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.

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