Long-term growth trends of Abies delavayi and its physiological responses to a warming climate in the Cangshan Mountains, southwestern China.

• Radial growth of Abies delavayi in southwest China is mainly limited by high temperatures and low moisture availability. • Tree growth rate of A. delavayi has been declining since the 1950s, accompanied with a remarkable increase of iWUE. • Xylem stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopic composition shifted to higher values from cold to warm years. • Growth declines of A. delavayi were driven by increasing drought stress and stomatal closure under a warming climate. Long-term tree growth trends and physiological responses to environmental changes, i.e., climate warming, and the rise of atmospheric [CO 2 ] (C a), need to be investigated across diverse forest ecosystems. We detected long-term trends in tree growth and δ¹³C derived intrinsic water use efficiency (iWUE) of Abies delavayi in the Cangshan Mountains, at its southern distribution edge in the Hengduan Mountains in southwestern China. We applied a dual-isotopic approach (δ¹³C, δ¹⁸O) to detect the shifts of tree physiological responses (photosynthetic, stomatal conductance) from colder to warmer conditions. Annual basal area increment (BAI) of A. delavayi in the Cangshan Mountains was decreasing since the 1950s, whilst the declining trend of tree growth was not obvious during the earlier period (1800–1951) after removing tree age- and size-related growth signals. Tree growth correlated negatively with temperatures and positively with relative humidity and self-calibrated Palmer drought severity index (scPDSI) during the early growing season (February to June), indicating a predominant growth limitation by moisture availability. Leaf inter-cellular [CO 2 ] (C i) derived from tree-ring δ¹³C of A. delavayi increased over time, with a rate slower than atmospheric C a rise, resulting in a decrease in the C i /C a ratio. Intrinsic water-use efficiency (iWUE) was increasing, especially after the 1950s. Dual-isotopic analyses revealed a reduction of stomatal conductance and stable or declined photosynthetic assimilation under warmer and drier environmental conditions. Recent growth decline of A. delavayi in the Cangshan Mountains was mainly related to the increase of warming-mediated drought stress and the resulting stomatal closure, while a positive effect of CO 2 fertilization was diminished under warming and drying environmental conditions. Our results serve as an early warning for tree growth and productivity of A. delavayi forests at the southern distribution limits under the ongoing warming climate. [ABSTRACT FROM AUTHOR]