Aboveground Carbon Storage and Net Primary Production in Human Impacted Forests Under Current and Future Climate Scenarios

Forested ecosystems contain the largest terrestrial carbon pools and fluxes on earth. Thus, forest ecosystem disturbances can result in large perturbations in global carbon cycling and subsequently climate. The objective of my dissertation was to investigate the role of forest burning and thinning, changing climate, and changing species compositions on carbon cycling in a subset of forests in the eastern United States. I examined processes that influenced net primary production (NPP) of forested ecosystems at different scales: First, I examined the sensitivity of budburst phenology on simulations of NPP and, based on the Harvard Forest phenology database, I optimized a subroutine, which included a chilling factor, for the prediction of budburst dates. Budburst phenology was a sensitive variable in PnET model simulations but leaf-off date was not. The phenology subroutine, when applied to southern Ohio forests, exhibited geographic specificity, highlighting the importance of local phenological data. Second, I examined the effects of mechanical thinning, prescribed burning, and multiple environmental factors on specific leaf weight (SLW) and leaf nitrogen content (N_mass) of seven common tree species. Increases in both leaf traits were positively associated with increases in potential NPP. Thinning increased SLW at the lower canopy by 22% and resulted in an 8% increase of modeled NPP. The effects of burning were not significant. Variations of both leaf traits were primarily explained by species differences. Third, I assessed the impacts of thinning and burning on aboveground carbon stocks and NPP using both field measurements and PnET model simulations in southern Ohio. Only thinning affected NPP (> -30%); however, the thinning effect on NPP was transient (1-2 years) according to both field and model results. Lastly, at a larger spatio-temporal scale, I used the PnET-II model to simulate the NPP consequences of potential tree species redistribution and future climate in four selected focal areas (40,000 km² each). The effects of potential species redistribution predicted in 2100 had moderate effects (-12% to 8%) on NPP compared to the impacts of future climatic change (-60% to 25%). As increasing temperatures negatively impact NPP, the reduction in forest carbon uptake capacity could accelerate changing climate.