Causes and Determinants of Ground-Layer Plant Species Turnover in Southern-European Old-Growth Forests

Old-growth forests represent an important reference point to develop sustainable forest management models. Due to their natural disturbance regime, these forests are structurally heterogeneous and have had a long ecological continuity. They are also characterized by high levels of local-scale floristic turnover, related to the occurrence of a wealth of microhabitats and to the presence of low-dispersal forest interior species that are patchily distributed. We aim to define which variables best explain ground-layer plant distribution patterns, and to quantify the relative importance of forest structural and environmental heterogeneity, and other spatially contagious processes, in determining variation in understorey floristic spatial turnover in Southern European old-growth forests. We surveyed 11 beech-dominated forests with old-growth features (Italy n=8, Balkan Peninsula n=2, Spain n=1). Each stand was sampled using a 1-hectare square plot containing a systematic grid of 25 quadrats (5m x 5m). Each quadrat was inventoried for ground-layer flora, overstorey structure (canopy closure, basal area, deadwood, spatial arrangement of large living trees) and abiotic variables (topography, photosynthetic photon-flux density, soil properties). We used multiple regression on dissimilarity matrices (stand-by-stand) and mixed-effect models (overall dataset) to quantify the relative contributions of differences in structural features, biotic and abiotic environmental variables, and spatial distances to explaining variation in understorey plant species turnover. Species turnover at the stand level was mostly related to structural and environmental heterogeneity. Total variation explained ranged between 2% and 56.5% (median 24.1%) and, after accounting for abiotic factors, it was mostly related to forest structure, both directly or indirectly. The importance of individual explanatory variables strongly differed among stands. Our results confirms that understorey plant species turnover is linked to overstorey structural heterogeneity, which affects ground-level environmental conditions. Silvicultural practices aimed at mimicking old-growth structure may thus be an effective strategy to increase understorey plant diversity levels in managed forests.