Methane emissions through trees in tropical and temperate forested wetlands

Pangala, Sunitha R.; Gauci, Vincent; Hornibrook, Edward R. C. and Gowing, David J. (2012). Methane emissions through trees in tropical and temperate forested wetlands. In: 9th INTECOL International Wetlands Conference: Wetlands in a Complex World, 03-08 Jun 2012, Orlando, Florida, p. 170.



Wetlands are the single largest natural source of methane (CH4) emissions to the atmosphere. CH4 dynamics in wetlands are generally well understood but recent reports of CH4 flux through trees suggests a need to revisit CH4 transport processes. Wetland trees possess morphological adaptations, such as hypertrophied lenticels, aerenchyma and adventitious roots, which develop in response to soil anoxia. Such adaptations facilitate gas transport, providing a means to aerate the rhizosphere and a conduit for escape of soil CH4 to the atmosphere. Approximately 60% of global wetlands are forested and many tropical forests are permanently or seasonally flooded. Hence tree-mediated CH4 release could be an important factor in the global CH4 budget. CH4 emission from the trunks of temperate tree species has been confirmed; however, CH4 emissions from tropical trees and processes that control tree-mediated CH4 emissions remain unclear.

This study investigated the role of trees in transporting soil-produced CH4 to the atmosphere and the principal mechanisms of tree-mediated CH4 emissions. CH4 fluxes from eight tropical tree species and two temperate tree species were measured in situ. The mechanisms and controls on tree-mediated CH4 emissions were studied using three-year-old common alder (Alnus glutinosa; 50 trees) grown under two artificially controlled water-table positions. The rates of CH4 emission from whole mesocosms, the soil surface and tree stems were measured using static closed chambers.

Both temperate and tropical tree species emitted significant quantities of CH4 and notably, trees dominated ecosystem level CH4 fluxes in a tropical forest peat swamp. In a temperate riparian wetland, CH4 transport mechanism and the quantity of CH4 emitted was dependent on tree species. In Alnus glutinosa, no correlation was observed between stomatal behaviour and tree-mediated CH4 emissions, although stem CH4 emissions were positively correlated with stem lenticel density and the concentration of dissolved CH4 in soil. CH4 was not emitted from leaf surfaces in Alnus glutinosa. These findings indicate that tree-mediated CH4 emissions should be included in flux measurement campaigns in forested wetlands to avoid underestimation of total CH4 emissions from such ecosystems.

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