How tree traits modulate tree methane fluxes: A review.

Trees can play different roles in the regulation of fluxes of methane (CH), a greenhouse gas with a warming potential 83 times greater than that of carbon dioxide. Forest soils have the greatest potential for methane uptake compared to other land uses. In addition to their influence on soil CH fluxes, trees can act directly as a source or sink of CH, by transporting CH produced in the soil and harbouring the key microorganisms involved in CH production and consumption (methanogens and methanotrophs). Tree CH fluxes can vary between species characterized by different traits that influence transport and modify the availability of CH reaction substrates as well as the habitat for methanogens and methanotrophs. Despite their important role in modulating CH fluxes from forest ecosystems, the identity and role of tree traits influencing these fluxes are poorly consolidated in the literature. The objectives of this paper are to 1) Review the functional traits of trees associated with their role in the regulation of CH emissions; 2) Assess the importance of inter-specific variability in CH fluxes via a global analysis of tree methane fluxes in the literature. Our review highlights that differences in CH fluxes between tree species and individuals can be explained by a diversity of traits influencing CH transport and microbial production of CH such as wood density and secondary metabolites. We propose a functional classification for trees based on the key traits associated with a function in CH emissions. We identified the fast-growing species with low wood density, species adapted to flood and species vulnerable to rot as functional groups which can be net sources of CH in conditions favorable to CH production. The global analysis further demonstrated the importance of taxonomy, with other factors such as land type and season in explaining variability in tree CH fluxes.