Effects of land-use and landscape structure on holm oak recruitment and regeneration at farm level in Quercus ilex L. dehesas

Successful regeneration of holm oaks is essential for the maintenance of biological diversity in the dehesas of Central Spain. In order to explore land-use and ecological site determinants of holm oak regeneration, we conducted a survey of oak recruitment and regeneration, physical features, and grazing impact indicators through a variety of sampling methods. Juvenile and sapling abundance was alarmingly low (51.2±69.1 and 85.0±85.2 plants ha−1), but varied considerably within and across study sites. Statistical analysis of spatial distribution of juveniles and saplings showed that they were highly clumped (tN=0.64 and 0.61), while mature trees were dispersed uniformly (tN=0.38). The clumping of juveniles and saplings results from the tendency for regeneration to occur on safe microsites protected from browsing and radiation (shrubs and next to rock outcrops) or from the capacity of ramets to grow vegetatively underneath canopies. We developed indicators for grazing pressure, but none of these showed any relationship with vegetation variables. Crop cultivated dehesas had lower mature tree densities (18.9±13.9 trees ha−1) than grazed (38.6±18.1 trees ha−1) and shrubby (56.4±27.5 trees ha−1) dehesas, indicating potentially negative consequences of cultivation on holm oak demography. Abundant oak regeneration was correlated with mature tree density and size, with woody vegetation cover, and with physical features such as slope, rock cover, and paddock extent. These results suggest that holm oak regeneration in dehesas is controlled by physiognomic features and long-term human impacts rather than directly by current grazing levels.