Spatiotemporal variability in tree ring’s δ13C of Picea crassifolia in the Qilian Mountains: climatic significance and responses to rising CO2

The carbon isotopic composition (δ¹³C) of tree rings was used to assess changes in intrinsic water-use efficiency (W_i) to increasing atmospheric CO₂ and climate change during the period of 1891–2003. Five Qinghai spruce (Picea crassifolia) stands were selected in the Qilian Mountains, growing along a precipitation gradient. All five δ¹³C were correlated to each other, but two sites (DDS and CLS), which are far from the main body of the mountains, show relative weak connections to other sites. Although trees at all sites had improved their W_i in response to increasing atmospheric CO₂ concentration, spruce growing in the regions far away from the main body of the mountains were less sensitive to improved W_i than those of other sites. Based on the correlation between carbon isotope discrimination (Δ) and Palmer Drought Severity Index (PDSI), the drought history covering the period of 1891–2003 was reconstructed in the study region. The two most severe drought epochs of the late 1920s and the last decade were caused by reduced precipitation and climate warming, respectively. Our results will be useful in assessing any further spatial climate-related bioclimatic information.