High-altitude tree growth responses to climate change across the Hindu Kush Himalaya

Aims Rapid warming at high altitudes may lead to a higher sensitivity in tree growth to temperature. The key factors constraining tree radial growth and to what extent regional tree growth has suffered from climatic changes are unclear. Methods Tree-ring width data were collected from 73 sites across the Hindu Kush Himalaya (HKH), including three dominant genera (Abies, Juniperus and Picea) at high altitudes over 3000 m. Dynamic time warping was introduced to develop subregional chronologies by considering the synchrony of annual tree growth among different sites. We quantified the contribution of the climate variables, and analyzed the spatiotemporal variation of the growth-climate relationship. Important Findings The site chronologies were grouped into three clusters, corresponding to the three distinct bioclimatic zones, i.e. the western HKH, central-eastern HKH and southeastern Tibetan Plateau (TP). Tree growth was positively correlated to winter and spring precipitation in the drier western HKH, and to winter temperature and spring precipitation in the humid southeastern TP. Tree growth was markedly constrained by the minimum temperature, especially in winter, with its importance increasing from the west toward the east. As shown by moving correlation analysis, the signal of winter temperature in tree growth was weakened in the western and central-eastern HKH, while it was enhanced in the southeastern TP following rapid warming since the 1980s. Our results highlight that continuous warming may cause forest recession due to warming-induced moisture deficit in the western HKH, but forest expansion in the southeastern TP.