Ditches, microtopographical hotspots and hot moments drive greenhouse gas emissions from a clear-felled conifer plantation on an organic soil

Abstract. In the United Kingdom (UK), forests on peaty gley, peaty podsol and deep peat soils contain ca. 50 % of the total forest soil C stock (Vanguelova, 2015). Many such forests were planted in the 1970s and 80s and are due for harvest, raising the question: what is the greenhouse gas (GHG) balance after felling?

Typically, planted forests in the wetter UK uplands contain a network of ditches and ridge-with-furrows resulting in a complex mosaic of microtopographical features. Measuring GHG exchange from such a complex landscape is challenging; methane (CH₄) and nitrous oxide (N₂O) fluxes can vary greatly in both space and time, and ditches have been highlighted as potentially important GHG sources although they are challenging to measure.

We used a combination of flux measurement techniques to quantify GHG fluxes and identify the drivers from the key microtopographies (ridges, hollows, ditches) within an upland forest in northern England immediately after clear felling. We deployed manual flux chambers, a SkyLine2D automated chamber system and two eddy covariance towers to measure carbon dioxide (CO₂), CH₄, and N₂O for an intensive campaign of five weeks. We used remote sensing to estimate the proportions of microtopographies and upscaled fluxes from the chamber to the forest block scale. We investigated the contribution of brash to the GHG emissions of harvest through a litter addition experiment.

Cumulative flux estimates based on the different techniques and the GHGs measured varied considerably. We found that CO₂ fluxes did not differ between microtopographies but the needle litter in harvesting residues increased CO₂ emissions by ca. 33 %. Soil moisture was an important driver of both CH₄ and N₂O fluxes. Ditches were the largest source of CH₄ fluxes, followed by hollows and then ridges. The opposite pattern was seen for N₂O fluxes, which were greatest from ridges and other drier areas within the landscape. Following heavy rainfall, emissions of all GHGs increased rapidly over the next 24 hours.