Seasonal divergence between soil water availability and atmospheric moisture recorded in intra-annual tree-ring δ¹⁸O extremes

Intra-annual variability of tree-ring oxygen stable isotopes (δ¹⁸O) can record seasonal climate variability and a tree's ecophysiological response to it. Variability of sub-annual tree-ring δ¹⁸O maxima and minima, which usually occur in different parts of the growing season, may exhibit different climatic signals and can help in understanding past seasonal moisture conditions, especially in Asian monsoon areas. We developed minimum and maximum tree-ring δ¹⁸O series based on sub-annual tree-ring δ¹⁸O measurements of Pinus massoniana at a humid site in southeastern China. We found that interannual variability in minimum tree-ring δ¹⁸O is primarily controlled by the July-September soil water supply and source water δ¹⁸O, whereas the maximum latewood tree-ring δ¹⁸O is primarily controlled by the relative humidity (RH) in October. The maximum of variability of earlywood tree-ring δ¹⁸O records the RH of October of the previous year. We used minimum and maximum tree-ring δ¹⁸O to develop two reconstructions (1900-2014) of seasonal moisture availability. The summer soil water supply (July-September self-calibrated Palmer drought severity index) and the RH in fall show contrasting trends, which may be related to late-growing seasonal warming leading to a high vapor capacity and high atmospheric moisture. Our findings are valuable for research that aims to explore seasonal moisture changes under anthropogenic climate change and the ecological implications of such contrasting trends.