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We sampled air in the canopy layer of undisturbed upland forests during wet and dry seasons at three sites in the Brazilian Amazon region and found that both methane (CH4) and carbon dioxide (CO2) mixing ratios increased at night. Such increases were consistent across sites and seasons. A canopy layer budget model based on measured soil-atmosphere fluxes of CO2 was constructed to estimate ecosystem CH4 emission. We estimate that net CH4 emission in upland forests ranged from 2 to 21 mg CH4 m(-2) d(-1). While the origin of this CH4 source is unknown, these ground based measurements are consistent with recent findings based on satellite observations that indicate a large, unidentified source of CH4 in tropical forest regions

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