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Abstract Understanding how mixed-species forests uptake subsurface water sources is critical to projecting future forest water use and stress. Variation in root water uptake (RWU) depths and volumes is common among trees but it is unclear how it is affected by species identity, local water availability or neighboring tree species compositions. We evaluated the hypothesis that RWU depths and the age of water (i.e., time since water entered soils as precipitation) taken up by red maples (Acer rubrum) varied significantly between two forested plots, both containing red maples, similar soils, topography and hydrologic conditions, but having different neighboring tree species. We measured soil moisture contents as well as stable isotopes (δ2H, δ18O) in plant xylem water and soil moisture across two years. These data were used to calibrate process-based stand-level ecohydrological models for each plot to estimate species-level RWU depths. Model calibration suggested significant differences in red maple tree RWU depths, transpiration rates and the ages of water taken up by maples across the two stands. Maple trees growing with ash and white spruce relied on significantly deeper and older water from the soil profile than maple trees growing with birch and oak. The drought risk profile experienced by maple trees differed between the plots as demonstrated by strong correlations between precipitation and model simulated transpiration on a weekly time scale for maples taking up shallow soil moisture and a monthly time scale for maples reliant on deeper soil moisture. These findings carry significant implications for our understanding of water competition in mixed-species forests and for the representation of forest rooting strategies in hydrologic and earth systems models. 

ABSTRACT Sampling of stable isotopes in plant xylem water (δ²H, δ¹⁸O) has become a ubiquitous technique to study spatiotemporal variations in the water taken up by plant roots; however, open questions remain concerning the most appropriate time of day to sample trees to obtain representative xylem water isotopic values (δ_XYLEM). We sampled the δ_XYLEMof oak and maple trees prior to solar midday (i.e., in a recommended sampling window) and then again after solar midday (i.e., outside of the recommended window) across 4 months. The paired root mean squared difference between AM and PM δ¹⁸O ranged from 1.00‰ to 1.16‰ for maples and 0.23‰ to 2.55‰ for oaks across all sampling dates. Xylem water seasonal origin index (SOI) values derived from AM and PM δ_XYLEMsamples were significantly different, though both SOI estimates supported the conclusion that maple and oak δ_XYLEMreflected summer precipitation on all sampling dates. We conclude that sampling time of day is a significant consideration in the design of δ_XYLEMsampling campaigns; however, our findings also support flexibility in the collection time of δ_XYLEMin field sites where sampling during the optimal time of day is challenging.