Variation in water supply leads to different responses of tree growth to warming

Background: Global climate change, characterized by changes in precipitation, prolonged growing seasons, and warming-induced water deficits, is putting increased pressure on forest ecosystems globally. Understanding the impact of climate change on drought-prone forests is a key objective in assessing forest responses to climate change. Methods: In this study, we assessed tree growth trends and changes in physiological activity under climate change based on measurements of tree ring and stable isotopes. Additionally, structural equation models were used to identify the climate drivers influencing tree growth for the period 1957-2016. Results: We found that the mean basal area increment decreased first and then increased, while the water use efficiency showed a steady increase. The effects of climate warming on tree growth switched from negative to positive in the period 1957-2016. Adequate water supply, especially snowmelt water available in the early critical period, combined with an earlier arrival of the growing season, allowed to be the key to the reversal of the effects of warming on temperature forests. The analysis of structural equation models (SEM) also demonstrated that the growth response of Pinus tabuliformis to the observed temperature increase was closely related to the increase in water availability. Conclusions: Our study indicates that warming is not the direct cause of forest decline, but does indeed exacerbate droughts, which generally cause forest declines. Water availability at the beginning of the growing season might be critical in the adaptation to rising temperatures in Asia. Temperate forests may be better able to withstand rising temperatures if they have sufficient water, with boosted growth even possible during periods of rising temperatures, thus forming stronger carbon sinks.