Methane emissions from tree stems: a new frontier in the global carbon cycle.

Tree stems from wetland, floodplain and upland forests can produce and emit methane (CH ). Tree CH stem emissions have high spatial and temporal variability, but there is no consensus on the biophysical mechanisms that drive stem CH production and emissions. Here, we summarize up to 30 opportunities and challenges for stem CH emissions research, which, when addressed, will improve estimates of the magnitudes, patterns and drivers of CH emissions and trace their potential origin. We identified the need: (1) for both long-term, high-frequency measurements of stem CH emissions to understand the fine-scale processes, alongside rapid large-scale measurements designed to understand the variability across individuals, species and ecosystems; (2) to identify microorganisms and biogeochemical pathways associated with CH production; and (3) to develop a mechanistic model including passive and active transport of CH from the soil-tree-atmosphere continuum. Addressing these challenges will help to constrain the magnitudes and patterns of CH emissions, and allow for the integration of pathways and mechanisms of CH production and emissions into process-based models. These advances will facilitate the upscaling of stem CH emissions to the ecosystem level and quantify the role of stem CH emissions for the local to global CH budget.