Forests and methane: looking beyond carbon for nature-based climate solutions

Forests, their preservation and expansion have been long championed as nature-based climate solutions for their capacity to remove carbon dioxide from the atmosphere and retain it as carbon within living and dead organic matter (Griscom et al 2017). However, as our understanding of forests evolves, with major forest biomes either at carbon saturation or experiencing loss (Hubau et al 2020) it becomes clear that focusing solely on their carbon dynamics overlooks their involvement in the cycling of other powerful greenhouse gases such as methane. The role of vegetation more broadly, and trees more specifically in the methane cycle is complex with attention for nearly two decades focussed on the function of lowland-wetland trees as potential sources or emission pathways for soil-produced methane (Pangala et al 2017, Bastviken et al 2023). This has clarified our understanding of flooded trees as point sources of wetland methane emissions responsible for up to half of all wetland methane emitted from the Amazon floodplain and elsewhere (Gauci et al 2022). We now need to consider methane cycling in the vast expanse of upland forest on free draining soils with low water tables. In these ecosystems, methane exchange has been more difficult to disentangle chiefly due to the difficulty of making in situ measurements of very small fluxes, in either direction, and identifying their source from a range of processes at scale and from within relatively inaccessible (for chamber measurement campaigns) tree crowns and canopies. These sources include aerobic sources that have wide uncertainties (Keppler et al 2006, Kohl et al 2023). However, in Gauci et al (2024), woody stem surfaces of upland trees were shown to be a significant locus of atmospheric methane removal (AMR), potentially as large or larger than the well-understood global soil sink (Dunfield 2007). Collectively, and in the tropics in particular, tree woody surface AMR via methanotrophy may add upwards of 10% to the climate benefit of trees via processes that are entirely independent of the carbon they contain (Gauci et al 2024). Here, I elaborate on this discovery to identify the opportunities, current uncertainties and areas for further investigation in an effort to use tree woody surface AMR as a nature-based solution to reduce growth in atmospheric methane.