Controls on CH₄ and CO₂ emissions along two moisture gradients in the Canadian boreal zone

Controls on the emission of CH 4 and CO 2 along moisture gradients in the northern study area of the Boreal Ecosystem‐Atmosphere Study (BOREAS), west of Thompson, Manitoba, were examined in the summer of 1994. A transect stretching from three forested upland sites to an open graminoid fen contained within a black spruce (OBS) palsa was compared with a similar transect from upland lichen and Sphagnum moss sites to wetland mire and pond sites adjacent to an active beaver pond (BP). Mean seasonal CH 4 (dark CO 2 ) fluxes were −0.51 (7.99), −0.14 (10.3), 0.15 (8.68), and 2.83 (8.22) mg/m 2 /d (g/m 2 /d) from the OBS feather moss, lichen, sphagnum moss, and fen sites. Mean seasonal CH 4 (dark CO 2 ) fluxes were −1.58 (9.16), 2.03 (7.62), 1144, and 153 mg/m 2 /d (g/m 2 /d) from the BP lichen, sphagnum moss, mire, and pond sites. Rates of CH 4 oxidation typical of uplands were observed at all sites except for the BP mire and pond. CH 4 efflux was observed in all sites. Rates varied by 4 orders of magnitude along the transition from the driest uplands to moist uplands to inundated wetlands. A high water table appears to be necessary for a strong CH 4 source. Low soil temperatures inhibit high CH 4 flux rates despite inundation. The potential for the conversion of upland sites from net sinks to sources of CH 4 by minor changes in the water table was observed. Prediction of CH 4 fluxes by plant community analysis appears to require knowledge of the thermal and hydrologic regime of particular sites over the course of the summer period. Dark CO 2 fluxes, by contrast, appear more uniform across similar plant community types.