Responses of Picea schrenkiana Tree-Ring Density to Climate Extremes at Different Elevations in the Kashi River Basin of the Western Tianshan Mountains

The frequency, intensity, and duration of extreme climate events will probably increase in various areas in the 21st century. This trend is especially critical for the western Tianshan Mountains, which are highly sensitive to climate variability. Here, we used tree core samples of Picea schrenkiana Fisch. & C.A.Mey. (P. schrenkiana) from the Kashi River Basin to establish standardized earlywood and latewood density chronologies, analyze correlations between the density chronologies and extreme climate indices, and explore the response of tree-ring density to climate and altitudinal variation. Earlywood and latewood densities were significantly correlated with major climatic indices: positively with warm nights (TN90p; June–August), Max Tmin (TNx; June–August), and Min Tmin (TNn; July), and negatively correlated with cold nights (TN10p; June–September). The climate has undergone significant changes particularly in minimum temperatures, and the sensitivity of tree-ring density to major extreme climate indices (TN90p, TNx, TNn, and TN10p) strengthened significantly in sample sites below 1900 m and above 2200 m during 1980–2015 and 1974–2009, respectively, reflecting the vulnerability of mid–high-altitude forests to drought stress. Combined with teleconnection analysis, the positive phase of the AMO is synergistic with the negative phase of NAO/AO, which aggravates the response of tree growth to drought. These results indicate that changes in extreme climate indices primarily exacerbate drought stress in the region’s vulnerable mountain ecosystems, suggesting that future forest management plans should accordingly strengthen their focus on P. schrenkiana.