Climate sensitivity of high altitude tree growth across the Hindu Kush Himalaya

Abstract High altitude tree growth is predominantly limited by low temperature, and thus more responsive to climate warming. The Hindu Kush Himalaya (HKH) has experienced the most rapid climate warming in the world, however, a regional study on climate sensitivity of high altitude tree growth is still lacking. We collected tree ring growth data of 21 high altitude tree species from 128 sites and used linear mixed-effects models to comprehensively quantify the best explanatory climate variables of tree growth and the spatio-temporal pattern of climate sensitivity across the HKH. Tree growth was negatively sensitive to spring temperature, and positively sensitive to spring precipitation and self-calibrated Palmer Drought Severity Index (scPDSI) in the westerly-dominated region, while positively sensitive to temperature, particularly in winter in the monsoon-dominated region. Tree growth was more sensitive to climate change in Juniperus and Picea than in Abies. But only the tree growth of Abies showed more sensitivity at higher- than at lower-altitudes. Climate sensitivity was significantly higher after the 1950s than before the 1950s, but this trend disappeared after the 1980s. Our results demonstrate the complexity of climate sensitivity of tree growth under a changing climate across the study region. Under the future IPCC climate change scenario, our study further suggests that spring drought caused by the warmer and drier climate will be detrimental to tree growth in the relatively drier region of the central and western HKH, while warming climate may be beneficial to tree growth in the relatively humid eastern HKH. Our findings indicate that regional studies of tree growth sensitivity to climate change should go beyond spatio-temporal patterns to consider the identity of taxa and the long-term divergent responses. The results advance the understanding of spatio-temporal patterns of tree growth sensitivity to climate change and contribute a valuable perspective of biogeographical vegetation change in the world highest elevation of the vulnerable Himalayas.