Abstract:

Forest soils are generally considered to be net sinks of methane (CH₄), but CH₄ fluxes vary spatially depending on soil conditions. Measuring CH₄ exchange with chambers, which are commonly used for this purpose, might not result in representative fluxes at site scale. Appropriate methods for upscaling CH₄ fluxes from point measurements to site scale are therefore needed. At the boreal forest research site, Norunda, chamber measurements of soils and vegetation indicate that the site is a net sink of CH₄, while tower gradient measurements indicate that the site is a net source of CH₄. We investigated the discrepancy between chamber and tower gradient measurements by upscaling soil CH₄ exchange to a 100 ha area based on an empirical model derived from chamber measurements of CH₄ exchange and measurements of soil moisture, soil temperature and water table depth. A digital elevation model (DEM) derived from high-resolution airborne Light Detection and Ranging (LiDAR) data was used to generate gridded water table depth and soil moisture data of the study area as input data for the upscaling. Despite the simplistic approach, modeled fluxes were significantly correlated to four out of five chambers with R > 0.68. The upscaling resulted in a net soil sink of CH₄of −10 μmol m⁻² h⁻¹, averaged over the entire study area and time period (June–September, 2010). Our findings suggest that additional contributions from CH₄ soil sources outside the upscaling study area and possibly CH₄ emissions from vegetation could explain the net emissions measured by tower gradient measurements.

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