Spatial Variability of Annual Estimates of Methane Emissions in a Phragmites Australis (Cav.) Trin. ex Steud. Dominated Restored Coastal Brackish Fen

Methane is a major greenhouse gas with a global warming potential 25 times higher than carbon dioxide over a 100 year time horizon. Determining annual emission estimates, usually specified for different vegetation types under particular land use, requires the use of chamber measurements. These emission estimates may be strongly biased towards the lower or upper end by the spatial arrangement of measurement spots in the ecosystem. Here, we analyze the spatial variability of annual methane emission estimates based on non-steady state closed chamber measurements in pure and mixed stands of Phragmites australis (Cav.) Trin ex Steud. in a restored coastal brackish fen. Annual methane emission estimates per measurement location vary largely between 46 and 1,329 kg ha−1 a−1 CH4 but they do not differ significantly between pure and mixed stands of P. australis. Mantel tests show a significant correlation of distances between spots and the variation in methane emission estimates (p < 0.05). Spatial correlation of water levels and annual methane emission estimates is not significant. Empirical variograms suggest that variance in annual methane emission estimates is not increasing with increasing spatial distance. However, spots that share larger distances may differ considerably in their annual methane emission estimates. At the same time, spots that share smaller distances–though these exceed the distances common to many closed chamber measurement setups–may differ less in their annual methane emission estimates. Thus, a huge part of the natural variation in annual methane emissions in a particular ecosystem or vegetation type may be missed when using the typical clustered arrangement of measurement locations in closed chamber studies. Therefore, we suggest that measurement locations should cover a wide spatial extent to improve the reliability of annual emission estimates per vegetation type and ecosystem, and that the number of measurement locations appropriate should be determined by pre-studies whenever possible.