Methane Emissions from Tree and Soils along a Wetland to Upland Transect

Forests are major sources of terrestrial CH4 and CO2 fluxes but not all surfaces within forests have been measured and accounted for. Stem respiration is a well-known source of CO2, but more recently tree stems have been shown to be sources of CH4 in wetlands and upland habitats. A study transect was established along a natural moisture gradient, with one end anchored in a forested wetland, the other in an upland forest and a transitional zone at the midpoint. Stem and soil fluxes of CH4 and CO2 were measured using static chambers during the 2013 and 2014 growing seasons, from May to October. Mean stem CH4 emissions were 68.8 ± 13.0 µg m-2 h-1 (mean ± standard error), 180.7 ± 55.2 µg m-2 h-1 and 567.9 ± 174.5 µg m-2 h-1 for the upland, transitional and wetland habitats, respectively. Mean soil methane fluxes in the upland, transitional and wetland were -64.8 ± 6.2 µg m-2 h-1, 7.4 ± 25.0 µg m-2 h-1 and 190.0 ± 123.0 µg m-2 h-1, respectively. Measureable CH4 fluxes from tree stems were not always observed, but every individual tree in our experiment released measureable CH4 flux at some point during the study period. These results indicate that tree stems represent overlooked sources of CH4 in forested habitats and warrant investigation to further refine CH4 budgets and inventories.