Vegetation heterogeneity and ditches create spatial variability in methane fluxes from peatlands drained for forestry

Drainage of peatlands for forestry starts a succession of ground vegetation in which mire species are gradually replaced by forest species. Some mire plant communities vanish quickly following the water-level drawdown; some may prevail longer in the moister patches of peatland. Drainage ditches, as a new kind of surface, introduce another component of spatial variation in drained peatlands. These variations were hypothesized to affect methane (CH₄) fluxes from drained peatlands. Methane fluxes from different plant communities and unvegetated surfaces, including ditches, were measured at the drained part of Lakkasuo mire, Central Finland. The fluxes were found to be related to peatland site type, plant community, water-table position and soil temperature. At nutrient-rich fen sites fluxes between plant communities differed only a little: almost all plots acted as CH₄ sinks (-0.9 to -0.4 mg CH₄ m-² d-¹), with the exception of Eriophorum angustifolium Honck. communities, which emitted 0.9 g CH₄ m-² d-¹. At nutrient-poor bog site the differences between plant communities were clearer. The highest emissions were measured from Eriophorum vaginatum L. communities (29.7 mg CH₄ m-² d-¹), with a decreasing trend to Sphagna (10.0 mg CH₄ m-² d-¹) and forest moss communities (2.6 mg CH₄ m-² d-¹). CH₄ emissions from different kinds of ditches were highly variable, and extremely high emissions (summertime averages 182-600 mg CH₄ m-² d-¹) were measured from continuously water-covered ditches at the drained fen. Variability in the emissions was caused by differences in the origin and movement of water in the ditches, as well as differences in vegetation communities in the ditches. While drainage on average greatly decreases CH₄ emissions from peatlands, a great spatial variability in fluxes is emerged. Emissions from ditches constantly covered with water, may in some cases have a great impact on the overall CH₄ emissions from drained peatlands.