Microbial Processes Related To Methane Fluxes In Temperate And Tropical Wetland Trees

Wetlands are the largest natural source of methane, a fast-growing atmospheric greenhouse gas. Most wetland ecosystems are forested but only a few studies have focused on methane emissions from trees, and little is known about the presence and role of methane-related microorganisms within them. Moreover, there are no studies on the spatial distribution of methane fluxes in wetland trees where the upper parts of tree stems are also included. Trees are known to play an important role in methane emissions and understanding how methane is cycled within and between the individual ecosystem components of forests is essential if we are to understand the role of forests, as a whole, in the global methane cycle.

We measured methane fluxes on tree surfaces in temperate and tropical wetland forests and quantified the processes within tree tissues and soils. Field campaigns were carried out in Kalimantan, Indonesia and Flitwick Moor, UK between March and August 2018 including measurements from the dominant tree species at each site. Stem methane fluxes were measured up to 3 m above the forest floor, and processes quantified from soil to depths of 50 cm. Production and oxidation rates were quantified from the same trees, tree heights, and soil depths, to identify the presence of methanogens and methanotrophs and their activities.

Tree stem methane fluxes averaged 0.11 ± 0.30 mg m^-2 d^-1 in the temperate site and 0.047 ± 0.5 mg m^-2 d^-1 in the tropical site. Average values for low and high affinity methane oxidation rates were respectively -232.2 ± 133.5 and -8.96 ± 8.85 mg m^-3 d^-1 in the temperate site and -359.6 ± 431.9 and -3.08 ± 3.30 mg m^-3 d^-1 in the tropical site. Methane production rates averaged 5.65 ± 2.11 mg m^-3 d^-1 in the temperate site and 5.82 ± 3.79 mg m^-3 d^-1 in the tropical site.

Evidence of methane uptake in the tree stems and of methane oxidation indicates that trees have the capacity to assimilate methane in wetland ecosystems. Samples from the same tree individuals and soils are currently being analysed to characterise methane-oxidising bacteria responsible for uptake we observed in the field.