity

Question: Seed banks often serve as reservoirs of taxonomic and genetic diversity that buffer plant populations and influence post-disturbance vegetation trajectories; yet evaluating their importance requires understanding how their composition varies within and across spatial and temporal scales (α- and β-diversity). Shifts in seed bank diversity are strongly governed by the deterministic role of differential seed longevities. Nevertheless, spatio-temporal variability in propagule depletion from and recruitment into seed banks may alter both α- and β-diversity. Here, we ask to what degree deterministic seed exhaustion, stochasticity in recruitment and mortality, or both, shape α- and β-diversity? Location: Temperate hardwood forest stands of varying ages in northern Pennsylvania, USA. Methods: We surveyed the seed bank and herbaceous vegetation communities at 39 sites and examined whether the species richness, abundance and composition of either community differs among and within sites of increasing age ranging from 43 to 106 yr old. We explored how α-diversity (species richness) and abundance (percentage cover, seed density) varied across the chronosequence age using regression analyses. We analyzed differences in β -diversity (community composition) using permutational multivariate analyses (i.e. PERMANOVA, PERMDISP). Finally, we tested whether community composition of the herbaceous layer and seed bank communities exhibited nestedness, where nestedness refers to the degree to which less species-rich sites are a non-random subset of more species-rich sites. Results: We found seed bank α-diversity and abundance consistently declined across a gradient of increasingly older sites. Moreover, nestedness analyses indicated species composition at older sites represented a subset of the species found at younger sites characterized by species with persistent seeds. Nevertheless, seed bank communities demonstrated divergent compositional trajectories, whereby older sites were increasingly dissimilar, not only from younger sites, but also from each other. Conclusions: Our results suggest that even in assemblages strongly structured by deterministic forces, such as seed banks, minor stochastic differences in colonization and extinction events may increase β-diversity over time. Therefore, we argue deterministic and stochastic processes are complementary factors governing post-disturbance turnover in species assemblages, and suggest that seed banks contribute to species’ persistence and overall forest community diversity across space and time