Are ghost forests a substantial source of methane from reservoirs?

Abstract. Methane (CH₄) is a potent greenhouse gas that is increasing in the atmosphere, being a major driver of 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 seasonal CH₄ and carbon dioxide (CO₂) emissions from a ghost forest 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 ghost forest stem CH₄ emissions, to the diffusive and ebullition flux pathways, at three within reservoir sites (North, Mid and South). The highest average ghost forest CH₄ fluxes occurred near the reservoir inflow site (South) during summer (1173 ± 338 µmol m^-2 stem d^-1). At the same location the average CH₄ fluxes from ghost forest trees and ebullition were significantly higher, 5.8 and 2.7 respectfully, during summer, compared to winter. Ghost forest 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 CH₄ emissions from reservoirs and encourage management strategies to balance CH₄ mitigation with other ecological benefits of standing ghost forests.