Using Deuterium and Oxygen-18 Isotopes to Understand Stemflow Generation Mechanisms

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

Stemflow is a nutrient-enriched type of rain partitioning that redirects intercepted water from the forest canopy down tree trunks, creating biogeochemical hotspots at tree bases. Few studies have examined species-specific effects of bark structure and storm meteorological conditions on stemflow generation via stable hydrogen (δD) and oxygen (δ18O) isotopic tracers. This study explores these relationships in an oak-hickory stand in central Mississippi. Species were chosen based on their unique bark characteristics and variable effects on rain partitioning. Stemflow volume and isotopic composition were measured over one year with objectives to determine (i) origins and pathways of stemflow water using stable isotopes, (ii) differences in stemflow generation mechanisms between tree species, and (iii) differences in stemflow generation mechanisms between storm events.

Stemflow collars were installed on 18 trees of six species. Water samples were collected within 24 hours of individual storm events. Laser ablation spectroscopy was used to analyze δD and δ18O in collected water samples. Results show that isotopic composition (δ2H) of stemflow (-20.08±10.18‰) is distinct from that of throughfall (-21.25±9.09‰) and precipitation (-15.49±10.03‰). The difference in isotopic composition of stemflow relative to throughfall and precipitation signifies evaporation, suggesting that this pathway is composed of both pre-event and event water. Bark thickness measurements were greatest in Quercus alba, followed by Q. stellata, Q. shumardii, Q. pagoda, Carya glabra, and C. ovata. Stemflow volumes per basal area followed a similar trend. Greater bark thicknesses correlate with lower stemflow volumes per basal area, advocating that interspecific bark characteristics play an intricate role in stemflow generation. A bark-wetting experiment showed bark water storage capacity (BWSC) per tree stem to be greatest in red oaks (Q. shumardii: 87.4±21.5 L and Q. pagoda: 85.4±21.5 L), then white oaks (Q. alba: 57.2±41.7 L and Q. stellata: 45.5±20.0 L), and hickories (C. ovata: 26.7±24.9 L and C. glabra: 18.6±6.5 L), respectively. Oak species with thick, continuous bark surfaces generate lower stemflow volumes and have higher BWSC; whereas hickory species tend to have thinner, irregular bark structures that lead to higher stemflow volumes and lower BWSC. Specifies-specific BWSC is therefore not only a determining factor for stemflow generation during an event, but also for how much pre-event water is contributing to this flux, both in terms of volume and chemistry. Thus these results show how stemflow significantly impacts forest hydrology and microclimate based on interspecific differences in bark thickness.

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