Seasonal divergence between soil water availability and atmospheric moisture recorded in intra-annual tree-ring δ 18 O extremes

Intra-annual variability of tree-ring δ18O can record seasonal climate variability and tree ecophysiological response to it. Variability of sub-annual tree-ring δ18O maxima and minima, which usually occur in different parts of growing season, may have different climatic signals and help to understand 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 July-September soil water supply and source water δ18O, whereas latewood maximum tree-ring δ18O is primarily controlled by October relative humidity. Earlywood maximum tree-ring δ18O variability records October relative humidity 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 (scPDSI)) and fall relative humidity show contrasting trends, which may be related to late-growing seasonal warming leading to 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.