LOW BOREAL WETLANDS AS A SOURCE OF ATMOSPHERIC METHANE

Boreal wetlands are thought to be a large source of atmosphereic methane, but this idea is based on very few measurements. Thus a regional survey in the low boreal forest region of central Ontario, Canada, consisting of 24 sites over 12 wetlands and 3 beaver ponds was conducted to determine the temporal and spatial trends in emissions and the net annual methane (CH4) flux. Conifer swamps represented nearly 50% of the wetland coverage, but emit a small amount of CH4 (seasonal means < 8 mg m-2 d-1). The significant emitters of CH4, in order from highest to lowest seasonal means, were beaver ponds (30 - 90 mg m-2 d-1), thicket swamps (0.1 - 88 mg m-2 d-1), and bogs (6 - 21 mg m-2 d-1). Mixed swamps, marshes, and fens emitted very little CH4 (< 3 mg m-2 d-1). Moisture saturation was the key determinant of high emissions, and when satisfied, differences in emissions could be explained by peat and sediment temperatures. On the basis of the areal extent of wetlands from peatland inventories we calculate that the low boreal region of Canada contributes approximately 0.15 Tg CH4 yr-1 to the atmosphere. This is an order of magnitude lower than the flux would be using the estimate of Aselmann and Crutzen (1989) for the same boreal region.