Journal Article Methane OP0UW79H66 Research Support, Non-U.S. Gov't Alnus glutinosa methane emissions stem CH4 emissions 059QF0KO0R Betula pubescens CH4 transport Soil Water gas transport global carbon cycle lenticels methane methane (CH4) methanogenesis seasonal variation soil-plant-atmosphere interactions spatial variability temporal variability tree stem CH4 fluxes tree stems tree-mediated CH4 emissions tree-mediated methane emissions upland forests wetland wetland forests wetlands