Contrasting the interannual variability in net ecosystem exchange of carbon dioxide in a northern peatland with the variability observed in northern forests

Peatlands of the boreal forest comprise only 17% of the land area, but contain up to 60% of the soil carbon (C) pool. Despite limited plant growth in these often cool and nutrient-poor wetlands, slow decomposition rates have resulted in a small but persistent net uptake of C over thousands of years. However, it has been suggested that climate warming may enhance respiration rates and cause peatlands to become significant annual sources of C. This hypothesis is based largely on observations in other ecosystem types and on laboratory or field experiments in peatlands lasting only a growing season or two. In this study we examine the sensitivity of C exchange processes to seasonal and annual variations in weather using five years of CO₂ flux measurements obtained using the eddy covariance technique at Mer Bleue, a 2800 ha low-shrub bog near Ottawa, Ontario. To emphasize the unique features of C exchange in this bog ecosystem, these results are contrasted with those from published studies of interannual C exchange at boreal forest sites.

During the five-year measurement period at Mer Bleue, seasonal temperatures varied by –4 to +2 degrees C from the long-term average. The annual net ecosystem exchange (NEE) was never a net loss of C. It varied from being either C neutral or a small C sink (up to 90 g C m^-2 y^-1), but the sink strength was dependant on the method used to gap-fill missing measurements. The responses of seasonal and annual NEE, ecosystem respiration (ER), and gross ecosystem production (GEP) to variations in weather were far more conservative in the Mer Bleue bog than in boreal aspen, black spruce, and jack pine forests. Linkages between production, respiration, and hydrology are also explored using water use efficiency calculations and ratios of ER, GEP, and NEE.