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SummaryUpland forests are traditionally thought to be net sinks for atmospheric methane (CH 4 ). In such forests, in situ CH 4 fluxes on tree trunks have been neglected relative to soil and canopy fluxes.We measured in situ CH 4 fluxes from the trunks of living trees and other surfaces, such as twigs and soils, using a static closed-chamber method, and estimated the CH 4 budget in a temperate upland forest in Beijing.We found that the trunks of Populus davidiana emitted large quantities of CH 4 during July 2014-July 2015, amounting to mean annual emissions of 85.3 and 103.1 lg m À2 h À1 on a trunk surface area basis on two replicate plots. The emission rates were similar in magnitude to those from tree trunks in wetland forests. The emitted CH 4 was derived from the heartwood of trunks. On a plot or ecosystem scale, trunk CH 4 emissions were equivalent to c. 30-90% of the amount of CH 4 consumed by soils throughout the year, with an annual average of 63%. Our findings suggest that wet heartwoods, regardless of rot or not, occur widely in living trees on various habitats, where CH 4 can be produced.