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The influence of forest management and spatio-temporal variation on greenhouse gas fluxes from riparian soils along headwater streams Silverthorn, Teresa K.

Abstract

Riparian zones of headwater streams have valuable ecosystem functions, and are particularly vulnerable to forest harvest in coastal British Columbia. Studies of greenhouse gas (GHG; CO₂, CH₄, N₂O) fluxes from these unique ecosystems, with fluctuating water tables, and high soil organic matter, remain limited. My first objective was to quantify the effects of forestry practices on GHG emissions from riparian forest soils, and determine the dominant driver(s) of emissions over the growing season. I compared sites that were clear-cut without a riparian buffer (“no buffer”) or with a buffer (“buffer”) to relatively undisturbed riparian zones (“reference”). I hypothesized that either a rise in the water table, increased soil temperatures, or disturbance of roots and microbes following forest harvest would have the greatest influence on GHG fluxes. My second objective was to examine the effects of temporal and spatial variation on annual GHG fluxes from relatively undisturbed riparian soils. I hypothesized that groundwater discharge (DIS) areas in the riparian zone would have high soil moisture and nutrients, resulting in greater anaerobically produced CH₄ and N₂O emissions compared to outside of these areas (ND). I further hypothesized that GHG fluxes would peak in the warmest and wettest months. I measured gas fluxes in situ alongside headwater streams using static chambers and gas chromatography. I found that N₂O emissions were 1.71 and 2.12 times lower at buffer and no buffer sites, respectively, than reference sites. Carbon dioxide fluxes were 1.16 and 1.09 times higher at buffer and no buffer sites, respectively, compared to reference sites. Methane fluxes were 1.34 and 2.89 times higher at buffer and no buffer sites, respectively, compared to reference sites. Additionally, CH₄ uptake during the growing season was 2.18 times higher at ND areas than DIS areas. Soil temperature, soil moisture, and depth to the groundwater were significant predictors of GHG emissions, and emission rates were highest in the spring and summer months. The results of my research provide information on the magnitude and drivers of GHG fluxes in riparian zones to help inform GHG budgets and forest management.

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