Divergent growth-climate responses of living and dying Picea crassifolia trees in the western Qilian Mountains: early-warning signals for tree mortality

Warming has increased the frequency and intensity of extreme drought and heatwaves, which have induced pervasive increases in tree mortality, threatening forest ecosystems. However, little is known about how growth-climate responses of living vs. dying trees change, hindering exploration of tree die-off mechanisms. To address this issue, we used tree rings from Qinghai spruce (Picea crassifolia) in the arid western Qilian Mountains. We compared the radial growth and climate-growth responses between living and dying trees before and after a warming shift in 1987, and tree-growth resilience under drought extremes. The results indicate that after the warming shift, dying spruce exhibited a significant decline in radial growth and an increased tree mortality rate. Compared with the pre-warming period, the main climatic limiting factor for radial growth of living trees shifted from the pre-growing season (from September of the previous year to March of the current year) SPEI_03 to precipitation during the early-mid growing season (April to June of the current year), whereas that of dying trees shifted to the early-mid growing season SPEI_03. In all drought extremes, dying trees exhibited lower resistance than living trees, but recovery depended on drought severity. Our results suggest that under persistent drought, physiological deterioration in trees before death, i.e., impaired hydraulic conductivity and reduced carbon reserves, suppressed radial growth and intensified differences in growth-climate response. Our findings improve the understanding of tree die-off mechanisms and provide a basis for conservation and management in arid mountainous forest ecosystems.