Divergent growth responses and future projections of Larix sibirica and Picea schrenkiana to extreme climate events at the Eastern Tianshan timberline

The timberline is a sensitive indicator of the health of alpine ecosystems. Forests in these ecosystems have been substantially affected by the increasing intensity of extreme climatic events and this trend is projected to escalate during the 21st century. However, the influence of extreme climatic events on tree growth remains poorly understood. In this study, we used Larix sibirica and Picea schrenkiana from the eastern Tianshan timberline to investigate the radial growth responses of these two species to extreme climatic events and predict growth trends under future climate scenarios. Our analysis of 1961–2009 climate data showed accelerated warming, particularly in extreme warm nighttime events (TN90P). The Iranian and western Pacific subtropical high-pressure systems may be key regulators of the variability of TN90P in the study area. The tree-ring chronologies of both species were positively correlated with extreme warm temperature indices and exhibited a weaker response to extreme precipitation. Larix sibirica growth was strongly correlated with extreme warm daytime events (r = 0.495, p < 0.01), whereas Picea schrenkiana primarily responded TN90P to (r = 0.640, p < 0.01). Lasso regression models incorporating CMIP6 climate projections predicted that both species would show an increase in growth, especially in SSP5–8.5. However, Larix sibirica (SSP2–4.5= 0.007/a; SSP5–8.5= 0.019/a) had a substantially greater growth trend than Picea schrenkiana (SSP2–4.5= 0.003/a; SSP5–8.5= 0.009/a). These findings highlight the emergence of day and nighttime warming events and species specificity as drivers of tree growth divergence in high-altitude ecosystems. They provide critical insights for predicting alpine forest dynamics under escalating climate extremes.