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.



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.

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