Qinghai spruce (Picea crassifolia) growth-climate response between lower and upper elevation gradient limits: a case study along a consistent slope in the mid-Qilian Mountains region

Tree-ring width chronologies are important records of climate change. Mounting evidence suggests that tree-ring climate response is elevation dependent. In this study, even-aged Qinghai spruce (Picea crassifolia Kom.) chronologies from six sites along a consistent slope in the Qilian Mountains region were selected to investigate altitudinal variability of growth-climate response. Results showed that growth-climate response of this species at different elevations was limited by identical factors. In the Qilian Mountains, P. crassifolia tree-ring growth was limited primarily by drought conditions which are determined by precipitation and temperature in the Qilian Mountains along the entire elevation gradient, exhibiting a positive correlation with PDSI in growth season, a positive correlation with precipitation in May and a negative correlation with the maximum temperature in June. Moreover, P. crassifolia recruitment dynamics coincided well with the temperature variations, suggesting that recruitment rates at different elevations were all primarily influenced by temperature. What was noteworthy was that the great decrease in the mean sensitivity and standard deviation of chronologies directly indicated that P. crassifolia climate-growth response weakened at higher elevations. Furthermore, tree-ring-climate correlation coefficients, and spatial correlation fields of tree-ring width indices to regional climate factors also significantly decreased at higher elevations, indicating that P. crassifolia climate-growth response at higher elevations was weaker than that at lower elevations. Additionally, response of P. crassifolia recruitment dynamics to changes in temperature decreased with increasing elevation, confirming results from chronology and response analyses that showed that P. crassifolia climate-growth response weakened at higher elevations.