Quantifying Effects of Seasonal Inundation on Methane Fluxes from Forested Freshwater Wetlands


Developing effective strategies for reducing methane and other greenhouse gas emissions requires a quantitative understanding of their global sources and sinks. Decomposition of organic matter in wet soils is one of the largest sources of methane to the atmosphere, but it is a highly variable process that remains difficult to quantify because we lack a predictive understanding of how environmental factors control methane emissions in wetlands. Hydrology is one of the most important factors controlling methane production wetlands along with temperature and vegetation, however it is unclear how to relate aspects of a wetland’s hydrologic regime to the timing, magnitude, and spatial extent of its methane emissions. Furthermore, discrepancies between the magnitude of global methane emissions calculated using different techniques indicate that current methods for measuring the extent and dynamics of wetland areas in global models may not adequately represent processes controlling methane cycling in wetlands and other small water bodies. I studied the role of seasonal hydrologic variability on methane emissions from forested mineral soil wetlands to inform modeling techniques at different scales. In Chapter 1, I show the importance of inundation extent and duration as major drivers of wetland methane emissions, that methane fluxes have a non-linear relationship with water level, and that methane fluxes are higher when water levels are falling rather than rising. In Chapter 2, I demonstrate a new technique for calculating methane emissions using high resolution satellite data to quantify wetland inundation time series, and some limits of current technology for modeling surface water dynamics in forested wetlands. Chapter 3 presents and applies a modeling framework for quantifying hydrologic fluxes of methane in the context of common forms of wetland restoration. In combination, these studies establish how and why quantifying the hydrologic regime of seasonally inundated forested wetlands enables a more accurate estimation of methane emissions at multiple scales, that water level drawdown associated with the natural hydrologic regime of forested wetlands considerably reduces methane producing areas, and that improved methods for detecting and modeling surface water dynamics in low relief landscapes will improve our ability to quantify methane emissions.

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