Most work on methane (CH 4 ) emissions from natural ecosystems has focused on wetlands because they are hotspots of CH 4 production. Less attention has been directed toward upland ecosystems that cover far larger areas, but are assumed to be too dry to emit CH 4 . Here we review CH 4 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 CH 4 because rates of CH 4

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... onsumption exceed CH 4 production. Production of CH 4 in upland soils occurs in microsites and may be common in upland forest soils. Some forests switch from being CH 4 sinks to CH 4 sources depending on soil water content. Plant physiology influences CH 4 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 CH 4 . The availability of O 2 (electron acceptor) is sensitive to changes in soil water content, and therefore, to transpiration rates. Recently, abiotic production of CH 4 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 CH 4 budget, but important to quantify for purposes of greenhouse gas accounting. A variety of observations suggest that our understanding of CH 4 sources in upland systems is incomplete, particularly in tropical forests which are stronger sources then expected.