Methane Emissions From Wetland Trees: Controls and Variability

Pangala, Sunitha Rao (2014). Methane Emissions From Wetland Trees: Controls and Variability. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f060

Abstract

Methane (CH₄) produced in wetland soil generally is thought to be released to the atmosphere primarily via diffusion, ebullition and transport through aerenchyma of herbaceous plants adapted to waterlogged soils. The role of trees as a conduit for CH₄ export from soil to the atmosphere has received limited attention despite laboratory studies of saplings demonstrating that wetland trees have a significant capacity to transport soil-produced CH₄ to the atmosphere.

In order to investigate the role of trees in transporting soil-produced CH₄ to the atmosphere and assess its ecosystem contributions, tree-mediated CH₄ flux was measured in situ from a temperate forested wetland (Flitwick Moor, UK) dominated by Alnus glutinosa and Betula pubescens and from a tropical forested wetland (Borneo, Indonesia). Mesocosm experiments complemented in situ data, in which CH₄ emissions were measured from Alnus glutinosa saplings subjected to two water-table treatments. In both the in situ and mesocosm studies, CH₄ emissions from trees were compared to CH₄ emissions from the soil surfaces.

Both temperate and tropical tree species released significant quantities of CH₄ from stem surfaces throughout the observation period. In Alnus glutinosa, CH₄ emissions from leaf surfaces were not detected and stem surfaces were the principle point of CH₄ egress. Stem-CU4 emissions from both Alnus glutinosa and Betula pubescens were less sensitive to small changes in water-table variations when compared to CH₄ emissions from soil surfaces, however, the quantity, temporal variability and CH₄ transport mechanisms differed between the two tree species. Stem-CH₄ emissions were controlled by a number of factors including tree physiology, abiotic factors and gas transport mechanisms. Wetland trees contributed significantly to ecosystem CH₄ flux (6-87%), with tropical trees dominating ecosystem level CH₄ fluxes. The results demonstrate that exclusion of tree-mediated CH₄ emissions from flux measurement campaigns conducted in forested wetlands can significantly underestimate ecosystem-wide CH₄ flux.