Access to a reliable water source plays an integral role in tree function and survival. Water is a critical component of tree physiological processes, with trees that experience water stress exhibiting lower stomatal conductance (Irvine et al. 1998, Panek and Goldstein 1999), reduced photosynthetic and growth rates (Grieu et al. 1988, DeLucia and Heckathorn 1989, Adams and Kolb 2005, Truettner et al. 2018), and increased risk of hydraulic failure (Brodribb and Cochrad 2009, Anderegg and Anderegg 2013). As droughts increase in frequency, severity, and duration globally (Allen et al. 2015), access to a reliable water source such as deep soil water or water stored in fractured bedrock (i.e, hydraulic refugia) may dictate how trees respond to periods of water stress (McDowell et al. 2019). For example, studies in the southwestern United States have indicated that trees with access to deeper, more reliable water sources experience lower rates of mortality during drought periods (Grossiord et al. 2017, McDowell et al. 2019). While the effect of water availability on tree function and survival is well studied, whether the primary water source used by trees varies through time and across topographic gradients remains poorly understood. Understanding variability in tree water use patterns may provide insight to how forests will respond to increasing water stress under climate change.

Tree core data were collected and individual growth years extracted and analyzed for oxygen isotopes.