Methane exchange in a boreal forest: : the role of soils vegetation and forest management

Popular Abstract in English Forests have a positive impact on the level of greenhouse gases in the atmosphere because they absorb carbon dioxide through photosynthesis and store carbon in the soil. Moreover, many forests function as a sink for the greenhouse gas methane (CH4). The uptake of CH4 takes place in well-aerated soils through oxidation by methanotrophic bacteria. However, there is a net production of CH4 in anaerobic soils, such as submerged soils, by microorganisms, archaeans. CH4 dissolved in ground water can be taken up by trees and discharged through stems or foliage. CH4 can also be formed in green plants and emitted, which mainly occurs when plants are exposed to high UV radiation, high temperature, or if the foliage is damaged. The work in this thesis focus on the CH4 exchange in a boreal forest, at the research station Norunda, in central Sweden. The aim is to quantify the net CH4 exchange at the site and investigate what factors constitute the main control of the exchange in soil and vegetation. The impact of clear-cutting, stump harvesting and thinning on the CH4 exchange was also studied. Chamber measurements in undisturbed and thinned forest show that the soil is a sink of CH4. The uptake generally benefits from a large groundwater depth, low soil moisture and high temperatures. Chamber measurements on clear-cut and stump harvested plots show net emissions, which is associated with increased groundwater levels and increased soil moisture as a result of harvesting. Branch chamber measurements on shoots of spruce, pine, birch and rowan also show an uptake of CH4, which is in contrast to previous studies, mostly done in laboratories, which found net emissions from plants. Uptake by vegetation correlated with photosynthetically active radiation, which indicates a coupling to photosynthesis. Unlike our measurements on soil and vegetation, results from our micrometeorological measurements above the canopy showed net emissions from the forests to the atmosphere. These measurements represented a larger area than the chambers and differences could therefore be explained by large CH4 production in small wet areas, which exceeds the uptake in the rest of the soil. However, scaling of soil chamber measurements based on elevation data, indicate that the soil in the main study area was a sink of CH4 regardless of some minor areas with CH4 production. CH4 emissions from vegetation or transport of CH4 from surrounding source areas are other possible explanations for this discrepancy