Methane emissions from upland forest soils and vegetation.

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) consumption 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.