Internal Water Movement and Residence Time Differ in Two Tree Species in a Temperate Deciduous Forest: Evidence From an In Situ D2O Isotope Tracer Study

To facilitate better predictive capabilities of hydrological fluxes in forested ecosystems, quantifying internal tree water movement at the tree species level is necessary. We measured short-term (daily) and long-term (weekly–monthly) water transport using isotopic breakthrough curves measured in real-time in tree species with different hydraulic anatomies (ring porous vs. tracheid). One gymnosperm species (loblolly pine; Pinus taeda L.) and one angiosperm species (southern red oak; Quercus falcata) were selected, which are common species across the Southeastern United States. Deuterated water (D₂O) was injected into the base of four trees per species (eight trees total) and tracked radially (within xylem tissues) and axially (along tree trunks) for 45 days within tree trunks and leaf water. At three heights along the main trunk, boreholes were installed to isolate distinct xylem tissues: (1) shallow sapwood (SS W); (2) deep sapwood (DSW); and (3) heartwood (HW). Initially, both species showed tracer enrichment in SSW. Subsequent tracer movement showed that oaks retained more tracer in HW and for a longer period of time than pines. Arrival time of tracer peaks ranged from 1 to 13 days in oaks (maximum δ²H = −109 to +7291) and from 1 to 4 days in pines (maximum δ²H = +22 to +8831), while time to recovery to baseline δ²H concentrations generally ranged from 7 to 38 days in oaks and from 2 to 6 days in pines. Accordingly, tracer residence time tended to be longer for all tissue depths measured in the oak trunks (1–9 days) and leaves (9–18 days) compared with pines (trunk: 0.2–3 days; leaf: 5–8 days), while tracer travel velocity was higher in pines for the trunk in the SSW and HW (2.8–5.6 m day⁻¹) compared with oaks (0.3–2.5 m day⁻¹), but not when estimated at the leaf. Although sapwood and HW are hydraulically connected, the degree of connectivity varied between tree species. Results from this study open the door for more focused studies and a greater understanding of internal water movement within mature trees.