Common growth signal and spatial synchrony of the chronologies of tree-rings from pines in the Baltic Sea region over the last nine centuries

The common growth signal and spatial synchrony of nine chronologies of tree rings (Pinus sylvestris L.) from the Baltic Sea region were studied using moving correlations with time windows of 50 and 100 years. Drifts from synchrony to asynchrony and again back to synchrony across several centuries were observed. The chronologies showed higher (or lower) coefficients of correlations and correspondingly gentler (or steeper) declines in spatial synchrony in certain periods. In accordance with research into other ecological phenomena, the origin of the spatial synchrony was found to relate to the intensity of the westerly circulation over the Atlantic and Europe as described by the North Atlantic Oscillation (NAO) index. The tree rings from all of the analysed subregions were found to be wider, identifying faster periods of growth, when there was a positive NAO during the extended winter season from December to March that preceded the period of growth. Previous studies showed that the NAO index can be linked with the growth of tree rings because of factors in the local climate that affect growth, such as temperature and precipitation. Moreover, the spatial synchrony showed multidecadal fluctuations that were correlated with variations of a similar scale in the reconstructed NAO index. However, a high degree of spatial synchrony was observed during the 20^th century in the presence of values of NAO index that were strongly negative. A more detailed view was provided by maps of the correlations in which spatial changes were demonstrated in the growth of pines between the 12^th and the 20^th century. These results could be used as background information in the development of improved proxy-based reconstructions of the NAO index (or other similar meteorological variables), which could also be inferred from tree ring data relating to earlier centuries.