Water Use Strategy of Riparian Conifers Varies with Tree Size and Depends on Coordination of Water Uptake Depth and Internal Tree Water Storage

Abstract

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.

Parameterizing Vegetation Traits With a Process‐Based Ecohydrological Model and Xylem Water Isotopic Observations

“…This finding is also in general agreement with observations of a reliance on shallow water uptake by conifers (Allen et al, 2019;Knighton et al, 2021). The lack of a significant correlation between K ROOT and DBH conflicts with empirical analysis of this data set (Li & Knighton, 2022). Numerical solutions of the land surface energy balance within EcH 2 O-iso are improved with thicker soil layers that can hold larger volumes of water.…”

“…Numerical solutions of the land surface energy balance within EcH 2 O‐iso are improved with thicker soil layers that can hold larger volumes of water. The requirement of soil layer thickness likely obscured DBH‐related variations in water uptake that were previously identified across the upper 20 cm (Li & Knighton, 2022).…”

“…Correlations between tree size, topographic position, and calibrated trait values demonstrated that δ XYLEM data reflect the ecohydrological characteristics and fluxes occurring within the specific trees that are sampled. Observations of δ XYLEM inform us only on soil‐water processes within the vicinity of the plant's roots and also likely reflect characteristics of the individual plant that has been sampled (e.g., species identity, topographic position, rooting structure, stomatal conductance, sensitivity to air temperature, phenology) (Gaines et al., 2016; Knighton, Conneely, & Walter, 2019; Knighton, Souter‐Kline, et al., 2019; Li & Knighton, 2022; Snelgrove et al., 2021). These observed correlations between vegetation parameters, DBH, and topographic position (Figure 5) can possibly support new scaling relationships to better simulate spatial variations of vegetation function where empirical measurements would be infeasible and global long‐term remotely‐sensed vegetation indices possibly too coarse.…”

“…Soils to 1 m depth are a uniform well‐drained fine sandy loam (Natural Resources Conservation Service, 2022). The forest cover is dominated by Eastern hemlock (Tsuga canadensis) with a basal area of 1.03 m 2 ha −1 (Li & Knighton, 2022).…”

“…All samples were run with three standards exceeding the range of measurements collected and screened for organic contamination. Further details on field sampling and lab analysis are documented (Li & Knighton, 2022). All precipitation, groundwater, soil, and xylem water isotopic data used in this study are publicly available (Knighton, 2022).…”