Studying Root Water Uptake Variations between Younger and Older Riparian Trees with Stable Water Isotopic Ratios

Recent empirical evidence indicates that trees vary considerably in their strategies for root water uptake (RWU). Previous studies have provided strong evidence that neighboring trees often use different water sources and support our understanding of global patterns in tree water use and transpiration. It is common to sample several individual trees within a study area to obtain a representative plot-scale xylem water isotopic composition, yet we lack a standardized methodology for selecting the individual trees to sample. Specifically, past studies generally disregard the influence of tree age/diameter, which is likely a critical consideration given its close relationship to plant rooting depths. We measured the monthly stable isotopic composition of groundwater, stream, soils, and riparian Eastern Hemlock (Tsuga Canadensis) xylem water across one growing season in the Northeastern US. We cored 30 Hemlock trees representing a wide range of diameters (5-80 cm), and therefore ages. Xylem water isotopic compositions show variations across tree ages, suggesting the need for reporting of tree ages and a standardization of field sampling protocols to support our understanding of tree water use strategies. This advance will support our understanding of hydrology and the refinement of process-based ecohydrological models.