Interspecific ecological and meteorological controls on forest canopy-derived hydrology and biogeochemistry in the southeastern United States

Background/Question/Methods

During storm events, as precipitation moves through the forest canopy it is transformed in both quantity and quality, thus delivering highly enriched water to the forest floor. Throughfall is spatially distributed beneath the forest canopy while stemflow is localized to the roots and soils in the immediate vicinity of individual tree trunks. Previous research has demonstrated that storm characteristics (e.g., intensity, duration, and magnitude), canopy structural parameters, and species composition have a significant control on canopy-derived nutrient fluxes. However, in the southeastern United States, contributions of the forest canopy to nutrient cycling have largely been overlooked, although the magnitude of tree biodiversity in the region separates these forests from their more-studied counterparts. Therefore, a field study was established in an oak-hickory forest in Mississippi to categorize the interspecific control on canopy-mediated nutrient cycling during precipitation events. Throughfall collectors and stemflow collars were located underneath the canopies of four oak (Shumard, Southern Red, Post, and White) and two hickory species (Shagbark and Pignut), with three replicates for each species. Hydrologic flux and nutrient samples were collected following individual precipitation events beginning in Fall 2014 and continue to present. Meteorological characteristics and precipitation chemistry were collected at a nearby open site.

Results/Conclusions

Preliminary results indicate interspecific differences were statistically significant for both throughfall and stemflow hydrologic partitioning (p=0.045 and p=0.021, respectively). Shumard oak, of the red oak family, partitioned an average of 78.7% of incident precipitation into throughfall and 1.3% into stemflow, the largest among all species. Mean concentrations of total nitrogen in throughfall were greatest in Shumard oak (1.44 mg/L) and post oak (1.39 mg/L) while stemflow concentrations were greatest in shagbark hickory (1.84 mg/L) and white oak (1.56 mg/L) and intermediate in Shumard oak (0.92 mg/L). These differences were not significant across species (throughfall: p=0.301, stemflow: p=0.459), but were significant across storm events (throughfall: p<0.001, stemflow: p=0.326). Results suggest that Shumard oak canopies facilitate the largest hydrologic fluxes in oak-hickory forests that correspond to intermediate biogeochemical fluxes of nitrogen, enabling this species to directly modify the substrata and its growing conditions. Additionally, the significant temporal differences in throughfall biogeochemistry highlight the importance of interspecific ecological traits in the phyllosphere to nutrient cycling in oak-hickory forests. Improved understanding of species-specific roles in nutrient cycles in highly diverse southern forests is critical to developing effective management strategies to mitigate shifts in species composition and ecosystem functions as regional climates change.