The role of trees, in addition to that of the soil, must be considered in CH4 budget for forests. Although trees can emit CH4 through their stems, there are uncertainties about the main factors that explain inter- and intraspecific variations, which impedes upscaling of measurements from the stem to the ecosystem level. This study aimed to characterize the variability in CH4 emissions (F-CH4) from stems between species and individuals, and within individuals. We measured F-CH4 in situ during the snow-free period in five species in a temperate mountain forest, using individuals of different sizes and chambers at different heights along the stems. One coniferous species emitted almost no CH4, whereas four broadleaved species exhibited high intraspecific variability in F-CH4 (0-3.7 nmol m(-2) s(-1)). Increasing trends in F-CH4 with tree diameter were observed for four species. The vertical patterns in F-CH4 were complex. Seasonal variations in F-CH4, measured on two trees per species, were well explained by air temperature with apparent temperature sensitivity coefficients (Q(10)) between 1.2 and 2, which were not related to the antecedent precipitation indices, whether calculated over 7 or 30 days. Potential CH4 production was detected in wood core segments incubated under anoxic conditions in the majority of individual trees of all species. Our results suggest that the CH4 emitted by trunks can originate either from soil or internal sources. Scaling F-CH4 from trees at the stand level and developing process-based models of F-CH4 will remain challenging until the sources of variation are better explained.