Micrometeorological measurements of methane and nitrous oxide exchange above a boreal aspen forest

This paper presents tower-based measurements of methane (CH₄) and nitrous oxide (N₂O) exchange between a boreal aspen stand and the atmosphere. Boreal ecosystems are a priority trace gas research area, and the work was conducted as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Methane and nitrous oxide fluxes were measured continuously between April 16 and September 16, 1994, in the Prince Albert National Park, Saskatchewan. The fluxes were determined using a high-resolution tunable diode laser Trace Gas Analysis System (TGAS) together with micrometeorological techniques. Both the CH₄ and the N₂O fluxes were small and required long averaging times to be resolved. Over the full experiment, small emissions of both CH₄ and N₂O were measured above the aspen stand. The mean flux of N₂O was 1.4 ± 0.7 ng m^-2 s^-1 or 1.9-2.5 ng m^-2 s^-1 when an enhancement factor to compensate for the breakdown of similarity theory just above forest canopies is included. Low rates of nitrification and denitrification throughout the growing season may explain the consistently small N₂O fluxes. The CH₄ flux averaged 15.7 ± 2.8 ng m^-2 s^-1, or 21-28 ng m^-2 s^-1, including the similarity theory enhancement factor. The CH₄ emissions were highest between late July and mid-September, and there was a strong correlation between the CH₄ flux and the soil temperature. Whereas CH₄ emission was measured from the above-canopy footprint, uptake was recorded close to the tower base. Overall, it appears that CH₄ emissions from anoxic wet patches located throughout the above-canopy footprint overwhelmed uptake from drier areas to produce a net emission of CH4 from the aspen site.