Global warming alters Himalayan hemlock's climate sensitivity and growth dynamics

Global warming may lead to increased tree growth but also reduced tree performance due to increased moisture stress. Under what conditions these conflicting responses occur remains uncertain, especially when growth is controlled by different climatic factors throughout the year. Here, we investigate the growth response of Tsuga dumosa, a Himalayan endemic tree species, to global climate change in unique old-growth forests that persist at around 3000 m elevation near Kanchenjunga, Nepal. Specifically, we are examining various growth parameters, including earlywood, adjusted latewood, maximum latewood density, and total basal area increment, to determine how different climatic factors influence growth dynamics during the year. First, we found that the response to climate change varied by season. Climate warming had contrasting effects on different components of tree growth. Higher temperatures during previous October and previous December and warming-induced moisture stress in April reduce earlywood and total annual increment. On the other hand, higher rainfall during June and July limits total tree ring width and adjusted latewood width, respectively. Second, warming is changing the sensitivity of tree growth to various climatic factors. Growth in the past two decades became more dependent on autumn conditions than on spring and summer conditions. Warming alleviates the cold limitation during the cool monsoon summer but reinforces the water deficit during autumn, winter, and pre-monsoon dry spring. Third, analysis of basal area increments, an indicator of aboveground biomass production, showed that accelerated warming in summer contributes to increased annual productivity. However, the growth reduction observed during warming spring and autumn months counteracts this effect. In conclusion, this study highlights the contrasting responses of Tsuga dumosa to climate warming in Himalayan montane forests. While accelerated warming makes the trees less sensitive to summer temperature limitations, warm autumns, dry springs, and less snowy winters become the main limiting factors for their growth. Understanding these complex and contrasting responses, as well as the sensitivity of different growth parameters, is essential for predicting the long-term impacts of climate change on mountain ecosystems.