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.

Most forests worldwide are located in upland landscapes. Previous studies have mainly focused on ground methane (CH4) flux in upland forests, and living tree stem‐based CH4 processes and fluxes are thus relatively poorly understood. This study investigated the relationship between CH4 concentration and water content in the heartwood of living trees in midtemperate, warm temperate, and subtropical upland forests and also measured seasonal changes of in situ stem CH4 flux and the CH4 concentration and water content in the heartwood of Populus davidiana Dode in a warm temperate upland forest. We found that approximately 4–13% of tree stems or approximately 8–31% of tree species had a substantial CH4 concentration of ≥10,000 μL L−1 in their heartwood across the three types of upland forests. The heartwood CH4 concentration was related to water content by a power function. A threshold of water content occurred beyond which CH4 was produced at high levels in the heartwood. The CH4 emissions from the breast height stems of P. davidiana ranged from 202.1 to 331.6 μg m−2 h−1 on a stem surface area basis during July–October 2016 and were significantly linearly correlated with the CH4 concentration or water content in the heartwood throughout the experimental period, but the linear correlation was not significant at daily and monthly scales. Temperature was not a limiting factor for CH4 production during July–October 2016, and thus, most of the CH4 production may be explained by water content in the heartwood of living trees in upland forests.