Trace Greenhouse Gas Fluxes in Upland Forests

Welch, Bertie William Courtney (2018). Trace Greenhouse Gas Fluxes in Upland Forests. PhD thesis The Open University.



Tree stems can act as a conduit for trace greenhouse gases (GHG) produced in the soil. However, the majority of studies describing tree stem fluxes of methane (CH₄) and nitrous oxide (N₂O) have focused on wetland ecosystems. Tree stem fluxes of GHGs on free-draining soils are understudied, but they are assumed to be a source of CH₄ and a weak source of N₂O. The work presented in this thesis aimed to determine how climatic variables, soil abiotic conditions, and tree species influence CH₄ and N₂O fluxes in forests on free-draining soil.

Soil and stem CH₄ and N₂O fluxes were measured in lowland tropical rainforest in Panama, Central America and temperate woodland in the UK, using chambers installed on the forest floor or strapped to individual stems of two common tree species. Air samples were collected every two to four weeks during 5 months in 2014 and during November 2015 at the tropical site, and between February 2015 and January 2016 at the temperate site.

Tree stem CH₄ fluxes differed significantly between species at both sites and stem N₂O fluxes also differed between species at the tropical site. However, there was little variation in soil CH₄ or N₂O fluxes. At both sites, tree-mediated CH₄ fluxes declined from positive values (emission) at the stem base to negative values (uptake) higher up. Stem CH₄ fluxes generally increased significantly with solar radiation, suggesting a link to photosynthetic activity mediated by tree water transport.

Collectively, these results show that trees on free-draining soils could act as net sinks for CH₄ and N₂O. These findings will improve GHG budgets because tree stem uptake is currently unaccounted for. In particular, if uptake of CH₄ by tree stems on free-draining soils is widespread, the global terrestrial CH₄ sink could be much larger than currently estimated.

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