A versatile gas flux chamber reveals high tree stem CH4 emissions in Amazonian peatland

Abstract

Tree stems can be a major source of CH4, altering the impact forests have on atmospheric radiative forcing. In the tropics stem flux monitoring has been limited, nevertheless, available field studies have observed high variability of stem CH4 flux rates between species and individuals and over the surface of individual tree stems. To evaluate the sources of variation and controls of CH4 stem fluxes, methods supporting large sampling campaigns are needed. We designed a portable, flexible, easily installed chamber that produces replicable flux measurements across broad species diversity. The chamber creates a strong seal on a range of stem sizes and surface roughnesses and extends radially, integrating surface flux variation so that small hot spots are not missed. Working in forested Amazonian peatlands, we used this chamber to measure stem fluxes on a variety of palms and trees with stems ranging from 10 cm to 85 cm DBH, both rough and smooth barked species. We compared a non-steady state, headspace recirculation method to a steady-state flow-through method and found the flow-through method likely yields more accurate estimates for high emissions. Our novel stem flux measurements from Amazonian peatlands reveal that stem CH4 emissions of the dominant palm (M. flexuosa) contrast markedly against dominant hardwood species, with palms emitting CH4 at much higher rates (up to 84 mg-C m−2 h−1 at 0.23 m stem height) that compare to the highest published rates of tree stem emission. Flux rates from M. flexuosa demonstrated exponential decay with stem height, rates at 0.5 m were 3 to 10 times higher than at 1.4 m. This pattern underscores the importance of quantifying CH4 flux rates over the lower 2 m of stem in order to accurately quantify stem CH4 fluxes. Considering their broad distribution and high density in Amazonian peatlands, M. flexuosa stems may be a major source of atmospheric CH4.