Site‐Specific Functional Response of the Tibetan Plateau's Forests to Climate Change Over the Past 200 Years – A Tree‐Ring Isotope Perspective

Global warming challenges forest resilience on the Tibetan Plateau by disrupting tree's physiological adjustments. However, understanding the physiological adjustments between autotrophic and heterotrophic metabolism under shifting moisture regimes remains challenging. We investigated 200‐year tree functioning across a moisture gradient using tree‐ring width and carbon isotope variation (δ 13 C) to indicate water stress, and the covariation between tree‐ring cellulose δ 18 O and δ 2 H (O▬H relationship) to indicate shifts in autotrophic and heterotrophic metabolism. The moist site exhibited strong and stable O▬H coupling, reflecting dominant hydroclimatic control, though recent droughts induced decoupling and reduced growth. In contrast, the semi‐arid site showed lower and variable O▬H relationship, reflecting repeated transitional functional state. The arid‐site forest showed increased growth due to recent wetting, yet persistent OH decoupling indicated ongoing physiological adjustments. Combined with the instrumental data, we found that O▬H coupling strength depended on site‐specific hydroclimatic baselines, appearing stronger in dry years at arid sites but in wet years at semi‐arid sites. Overall, our framework provides a quantitative, time‐resolved perspective on trees' physiological response across moisture regimes. The O–H relationship serves as an early‐warning tool for detecting forests undergoing functional shifts under ongoing climate change.