Mid-latitude, deciduous forests have become increasingly important global carbon (C) and nitrogen (N) sinks as human-related emissions from fuels, biomass burning, fertilizer use, and agricultural operations grow and tropical forests are harvested. Many studies have examined C and N storage within forested ecosystems, yet few have characterized the ecohydrological dynamics of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) derived from canopy sources. No known study has holistically examined the intra-storm variability of all canopy-derived DOC and DON fluxes for mid-latitude deciduous forests, although storm events being a critical period for the mobilization and transport of DOC and DON. Data quantifying the overall fate and transport of DOC and DON from canopy sources with respect to meteorological conditions and catchment characteristics are scant. To fill these data gaps, this doctoral dissertation research project will sample throughfall, stemflow, and litter leachate for two co-dominant tree species of contrasting canopy morphologies at the intra-storm scale and characterize the influence of meteorological conditions and catchment conditions on DOC and DON. Samples also will be analyzed for base cations for use in end-member mixing analyses (EMMA). EMMA will be used to evaluate hydrologic relationships between canopy-derived fluxes, variable source areas (VSA), and streamflow. Previous EMMAs have evaluated the influence of throughfall and litter leachate on streamflow chemistry and VSAs but neglected stemflow. The stemflow of many broadleaved deciduous trees gains preferential access to subsurface lateral flow. Hence, it is likely that the stemflow of certain species may contribute to streamflow and/or VSAs. This study will be the first to evaluate stemflow as an end-member in end-member mixing analysis and the first to monitor all canopy-derived DOC and DON fluxes for a single catchment at the intrastorm scale.
Results will be used to review bulk sampling practices and assumptions made by current watershed models that generally disregard the influence of intrastorm variability and stemflow on the hydrology and water quality of forested catchments. If intrastorm variability or stemflow significantly contribute to the chemistry or hydrology of subsurface flux, VSAs, and/or streamflow for forested catchments, then widely-used models for civil engineering and environmental research and design that employ these assumptions will need to be corrected. Current bulk sampling practices also may need to be corrected if intrastorm variability significantly influences biogeochemical processes in forested catchments. Investigation of the intrasystem cycling dynamics of DOC and DON within mid-latitude deciduous forests during discrete storm events nonetheless will inform relevant theory in physical geography and hydrology that seeks to better understand the cycling of water and solutes in forests. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.
A better understanding of the cycling of dissolved organic carbon and nitrogen is necessary to gauge the effects of humankind on the functional ecology of forested ecosystems. This funded work collected data that has elucidated the complicated manner in which carbon and nitrogen are transported through forests. Specifically, the intellectual merit of this work has been to more fully understand the transport of dissolved carbon and nitrogen (among other solutes) within discrete rain events. Our sequential data has allowed us to begin to disentangle the complex mobilization of solutes as a forest canopy wets-up and becomes saturated. This work will help inform scientists interested in examining the flux of solutes from forests and its impact on water quality and pollutant cycling. The articles published from this work to date are readily available in international journals. This project also benefited a young scholar, enhancing the quality of his dissertation and his publication record. The training provided to the student in this project has helped prepare him for a career in academia so that he, in turn, can further educate generations of future students. Thus, the broader impact of the project is threefold: (1) in the near term, it has improved the quality of the student’s dissertation and provided an invaluable research experience; (2) in the longer term, the student’s knowledge gleaned from this experience will positively impact future students; and (3) the scientific findings will better inform theories of solute transport through forests which ultimately could be used to improve water quality and minimize the deleterious effects of pollutants.
