Methane efflux from boreal wetlands: Theory and testing of the ecosystem model Ecosys with chamber and tower flux measurements

A robust model of CH 4 emission from terrestrial ecosystems should be capable of simulating the temporal and spatial variability that characterizes field measurements. Such a model should couple a biologically based treatment of microbial CH 4 transformations with a physically based treatment of heat, solute, and gas transfer vertically and laterally through soils. These processes are coupled in the ecosystem model Ecosys, which was tested against CH 4 effluxes measured with surface chambers and a flux tower at a beaver pond in the BOREAS Northern Study Area. Spatial and temporal variation of CH 4 effluxes in the model encompassed that measured by surface chambers and the flux tower. Both modeled and measured CH 4 effluxes rose from <0.05 μmol m −2 s −1 and <0.01 g C m −2 d −1 at sites above the pond to >1.0 μmol m −2 s −1 and >0.5 g C m −2 d −1 at the pond margin. Larger effluxes occurred in the model when warming pond sediments generated episodic bubbling events. Annual CH 4 effluxes in the model rose from <1 g C m −2 at sites above the pond to 76 g C m −2 at the pond margin. Annual totals included several brief but rapid efflux events during thawing and warming of soil and pond sediments that are frequently missed by surface measurements. Annual CH 4 effluxes predicted after 100 years under an IS92a‐driven climate change scenario rose by ∼20% from the pond, but changed little from the surrounding landscape, indicating topographic variation in response of CH 4 effluxes to climate change.