Extreme tree rings in spruce (Picea abies [L.] Karst.) and fir (Abies albaMill.) 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 or der to investigate the geographical extension of climate influences on tree -rings. Two common species, Norway spruce (Picea abies Karat.) and white fi r (Abies alba Mill.) were compared. The regional study was carried out on 3 3 populations located in four alpine valleys along a climatic gradient of s ummer aridity (Tarentaise, Maurienne, and Brianconnais, in France, and Susa valley in Italy). For most of species and regions, several negative ring-w idth pointer years with abrupt growth reductions such as 1976, 1922, 1986, and 1944 were common (listed in order of decreasing importance). However, s pruce growth was more reduced in 1948 than that of fir. At the country scal e, 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 s evere winter frosts, unusual summer droughts, or excessive wet and cold spr ings can explain most of the negative pointer years. Conversely, most posit ive growth responses are caused by a local combination of favorable climati c factors rather than simple extreme events, and therefore are less efficie nt for wood dating.