Biogeochemical Controls and Spatial Modeling of CO2 and CH4 Fluxes in a Complex Forest Landscape

• Main Author: Warner, Daniel L
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2018
• Subjects: Biogeochemistry; Ecology; Soil sciences
• ISBN: 0438595580; 9780438595583

Forest ecosystems store massive quantities of carbon in the form of living biomass, dead wood, and soils. Additionally, large quantities of carbon are exchanged between these carbon pools and the atmosphere in the form of greenhouse gases, CO2 and CH4. Small changes in the amounts of carbon storage and exchange may have major consequences for global CO 2 and CH4 dynamics. This dissertation consists of three original studies that investigate the spatiotemporal variability of CO2 and CH4 fluxes from multiple carbon pools within a temperate forested watershed in the Maryland Piedmont. Chamber techniques were employed for measuring fluxes, coupled with measurements of soil chemical and physical properties, tree species and coarse woody debris surveys, and GIS analyses. The first study focused on CO2 and CH4 fluxes across soils, coarse woody debris, and living tree stems within a forest plot, with the goal of identifying the relative contributions of these ecosystem components to plot scale fluxes. Soils acted as the dominant component of both CO2 and CH4 fluxes, and were the focus of subsequent chapters. This study also documented some of the first in situ observations of CH4 emissions from living tree stems and coarse woody debris in existing literature. Emissions varied with tree species and with the level of decay in coarse woody debris, suggesting potential implications of forest management strategies for ecosystem scale CO2 and CH 4 exchange. The second study expanded the scope of soil CO2 and CH 4 fluxes from a plot to the entire watershed, with the goal of identifying the relationships of fluxes to the biogeochemical characteristics of the soil. Sampling locations were distributed across hillslope gradients to include flat upland areas, steep transitional slopes, and valley bottom flats. Fluxes were measured across seasons for two years, along with an array of soil biogeochemical properties such as carbon content, sorption capacity, porewater chemistry, and soil structure. Although soils on transitional slopes had been documented to act as landscape hotspots of CO2 emission, this study found them to act as consistent hotspots of CH4 uptake as well. The well-drained, carbon and clay-rich soil environment supported high rates of CH4 uptake relative to other landscape positions across all seasons. The third study built upon the finding of topographic influence on spatial distributions of soil CO2 and CH4 fluxes, with the goal of developing a modeling framework for upscaling chamber measurements across complex landscapes. Digital terrain analysis and soil mapping techniques were employed to upscale point observations of fluxes to a high resolution continuous distribution across the landscape. This novel modeling approach provided reliable, transparent estimates of seasonal mean soil CO2 and CH4 fluxes across the topographically complex landscape. Unlike conventional upscaling techniques, this approach preserved the inherent spatial variability of fluxes across the watershed, which revealed shifting spatial distributions of fluxes in response to seasonal changes in temperature and precipitation. Findings suggested that steeply sloping areas may act as greater sources of CO 2 but also greater sinks of CH4 under warmer future climates, while valley bottom areas may have complex responses to changing precipitation patterns. This dissertation provides novel insights into CO2 and CH 4 dynamics within temperate forest ecosystems, the biogeochemical controls on these gas fluxes, and modeling techniques for making large-scale estimates of soil CO2 and CH4 fluxes in complex terrain. The findings will be of interest to climate scientists, land managers, and the biogeosciences community at large.