Tree methane exchange in a changing world

Tree surfaces facilitate methane (CH4) exchange between terrestrial systems and the atmosphere. In this Perspective, I consider methane emission and uptake in trees, the underlying mechanisms and their response to environmental changes. Methane emitted from trees predominantly originates in soils, being transferred through the stem. The highest tree methane emissions occur in waterlogged soil conditions. As such, trees in wetland and riparian forests are a net source of methane, with topical wetland trees emitting up to ~44 Tg CH4 yr−1. By comparison, trees on free-draining upland soils are a net sink of methane on the order of 50 Tg CH4 yr−1, with microbially mediated methanotrophy along the soil–tree continuum dominating the vertical attenuation of soil-derived methane fluxes. Latitude (temperature) and nutrient status also influence net methane exchange. For example, tree methane emissions in the more nutrient-replete Amazon floodplain are up to 1,000-fold higher than those in ombrotrophic peat swamps of Panama and Borneo. Elevated atmospheric CO2 concentrations are predicted to enhance wetland tree methane emissions and reduce upland tree net methane uptake, with the latter effect probably being strongest. Evidence from ice core records suggests that notable changes in global forest cover in the Americas in the 1500s following European first contact might have reduced global atmospheric methane concentrations by up to 50 ppb, indicating that forest area influence over the global methane budget is potentially substantial. Future research should better quantify tree methane exchange responses to environmental changes and reduce uncertainty in the global methane budgets. Tree methane exchange is an important component of the terrestrial and global methane budget. This Perspective explores the mechanisms underlying why wetland trees are a net source of methane to the atmosphere and upland trees are a net sink, and the effect environmental changes will have on these processes.