Temperate Forest Response to Multiple Ecosystem Change Drivers in the Eastern United States: Implications for Forest Carbon and Water Balance.

Temperate forests of the eastern United States are a globally important sink for atmospheric CO2 and provide other vital ecosystem services, including regulation of water resources. In recent decades, these forests have been subjected to environmental changes including elevated atmospheric CO2 concentrations, changes in acidic air pollution, and changing temperature and precipitation regimes. How these concurrent changes have impacted forest carbon and water balance remain unclear. The research in this dissertation examines how changes in climate and atmospheric deposition of nitrogen (N) and sulfur (S) have individually and interactively affected forest productivity and water use efficiency (WUE) in montane forests of the eastern United States, against a backdrop of monotonically increasing atmospheric CO2. In Chapter 2, I utilize remote sensing data to examine when and where temperate broadleaf forests are most sensitive to water stress by examining effects of topography, soil properties, and canopy height on forest climate sensitivity in Shenandoah National Park. In Chapter 3, I examine whether experimental N and S additions to a forested watershed alter tree growth and sensitivity to climate in a paired watershed experiment at the Fernow Experimental Forest in West Virginia. Chapter 4 examines how tree growth and intrinsic water use efficiency (iWUE) responded to soil nutrient manipulations in three paired watershed experiments in the northeastern United States: Hubbard Brook Experimental Forest, where soil calcium (Ca) was restored to preindustrial levels, and the Bear Brook Watersheds and Fernow Experimental Forest, where soils were acidified via experimental N and S additions for 25+ years. In Chapter 5, I assess trends in, and drivers of, tree iWUE derived from tree ring data in 13 evergreen and broadleaf deciduous species along climate and atmospheric deposition gradients in the eastern United States. Combined, this work may improve our understanding of how recent environmental changes have affected tree growth and physiology in the eastern United States, and inform efforts to model and manage forest ecosystems in the future.