Tree stem greenhouse gas emissions from forested closed landfill sites

Tree planting has the potential to increase carbon sequestration and is used as a common management strategy on former landfill sites to improve their visual appeal and manage issues such as leachates from decomposing organic matter. Tree stems mediate methane (CH₄) emissions to the atmosphere from anaerobic soils, bypassing bacterial populations that would otherwise break down CH₄ before it is released to the atmosphere. This process has been observed in wetland forests but has yet to be measured in a landfill context. We examined whether trees emitted more CH₄ and carbon dioxide (CO₂) on a closed UK landfill site relative to a more natural, comparable control site to determine the importance of this natural phenomenon in a managed environment. CH₄ and CO₂ fluxes from tree stem and soil surfaces were measured using flux chambers and an off-axis integrated cavity output spectroscopy analyser. Temporal and seasonal variations in greenhouse gas emissions from landfill tree stems were also investigated, as well as the impact of different landfill management techniques including site closure methods and tree species planted. Analyses showed that tree stem emissions from landfill were larger than from trees in the non-landfill control site. However, there was high variability in the greenhouse gas fluxes from trees on the landfill. Findings from this investigation suggest that conditions associated with landfill construction may increase CH₄ emissions from trees planted on their surface after closure of the site. Trees planted on former landfill sites may therefore result in additional CH₄ emissions to the atmosphere.