| Summary: | Forests of the northeastern U.S. help maintain water and air quality by reducing losses of nitrogen (N) into nearby waterways and removing carbon dioxide (CO2) from the atmosphere. However, carbon (C) and N retention in northeastern forests may decrease in response to projected changes in climate, including reductions in winter snowpack and increased soil freezing. Together, these climatic changes may damage tree roots and alter soil processes. Few studies have investigated the extent to which snowpack and soil frost drive C and N fluxes during spring snowmelt, a biogeochemically important period. Similarly, little is known about how changes in winter climate affect above- and belowground CO2 fluxes to the atmosphere. My dissertation combines laboratory and field experiments to quantify the effects of reduced snowpack and increased soil freezing on C and N cycling in northeastern forests. I conducted a laboratory experiment to study the effects of soil freezing on C and N losses during snowmelt. Organic horizon soils collected from mixed Acer saccharum-Fagus grandifolia and Picea rubens-Abies balsamea forests were incubated in severe, mild, and no soil frost conditions prior to snowmelt. Results show that losses of N in leachate, as well as total C and N fluxes (gases + leachate), were reduced following severe soil frost, indicating the response to winter climate depends on both the presence and severity of soil frost. I also implemented a snow removal experiment in a mixed Quercus rubra-Acer rubrum forest at Harvard Forest, MA to quantify the effects of depth and duration of snowpack and soil frost on CO2 losses from tree stems and soils. This study provides evidence that reduced snowpack and increased soil freezing may increase annual soil CO2 efflux, but have no significant effect on tree stem CO2 efflux. Taken together, results from my dissertation highlight the importance of winter climate as a driver of C and N fluxes in northeastern forests and suggest that while soil frost reduces C and N losses during snowmelt, annual losses of CO2 may increase Future studies investigating controls on C and N cycling in northeastern forests should account for changes in winter climate.
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