The spatiotemporal dynamics of plant water sources are hidden and poorly understood. We document water source use of Quercus garryana growing in Northern California on a profile of approximately 50 cm of soil underlain by 2–4 m of weathered bedrock (sheared shale mélange) that completely saturates in winter, when the oaks lack leaves, and progressively dries over the summer. We determined oak water sources by combining observations of water stable isotope composition, vadose zone moisture and groundwater dynamics, and metrics of tree water status (potential) and use (sapflow). During the spring, oak xylem water is isotopically similar to the seasonal groundwater and shallow, evaporatively enriched soil moisture pools. However, as soils dry and the water table recedes to the permanently saturated, anoxic, low‐conductivity fresh bedrock boundary, Q. garryana shifts to using a water source with a depleted isotopic composition that matches residual moisture in the deep soil and underlying weathered bedrock vadose zone. Sapflow rates remain high as late‐summer predawn water potentials drop below −2.5 MPa. Neutron probe surveys reveal late‐summer rock moisture declines under the oaks in contrast to constant rock moisture levels under grass‐dominated areas. We therefore conclude that the oaks temporarily use seasonal groundwater when it occupies the weathered profile but otherwise use deep unsaturated zone moisture after seasonal groundwater recedes. The ample moisture, connected porosity, and oxygenated conditions of the weathered bedrock vadose zone make it a key tree water resource during the long summer dry season of the local Mediterranean climate.

中文翻译：

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夏季干燥季节从风化的基岩渗流带提取的橡木蒸腾作用

植物水源的时空动态是隐藏的，人们对此知之甚少。记载了栎属植物garryana的水源利用它生长在北加利福尼亚约50 cm的土壤剖面上，并有2–4 m的风化基岩（带剪切的页岩混杂岩），在冬季，当橡树缺少叶子时，该岩层完全饱和，并在夏季逐渐变干。我们通过结合水稳定同位素组成，渗流带湿度和地下水动力学以及树木水状态（潜力）和使用量（树液流量）的度量来确定橡木水源。在春季，橡木木质部水的同位素与季节性地下水相似，且蒸发量较浅，土壤水分丰富。但是，随着土壤干燥和地下水位逐渐下降到永久饱和，缺氧，低电导率的新鲜基岩边界，Q。garryana转向使用同位素组成耗尽的水源，该同位素组成与深层土壤和下层风化基岩渗流带中的残留水分相匹配。由于夏末黎明前的水势下降到− 2.5 MPa以下，树汁流量仍然很高。中子探测调查显示，橡树下的夏末岩石水分下降，而草为主的地区则保持恒定的岩石水分水平。因此，我们得出的结论是，橡树在占据风化剖面时暂时使用季节性地下水，但在季节性地下水消退之后，则使用深层的不饱和带水分。风化的基岩渗流带充沛的水分，连通的孔隙度和充氧条件使其成为当地地中海气候漫长的夏季干旱季节的重要树木水资源。