Effects of fragmentation on vascular plant diversity in a Mediterranean forest archipelago

We analysed the effects of patch size and isolation on vascular plants in Quercus cerris forest surrounding Rome (Italy). We randomly sampled 96 plots within 18 forest patches with homogeneous environmental variables; the patches ranged from 1.4 ha to 424.5 ha and were divided into four size classes. We performed the analyses at the patch level using linear regression. At the size class level, the analysis of species richness response to fragmentation (area effect) was performed with ANOVA, while the effect on community composition was analysed by means of PERMANOVA. We also investigated which species could be used as indicator species for each size class. Lastly, to evaluate the advantages of conserving several small patches as opposed to few large ones, we used a cumulative area approach ranking forest fragments. The correlation between species richness and patch area was positive, with a significant difference between the olargeo and osmallo size classes, while analysis on community composition showed that olargeo versus omediumo and olargeo versus osmallo were significantly different. Nemoral species were recognised as indicators in the olargeo class, and shrub and edge species in the osmallo class. Our results indicate that 10 ha may be a suitable forest size threshold for planning and conservation.