Controls on the emission of CH4 and CO2 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 CH4 (dark CO2) fluxes were -0.51 (7.99), -0.14 (10.3), 0.15 (8.68), and 2.83 (8.22) mg/m2/d (g/m2/d) from the OBS feather moss, lichen, sphagnum moss, and fen sites. Mean seasonal CH4 (dark CO2) fluxes were -1.58 (9.16), 2.03 (7.62), 1144, and 153 mg/m2/d (g/m2/d) from the BP lichen, sphagnum moss, mire, and pond sites. Rates of CH4 oxidation typical of uplands were observed at all sites except for the BP mire and pond. CH4 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 CH4 source. Low soil temperatures inhibit high CH4 flux rates despite inundation. The potential for the conversion of upland sites from net sinks to sources of CH4 by minor changes in the water table was observed. Prediction of CH4 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 CO2 fluxes, by contrast, appear more uniform across similar plant community types. ¿ 1997 American Geophysical Union