Winter North Atlantic oscillation effects on the tree rings of the Italian beech (Fagus sylvatica L.).

Climatic signals in beech tree-ring width series from Central Italy have been studied over different periods of time. Prewhitened tree-ring chronologies respond mainly to summer precipitation and they do not correlate in a significant manner with the winter North Atlantic oscillation (NAO) index. In this high-frequency pattern the NAO signs are only found on a small number of rings characterized by being very narrow or wide. By contrast, tree-ring width chronologies in which all the frequency components are conserved were significantly related to the NAO. The significant inverse correlation between actual measurements of ring width and NAO is a consequence of the availability of water in the soil at the beginning of the growing season. In fact, in the Mediterranean area the recharging of soil moisture depends on the amount of winter precipitation, which is inversely correlated with the NAO. Strong signals of winter precipitation and NAO are found in the low-frequency components of tree-ring growth.