Water and carbon uptake and soil nitrogen cycling in a northern hardwood forest under a changing climate
MetadataShow full item record
Projected changes in climate for the northeastern U.S. over the next century include increased air temperatures and reduced snowpack, leading to increased frequency of soil freeze/thaw cycles (FTC) in winter. Forests of the northeastern U.S. currently offset up to 35% of regional carbon (C) emissions and water uptake by trees in these forests constitutes the majority of evapotranspiration. In addition, nitrogen (N) is an essential element and often limiting nutrient for net primary production in temperate ecosystems, but recent declines in atmospheric deposition of N and changes in climate have led to concerns about N supply not meeting demands by temperate forest trees in the future. While much is known about the effects of climate change in winter and the growing season independently on water, C, and N cycling in temperate forests, little is known about the combined effects on these processes. In Chapter 2, I utilize a soil temperature manipulation experiment and demonstrate that rates of transpiration and leaf-level C uptake by Acer rubrum increase with rising growing season soil temperatures, but increased rates of C uptake are offset by increased frequency of FTCs, while increased transpiration rates are maintained. In Chapter 3, I demonstrate that net N mineralization and foliar N in trees are elevated with soil warming and not affected by FTCs. In Chapter 4, I show that trees access shallow water (< 30 cm depth) in the early growing season. Further, trees that experience FTCs take up equal amounts of water from all soil depths, while those without FTCs switch to a deeper source (> 90 cm depth) that has greater water potential in the late growing season. In Chapter 5, my synthesis of the published literature demonstrates that the majority of studies utilizing stable isotopes of water to determine water sources for vegetation occurred using no experimental manipulation, in forests and grasslands, and in arid climates. Overall, results of my dissertation demonstrate that biogeochemical cycling of C, N, and water are affected by projected changes in climate across seasons in ways that would not have been apparent from examining only one season alone.
RightsAttribution 4.0 International