Simulating the Carbon Cycling of Northern Peat lands Using a Land Surface Scheme Coupled to a Wetland Carbon Model (CLASS3W-MWM)

Northern peatlands store approximately one-third of the terrestrial soil carbon (C), although they cover only 3% of the global land mass. Northern peatlands can be subdivided into bogs and fens based on their hydrology and biogeochemistry. Peat/and hydrology and biogeochemistry are tightly coupled to climate and, therefore, may be very sensitive to climate variability and change. To address the fate of the large peatland soil C storage under a future changed climate, a peatland C model, the McGill Wetland Model (MWM), was coupled to a land surface climate model (the wetland version of the Canadian Land Surface Scheme, CL4SS3W), referred as CLASS3W-MWM. We evaluated the CL4SS3W-MWM for a bog (Mer Bleue, located at 45.41 degrees N, 75.48 degrees W, in eastern Canada) and a poor fen (Degero Stormyr, located at 64 degrees 11'N, 19 degrees 33'E, in northern Sweden). CLASS3W-MWM captured the magnitude and direction of the present day C cycling very well for both bogs and fens. Moreover, the seasonal and interannual variability were reproduced reasonably well. Root mean square errors (RMSE) were <0.65 and the degree of agreements (d*) were >0.8 for the components of net ecosystem production (NEP) for both the Mer Bleue bog and the Degero Stormyr fen. The performance of the coupled model for both bog and fen is similar to that of the stand-alone MWM driven by observed weather rather than simulated surface and soil climate. This modelling study suggests that northern peatlands are hydrologically and thermally conservative ecosystems. It was also shown that C cycling for bogs and fens was more sensitive to changes in air temperature than precipitation. Changes in temperature within the Intergovernmental Panel on Climate Change (IPCC) projected range switch the peatlands from a present-day C sink to a source, but projected changes in precipitation still maintain the peatlands as a C sink, although to a somewhat lesser degree. Increase in atmospheric CO2 concentration enhances C sequestration for both bogs and fens. Our sensitivity analysis suggests that northern peatlands respond to changes in temperature, precipitation and doubled CO2 concentration in a highly non-linear way. The sensitivity of C cycling in northern peatlands with respect to changes in air temperature, precipitation and the concentration of atmospheric CO2 together is not a simple addition or subtraction of the sensitivity of the individual changes. Therefore, the sensitivity of a combination of changes in temperature, precipitation and doubled CO2 concentration is very different from the sensitivity of peatlands to each environmental variable on their own. Our sensitivity analysis suggests that fens have a narrower tolerance to climate changes than bogs.