Abstract. Trees employ mechanisms to maintain safe xylem water transport including variations in trunk water storage and the depth of root water uptake. We tested the hypotheses that 1) trunk water storage is correlated with root water uptake in Eastern hemlock, 2) and that water use strategy varies with tree size. High spatiotemporal sampling of soil and hemlock xylem (30 trees) water isotopic ratios (2H, 18O) and tree tissue Relative Water Content (RWC) was conducted across seven months. Hemlock accessing more evaporatively enriched water from shallow soils stored less water within their trunks during dry periods, and more during wet periods. Soil and xylem water isotopic compositions revealed older and lower elevation hemlock primarily sourced water uptake from the upper 10 cm of soils, whereas younger and higher elevation trees sourced some water uptake from deeper soil layers. Larger diameter hemlock showed significant temporal changes in trunk RWC. In contrast, smaller diameter trees exhibited more temporally stable RWC. Observed species-level heterogeneity in xylem water isotope composition suggests the need for reporting of tree ages and a standardization of field sampling protocols to support our understanding of tree water use strategies. Our results inform the development of plant hydraulic strategies in ecohydrological- and terrestrial biosphere-models to understand forest responses to external stressors.

<p>Plant water use in hydrologic, land-surface, and earth system models is frequently estimated by a series of equations reliant on unknown model parameters controlling plant hydraulic function. Estimating these plant hydraulic traits is critical for accurate simulation of terrestrial water storage, flow paths, tree resistance to drought, and ultimately, ecosystem response to climate change. Despite the prevalence of δ<sub>XYLEM </sub>observations, few studies have used δ<sub>XYLEM</sub> to estimate plant traits numerical ecohydrologic models We calibrated EcH<sub>2</sub>O-iso, an isotopic-enabled, fully distributed ecohydrologic model, with δ<sub>XYLEM</sub> observations of 30 Eastern Hemlock (<em>Tsuga canadensis</em>) trees across seven months. Calibrated values for maximum stomatal conductance, canopy light interception, and rooting depths were validated with independent datasets of latent heat flux, canopy light interception, and δ<sub>XYLEM</sub> from a nearby hemlock stand. Results indicate significant correlations between tree diameter (DBH), topographic position, and the calibrated values of several vegetation traits. Our results demonstrate that δ<sub>XYLEM</sub> data can be used to accurately parameterize plant traits; however, the locations and sizes of the sampled trees should be considered when upscaling measured or calibrated plant-traits from individual trees into larger horizontal scales.</p>

Xylem water isotopic compositions (2H, 18O; δXYLEM) can be used to estimate plant water uptake depths; however, environmental heterogeneity in these measurements may prevent reaching robust conclusions. Bayesian mixing models used to estimate plant water uptake depths often assume that measurements of δXYLEM and candidate water uptake sources are normally and identically distributed. We tested if δXYLEM measured across 30 Eastern hemlock (Tsuga canadensis (L.) Carrière) trees met these assumptions. Bootstrap simulations suggested that the distributions of hemlock δXYLEM data were non‐normal in March, April, June, and July and that between 15 and 26 hemlock δXYLEM samples were required to reject the assumption of normality. In June, July, and August, δXYLEM was significantly predicted by a multivariate linear regression with tree sapwood depth or elevation, rejecting the assumption of independently distributed observations. A comparison of dry season hemlock water uptake depth estimates between a Bayesian mixing model and a process‐based ecohydrological model calibration showed differences, with the Bayesian model estimating a substantially greater proportion of shallow water uptake. These results highlight the need for standardized field sampling protocols for δXYLEM and analytical methods that will lead to more robust estimates of plant water uptake depths. These findings also suggest that water uptake functions conditioned on landscape and tree structural variables could substantially advance the representation of plants in ecohydrological models.

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