Forest ecosystem changes from annual methane source to sink depending on late summer water balance

Forests dominate the global carbon cycle, but their role in methane (CH4) biogeochemistry remains uncertain. We analyzed whole‐ecosystem CH4fluxes from 2 years, obtained over a lowland evergreen forest in Maine, USA. Gross primary productivity provided the strongest correlation with the CH4flux in both years, with an additional significant effect of soil moisture in the second, drier year. This forest was a neutral to net source of CH4in 2011 and a small net sink in 2012. Interannual variability in the summer hydrologic cycle apparently shifts the ecosystem from being a net source to a sink for CH4. The small magnitude of the CH4fluxes and observed control or CH4fluxes by forest productivity and summer precipitation provide novel insight into the CH4cycle in this globally important forest ecosystem.