Contrasting tree-ring growth to climate responses of Abies alba toward the southern limit of its distribution area

Silver fir Abies alba is an indigenous tree species present in many southern European mountain forests. Its distribution area and its adaptive capacity to climate variability, expressed in tree-ring growth series, make it a very suitable target species for studying responses to climate particularly in a complex area like the Mediterranean basin where significant changes are expected. We used a set of 52 site chronologies (784 trees) in the Italian Alps and Apennines (38.1 degrees-46.6 degrees N and 6.7 degrees-16.3 degrees E) and temperature and precipitation monthly data for the period 1900-1995. Principal component analyses of the tree-ring site network was applied to extract common modes of variability in annual radial growth among the chronologies. Climate/growth relationships and their stationarity and consistency over time were computed by means of correlation and moving correlation functions. Tree-ring chronologies show a clear distinction between the Alpine and the Mediterranean sites and a further separation of the Alpine region in western and eastern sectors. Accordingly, we found different transient and contrasting regional responses in time with the trends found in the Mediterranean sites marking a relaxation of some of the major climate limiting factors recorded prior to the last decades. Species' sensitivity to global change may result in distinct spatial responses reflecting the complexity of the Mediterranean climate, with large differences between various areas of the basin. It is still unclear if these contrasting tree-ring growth to climate responses of Abies alba are due to the corresponding separation between the Alpine and Mediterranean climate modes, the atmospheric CO2 fertilization effect, the environmentally most fitted genetic pools of the southern fir ecotypes or a combination of all factors. Climate-growth analysis based on a wide site network and on long-term weather records confirmed to be excellent tools to detect spatial and temporal variability of species' responses to climate.