Contrasting effects of water availability on <Emphasis Type="Italic">Pinus pinaster</Emphasis> radial growth near the transition between the Atlantic and Mediterranean biogeographical regions in NW Spain

Summer water stress is the main limiting factor for Pinus pinaster growth under Mediterranean climate, but no information on limiting physiographic and climatic factors under Atlantic conditions is available for this species. We hypothesize that water availability during the active period limits P. pinaster growth, with stands nearby the Atlantic–Mediterranean boundary being more sensitive to summer drought stress. We analyzed the inter- and intra-annual climatic response of this species at ten monospecific stands in a transitional area between the Atlantic and Mediterranean biogeographical regions in Galicia, NW Spain. Mean sensitivity of radial growth consistently decreased with increasing elevation, and growth variation was quite similar for the earlywood, latewood or total ring widths, suggesting a strong intra-annual coherency of radial growth. Growth patterns were site dependent and geographically structured, with three groups of stands showing different climatic responses. As expected for sites suffering from summer drought stress, water availability enhances growth in western and southern Galicia, showing negative responses to maximum temperatures and positive to water availability. In northern Galicia, away from the Atlantic-Mediterranean boundary, water surplus in the rainy seasons negatively influenced growth. This was probably due to the combined effects of seasonal water-logging stress and the reduction of solar radiation associated with cloudiness, which would limit photosynthetic rates in winter and spring. Local variations of water availability strongly controlled the physiological processes that determine growth dynamics of P. pinaster in NW Spain, contributing to its geographical structure and contrasting sensitivity.