Abstract

Most work on methane (CH₄) emissions from natural ecosystems has focused on wetlands because they are hotspots of CH₄ production. Less attention has been directed toward upland ecosystems that cover far larger areas, but are assumed to be too dry to emit CH₄. Here we review CH₄ production and emi... Show moreMost work on methane (CH₄) emissions from natural ecosystems has focused on wetlands because they are hotspots of CH₄ production. Less attention has been directed toward upland ecosystems that cover far larger areas, but are assumed to be too dry to emit CH₄. Here we review CH₄ production and emissions in upland ecosystems, with attention to the influence of plant physiology on these processes in forests. Upland ecosystems are normally net sinks for atmospheric CH₄ because rates of CH₄ consumption exceed CH₄ production. Production of CH₄ in upland soils occurs in microsites and may be common in upland forest soils. Some forests switch from being CH₄ sinks to CH₄ sources depending on soil water content. Plant physiology influences CH₄ cycling by modifying the availability of electron donors and acceptors in forest soils. Plants are the ultimate source of organic carbon (electron donor) that microbes process into CH₄. The availability of O₂ (electron acceptor) is sensitive to changes in soil water content, and therefore, to transpiration rates. Recently, abiotic production of CH₄ from aerobic plant tissue was proposed, but has not yet been verified with independent data. If confirmed, this new source is likely to be a minor term in the global CH₄ budget, but important to quantify for purposes of greenhouse gas accounting. A variety of observations suggest that our understanding of CH₄ sources in upland systems is incomplete, particularly in tropical forests which are stronger sources then expected. Show less