Root water uptake (RWU) is fundamental to plant function and ecosystem processes, yet isotopic inconsistencies between plant compartments challenge interpretations of water sources. In this study, we examined RWU patterns of three urban endemic species: boxelder maple (Acer negundo), cherry laurel (Prunus caroliniana), and elderberry (Sambucus canadensis), within a humid, subtropical environment of north-central Texas (USA). Mobile water was extracted from stems, roots, and soils using centrifugation, and analyzed for δ¹⁸O, δ2H, and d-excess. Seasonal precipitation variability was evident across all species, with higher total water content and mobile extractable water in spring and reduced extracted values during summer months. Isotopic values within roots and stems were strongly modulated by seasonal hydroclimatic conditions, particularly (a) larger precipitation inputs and soil moisture availability during the spring season and (b) higher evaporative demand during summer and early fall. Across all species, precipitation inputs during the spring growing season promoted relatively small δ¹⁸O offsets (±1 ‰) between throughfall, soils, roots, and stems, suggesting a strong connectivity between meteoric inputs, soil moisture availability, and RWU. During summer and early fall, evapoconcentration resulted in higher δ¹⁸O offsets between throughfall, roots, and stems. In winter, deciduous species (boxelder maple and elderberry) showed larger δ¹⁸O offsets between throughfall, roots, and stems. This indicates the existence of a 'legacy-type effect' (i.e. older water stored) from the summer season. Longitudinal analysis also revealed δ¹⁸O and d-excess heterogeneity along the root system. These findings highlight the potential of centrifugation as a reliable method for extracting mobile water in stems and roots in a sub-tropical environment, while also emphasising the natural complexity of isotopic heterogeneity within plant compartments.
Shrestha et al. (Fri,) studied this question.