Tree-ring δ18O identifies similarity in timing but differences in depth of soil water uptake by trees in mesic and arid climates

Water availability and uptake by trees will be important to predict tree growth, especially in regions with increasingly drying climate. Tree-ring δ18O (δ18OTR) can be influenced by moisture, so it may be a useful tool for tracking soil moisture content (SMC) variation with respect to the depth from which water is taken up and the timing of water uptake. Here we analyzed the relationship between δ18OTR of conifers and monthly SMC at different depths (0–10 and 0–100 cm) at three dry and three humid sites in China. We found that SMC outperformed local precipitation amount in terms of strength of correlations with δ18OTR in most months during the growing season. At the dry sites, δ18OTR was significantly correlated with topsoil SMC (0–10 cm) but more weakly related to SMC at 0–100 cm during the main growing season (June-July-August), implying that the trees were primarily utilizing shallow water. Whereas humid sites had similar correlations at both depths, suggesting that these trees utilized water over depth. At both sites, the highest correlation between δ18OTR and SMC occurred in months with the strongest monsoonal precipitation (June for the humid sites and July/August for the dry sites), which may indicate rapid cycling of precipitation into surface soils and then into tree roots. These findings are supported by the root distribution at the dry and humid sites, but are contrary to Walter's tree-grass two-layer model that stressed the uptake of deep soil water by trees in dry regions. In summary, our study suggests similar soil water-use seasonality but different water uptake depths of trees under dry and humid climates. This implies that δ18OTR could be a reliable proxy for SMC variation, which may provide important evidence for understanding tree water-use adaptation to changing climate.