Methane exchange in a boreal forest estimated by gradient method

Forests are generally considered to be net sinks of atmospheric methane (CH₄) because of oxidation by methanotrophic bacteria in well-aerated forests soils. However, emissions from wet forest soils, and sometimes canopy fluxes, are often neglected when quantifying the CH₄ budget of a forest. We used a modified Bowen ratio method and combined eddy covariance and gradient methods to estimate net CH₄ exchange at a boreal forest site in central Sweden. Results indicate that the site is a net source of CH₄. This is in contrast to soil, branch and leaf chamber measurements of uptake of CH₄. Wetter soils within the footprint of the canopy are thought to be responsible for the discrepancy. We found no evidence for canopy emissions per se. However, the diel pattern of the CH₄ exchange with minimum emissions at daytime correlated well with gross primary production, which supports an uptake in the canopy. More distant source areas could also contribute to the diel pattern; their contribution might be greater at night during stable boundary layer conditions.