How does plant species composition change from year to year? A case study from the herbaceous layer of a submediterranean oak woodland

Over millions of years there is a long-term increase in species richness, accompanied by substantial turnover in species composition. However, little is known about species temporal turnover over shorter, ecologically relevant time periods, such as years. In the present study, we examine the inter-annual temporal turnover in species composition in 100 m² plots of the herbaceous layer in a submediterranean oak woodland over six years. We found that approximately half of the accumulated number of species over the six years is accommodated as temporal turnover. We also found that species temporal turnover in undisturbed control plots was not significantly different from that in plots where vegetation was recovering naturally without assistance, i.e., plots undergoing ecological succession. Only in the most disturbed (continuously overgrazed) plots temporal turnover was low to non-existent. We therefore suggest that diversity estimates based on a single year of observations may seriously underestimate species richness or the detrimental effects of disturbance, at least at the 100 m² scale. Furthermore, we found that, with the exception of the heavily grazed plots, short-lived species (annuals and biennials) did not display significantly greater temporal turnover than long-lived (perennial) species. Our analysis also supports that the space for time substitution applies in the patterns of species turnover. Spatial species turnover was comparable to temporal turnover. Species that are observed in many plots are also present in many years, and vice versa. Also, the similarity in species composition decreased as the time period between observations increased, as is the case with distance decay. Overall we conclude that the patterns of species turnover in time resemble those in space, and thus temporal turnover makes an important contribution to total biodiversity that should not be ignored.