Intra-annual variability of tree-ring oxygen stable isotopes (delta O-18) can record seasonal climate variability and a tree's ecophysiological response to it. Variability of sub-annual tree-ring delta O-18 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 delta O-18 series based on sub-annual tree-ring delta O-18 measurements ofPinus massonianaat a humid site in southeastern China. We found that interannual variability in minimum tree-ring delta O-18 is primarily controlled by the July-September soil water supply and source water delta O-18, whereas the maximum latewood tree-ring delta O-18 is primarily controlled by the relative humidity (RH) in October. The maximum of variability of earlywood tree-ring delta O-18 records the RH of October of the previous year. We used minimum and maximum tree-ring delta O-18 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.

Arctic precipitation (P-G) that occurs as rainfall (P-rain) or snowfall (P-snow) depending on the prevailing climatic conditions results in seasonally specific hydrological events. Climate change can affect the P-G- and permafrost-originated water (P-ice) regimes, resulting in change to ecohydrological processes. However, the relative influences of source waters (i.e., P-rain, P-snow, and P-ice) on terrestrial hydrological processes have not yet been fully established. Here, we report the development and implementation of a numerical water tracer model designed to quantify changes in the storages and fluxes of the source waters and the hydrogen and oxygen isotopic tracers associated with hydrometeorological events. The presented tracer model was used to illustrate the spatiotemporal variability of the tracers in the surface-subsurface system of a deciduous needleleaf boreal forest and to separate the contribution rates of the tracer waters to evapotranspiration (ET). Although P-snow accounted for 22%-57% of ET and the subcomponents, the contribution rates to soil evaporation and transpiration were significant only during spring. The major source water for soil moisture was P-rain, which accounted for 69.2% of ET and showed an increasing trend during 1980-2016. Additionally, P-rain also accounted for 77.2% of transpiration. Under the present conditions of warming permafrost, P-ice demonstrated negligibly low impact on ET. The tracer model was shown capable of quantifying the contribution rates of tracer waters to ET, highlighting the advantages of the tracer model for a similar quantitative separation regarding future climate change.