Josep Barba

Abstract Recent reintroductions of the Eurasian beaver (Castor fiber) across Europe represents an ecological shift with potential implications for carbon cycling in stream corridors. However, the capacity of beaver impacts to influence short- and long-term carbon fluxes, and the mechanisms that govern these changes, remains poorly understood.

Until recently, tree stem surfaces were thought to be inert to methane (CH4) exchange with the atmosphere. However, technological advances in CH4 analysis techniques and the development of stem chamber systems for gas flux determination have enabled intensive field measurements of CH4 fluxes not only from soils but also from tree stem surfaces.

Summary Tree stems from wetland, floodplain and upland forests can produce and emit methane (CH4). Tree CH4 stem emissions have high spatial and temporal variability, but there is no consensus on the biophysical mechanisms that drive stem CH4 production and emissions. Here, we summarize up to 30 opportunities and challenges for stem CH4 emissions research, which, when addressed, will improve estimates of the magnitudes, patterns and drivers of CH4 emissions and trace their potential origin.