Increased Methane Emissions by an Introduced Phragmites australis Lineage under Global Change

North American wetlands have been invaded by an introduced lineage of the common reed, Phragmites australis. Native lineages occur in North America, but many populations have been extirpated by the introduced conspecific lineage. Little is known about how subtle changes in plant lineage may affect methane (CH4) emissions. Native and introduced Phragmites were grown under current and predicted future levels of atmospheric CO2 and nitrogen(N) pollution in order to understand how CH4 emissions may vary between conspecific lineages. We found introduced Phragmites emitted more CH4 than native Phragmites, and that CH4 emissions increased significantly in both with CO2+N treatment. There was no significant difference in CH4 production potentials, but CH4 oxidation potentials were higher in soils from the introduced lineage. Intraspecific plant responses to resource availability changed CH4 emissions, with plant density, root mass, and leaf area being significantly positively correlated with higher emissions. The absence of CO2-only or N-only effects highlights a limitation on the generalization that CH4 emissions are proportional to plant productivity. Our data suggest that intraspecific changes in plant community composition have important implications for greenhouse emissions. Furthermore, global change-enhanced invasion by introduced Phragmites may increase CH4 emissions unless these factors cause a compensatory increase in carbon sequestration.