Are ghost forest trees a substantial source of methane from reservoirs?

Abstract. Methane (CH4) is a potent greenhouse gas that is increasing in the atmosphere, driving climate change. Tree stem CH4 emissions are a rapidly advancing research field, however emissions from dead trees remain poorly studied. This is of particular concern in reservoir “ghost forests”, where large areas of standing dead trees can form, and remain submerged in CH4-enriched waters, providing a potential CH4-flux pathway along the soil-tree-atmosphere continuum, for many decades. This study quantified the drivers of CH4 emissions from ghost forest trees within a subtropical reservoir alongside diffusive and ebullition fluxes, across two seasons. We compared the influence of sediment organic carbon, water level and temperature fluctuations on all three CH4 flux pathways, at three sites within the reservoir (North, Mid and South). The highest average ghost forest tree CH4 fluxes occurred near the reservoir inflow site (South) during summer (1173±338 µmol m−2 stem d−1). At the same location, average CH4 fluxes from ghost forest trees and ebullition were significantly higher in summer than in winter, by 5.8 and 2.7-fold, respectfully. Ghost forest tree CH4 fluxes contributed an additional ∼ 15 % to the overall reservoir greenhouse gas budget, beyond conventional methods – which generally only consider ebullition and diffusive flux pathways. Our findings reveal the need to recognise ghost forest tree CH4 emissions from reservoirs and encourage management strategies to balance CH4 mitigation with other ecological benefits of standing ghost forest trees.