Uncertainties in modelling CH₄ emissions from northern wetlands in glacial climates: the role of vegetation parameters

Abstract. Marine Isotope Stage 3 (MIS 3) interstadials are marked by a sharp increase in the atmospheric methane (CH₄) concentration, as recorded in ice cores. Wetlands are assumed to be the major source of this CH₄, although several other hypotheses have been advanced. Modelling of CH₄ emissions is crucial to quantify CH₄ sources for past climates.

Vegetation effects are generally highly generalized in modelling past and present-day CH₄ fluxes, but should not be neglected. Plants strongly affect the soil-atmosphere exchange of CH₄ and the net primary production of the vegetation supplies organic matter as substrate for methanogens. For modelling past CH₄ fluxes from northern wetlands, assumptions on vegetation are highly relevant since paleobotanical data indicate large differences in Last Glacial (LG) wetland vegetation composition as compared to modern wetland vegetation. Besides more cold-adapted vegetation, Sphagnum mosses appear to be much less dominant during large parts of the LG than at present, which particularly affects CH₄ oxidation and transport. To evaluate the effect of vegetation parameters, we used the PEATLAND-VU wetland CO₂/CH₄ model to simulate emissions from wetlands in continental Europe during LG and modern climates.

We tested the effect of parameters influencing oxidation during plant transport (f_ox), vegetation net primary production (NPP, parameter symbol P_max), plant transport rate (V_transp), maximum rooting depth (Z_root) and root exudation rate (f_ex). Our model results show that modelled CH₄ fluxes are sensitive to f_ox and Z_root in particular. The effects of P_max, V_transp and f_ex are of lesser relevance. Interactions with water table modelling are significant for V_transp.

We conducted experiments with different wetland vegetation types for Marine Isotope Stage 3 (MIS 3) stadial and interstadial climates and the present-day climate, by coupling PEATLAND-VU to high resolution climate model simulations for Europe. Experiments assuming dominance of one vegetation type (Sphagnum vs. Carex vs. Shrubs) show that Carex-dominated vegetation can increase CH₄ emissions by 50% to 78% over Sphagnum-dominated vegetation depending on the modelled climate, while for shrubs this increase ranges from 42% to 72%. Consequently, during the LG northern wetlands may have had CH₄ emissions similar to their present-day counterparts, despite a colder climate. Changes in dominant wetland vegetation, therefore, may drive changes in wetland CH₄ fluxes, in the past as well as in the future.