Topography-based modelling reveals high spatial variability and seasonal emission patches in forest floor methane flux

Abstract. Boreal forest soils are globally an important sink for methane (CH₄), while these soils are also capable to emit CH₄ under favourable conditions. Soil wetness is a well-known driver of CH₄ flux, and the wetness can be estimated with several terrain indices developed for the purpose. The aim of this study was to quantify the spatial variability of the forest floor CH₄ flux with a topography-based upscaling method connecting the flux with its driving factors. We conducted spatially extensive forest floor CH₄ flux and soil moisture measurements, complemented with ground vegetation classification, in a boreal pine forest. We then modelled the soil moisture with a Random Forest model using topography, based on which we upscaled the forest floor CH₄ flux – this was performed for two seasons: May–July and August–October. Our results demonstrate high spatial heterogeneity in the forest floor CH₄ flux, resulting from the soil moisture variability, as well as on the related ground vegetation. The spatial variability in the soil moisture and consequently in the CH₄ flux was higher in the early summer compared to the autumn period, and overall the CH₄ uptake rate was higher in autumn compared to early summer. In the early summer there were patches emitting high amounts of CH₄, however, these wet patches got drier and smaller in size towards the autumn, which was enough for changing their dynamics to CH₄ uptake. The results highlight the small-scale spatial variability of the boreal forest floor CH₄ flux, and the importance of soil chamber placement in order to obtain spatially representative CH₄ flux results. We recommend that a site of similar size and topographical variation would require 15–20 sample points in order to achieve accurate forest floor CH₄ flux.