A comparison of methane flux in a boreal landscape between a dry and a wet year.

We used field measurements of methane (CH₄) flux from upland and wetland soils in the Northern Study Area (NSA) of BOREAS (BOReal Ecosystem-Atmosphere Study), near Thompson, Manitoba, during the summers of 1994 and 1996 to estimate the overall CH₄ emission from a 1350 km² landscape. June-September 1994 and 1996 were both drier and warmer than normal, but summer 1996 received 68 mm more precipitation than 1994, a 40% increase, and had a mean daily air temperature 0.6°C warmer than 1994. Upland soils consumed CH₄ at rates from 0 to 1.0 mg m^-2 d^-1, with small spatial and temporal variations between years, and a weak dependence on soil temperature. In contrast, wetlands emitted CH₄ at seasonal average rates ranging from 10 to 350 mg CH₄ m^-2 d^-1, with high spatial and temporal variability, and increased an average of 60% during the wetter and warmer 1996. We used Lands at imagery, supervised classification, and ground truthing to scale point CH₄ fluxes (<1 m²) to the landscape (>1000 km²). We performed a sensitivity analysis for error terms in both areal coverage and CH₄ flux, showing that the small areas of high CH₄ emission (e.g., small ponds, graminoid fens, and permafrost collapse margins) contribute the largest uncertainty in both flux measurements and mapping. Although wetlands cover less than 30% of the landscape, areally extrapolated CH₄ flux for the NSA increased by 61% from 10 to 16 mg CH₄ m^-2 d^-1 between years, entirely attributed to the increase in wetland CH₄ emission. We conclude that CH₄ fluxes will tend to be underestimated in areas where much of the landscape is covered by wetlands. This is due to the large spatial and temporal variability encountered in chamber-based measurements of wetland CH₄ fluxes, strong sensitivity of wetland CH₄ emission to small changes in climate, and because most remote sensing images do not adequately identify small areas of high CH₄ flux. [ABSTRACT FROM AUTHOR]