Determinants of distribution of six Pinus species in Catalonia, Spain

This study explores the determinants of distribution. abundance and regener ation of six Pinus species (P. halepensis. P. nigra, P. pinaster, P. pinea, P, sylvestris and P, uncinata) that occur naturally in Catalonia. northeas tern Spain. The aim of this study was to generate accurate predictions of t he distribution of each species using simple and readily available environm ental variables. We used recursive partitioning and GIS analyses to relate the data base of 10600 field plots from the Forestry Inventory of Catalonia with abiotic and biotic characteristics of each plot. We present general p atterns of distribution, dominance and regeneration for the six species and then focus on P. halepensis. the most abundant pine species in the western Mediterranean Basin. For all six species, the models correctly classified more than 80% of the distribution using abiotic factors, mainly altitude an d rainfall variability, Biotic factors such as the basal area of other pine species were necessary to accurately predict patterns of pine species domi nance. Biotic factors. especially tile basal area of evergreen species (mai nly Quercus ilex). were of overriding importance when predicting patterns o f seedling occurrence. Potentially important factors such as land use and f ire history were of little significance for predicting distribution at the scale of our study. Our models failed to predict accurately which species ( and in which numbers) co-occur with P. halepensis. Factors not included in this study, such as stand age. disturbance (cutting, clearing) and other hu man-induced factors. are probably the main determinants of co-existence pat terns.