Overlooked sources of methane emissions from trees: branches and wounds

Tree stems have been identified as globally significant methane (CH4) sources; however, little information exists on emissions from tree wounds and branches. CH4 emissions can occur from the decomposition of anaerobic heartwood, which is also associated with wounds; CH4 may also be transported through the transpiration stream and emitted from branches. We compared CH4 emissions between tree stems and branches and assessed whether trees with major wounds emit more than those without. CH4 fluxes were measured from stems, branches, and wounds (classified as major or minor) of two dominant tree species in an upland temperate forest, and from the soil, and scaled up to the stand level. Branches and stems of both species emitted CH4, and the per unit area emission rates from branches were similar to (or in some cases greater than) stems. Trees with major wounds had greater CH4 emission rates than those without, from unblemished sections of their stems and from the wounds. At the stand scale, branches, stems, and wounds accounted for 83%, 9%, and 8% of net CH4 emissions from trees, respectively, and collectively offset 63% of the soil CH4 sink. These results indicate that tree branches and wounds can be important CH4 sources in forests.