Using deuterium and oxygen-18 stable isotopes to understand mechanisms of stemflow generation as a function of tree species and climate

Author(s): Siegle-Gaither, M.; Siegert, C.; Keim, R.

Stemflow (SF) is a type of rain partitioning from the forest canopy that redirects intercepted water down tree trunks. Through this mechanism, SF leaches nutrients from the canopy and bark to deliver highly enriched water to the base of the tree. Few studies have examined the species-specific effects of bark structure and storm meteorological conditions on SF generation by means of stable hydrogen ( δD) and oxygen δ18O) isotopic tracers. This study explores these relationships in an oak-hickory stand and a pine plantation in central Mississippi. The species selected for this project have unique bark characteristics and variable effects on rainfall partitioning centered on their geographic distribution. Specifically, SF volume and isotopic composition are measured over a one-year period. The objectives of this study are (i) to determine origins and pathways of stemflow water using stable water isotopes, (ii) to identify differences in stemflow generation mechanisms between tree species, and (iii) to identify differences in stemflow generation mechanisms between storm events.

Stemflow collars were installed on seven species of trees with three trees per species. Water samples were collected within 12-24 hours after individual storm events. Tree characteristics such as species, height, and bark thickness were measured. Laser ablation spectroscopy was used to analyze δD ad δ18O in the water samples collected. Preliminary results show that bark thickness was greatest in Pinus taeda (1.74 ±0.09cm), followed by Quercus alba (1.56 ±0.08cm), Q. stellata (1.19 ±0.13cm), Q. shumardii (0.95 ±0.08cm), Q. pagoda (0.95 ±0.05cm), Carya glabra (0.83 ±0.09cm), and C. ovata (0.56 ±0.10cm) (n=24 for all species except P. taeda where n=40). Additional preliminary results suggest that the isotopic composition of stemflow is distinct from that of throughfall and bulk precipitation. A better understanding of isotopic composition (δD and δ18O) and stemflow generation mechanisms will allow for more accurate hydrological and biogeochemical models to be established.

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