What trees tell us: dendrochronological and statistical analysis of the data

Abstract

Trees are a great bank of data, named sometimes for this reason as the "silent witnesses" of the past. Due to annual formation of rings, which is normally influenced directly by of climate parameters (generally changes in temperature and moisture or precipitation) and other environmental factors; these changes, occurred in the past, are "written" in the tree "archives" and can be "decoded" in order to interpret what had happened before, mainly applied for the past climate reconstruction. Using dendrochronological methods for obtaining samples of Pinus nigra from the Catalonian PrePirineous region, the cores of 15 trees with total time spine of about 100 - 250 years were analyzed for the tree ring width (TRW) patterns and had quite high correlation between them (0.71 ¿ 0.84), corresponding to a common behaviour for the environmental changes in their annual growth. After different trials with raw TRW data for standardization in order to take out the negative exponential growth curve dependency, the best method of double detrending (power transformation and smoothing line of 32 years) were selected for obtaining the indexes for further analysis. Analyzing the cross-correlations between obtained tree ring width indexes and climate data, significant correlations (p<0.05) were observed in some lags, as for example, annual precipitation in lag -1 (previous year) had negative correlation with TRW growth in the Pallars region. Significant correlation coefficients are between 0.27- 0.51 (with positive or negative signs) for many cases; as for recent (but very short period) climate data of Seu d¿Urgell meteorological station, some significant correlation coefficients were observed, of the order of 0.9. These results confirm the hypothesis of using dendrochronological data as a climate signal for further analysis, such as reconstruction of climate in the past or prediction in the future for the same locality.