Extreme tree rings in spruce (Picea abies [L.] Karst.) and fir (Abies alba Mill.) stands in relation to climate, site, and spate in the southern French and Italian Alps

The similarity over long distances of dendroecological pointer years (with extreme ring-widths) were studied at both regional and country scales in order to investigate the geographical extension of climate influences on tree-rings. Two common species, Norway spruce (Picea abies Karat.) and white fir (Abies alba Mill.) were compared. The regional study was carried out on 33 populations located in four alpine valleys along a climatic gradient of summer aridity (Tarentaise, Maurienne, and Brianconnais, in France, and Susa valley in Italy). For most of species and regions, several negative ring-width pointer years with abrupt growth reductions such as 1976, 1922, 1986, and 1944 were common (listed in order of decreasing importance). However, spruce growth was more reduced in 1948 than that of fir. At the country scale, some of the strongest positive (e.g., 1932, 1964, 1969) and negative (e.g., 1956, 1962, 1976, 1986) pointer years extended over the whole of France, whereas the geographic variability was explainable by the autoecology of species. At both studied scales, evident climatic interpretations such as severe winter frosts, unusual summer droughts, or excessive wet and cold springs can explain most of the negative pointer years. Conversely, most positive growth responses are caused by a local combination of favorable climatic factors rather than simple extreme events, and therefore are less efficient for wood dating.