Methane emissions from woody stems of tropical and temperate wetland trees

Pangala, S. R.; Hornibrook, E. R.; Gowing, D. J.; Bastviken, D.; Enrich-Prast, A. and Gauci, V. (2013). Methane emissions from woody stems of tropical and temperate wetland trees. In: AGU Fall Meeting, 9-13 Dec 2010, San Francisco, CA, USA.



Wetland-adapted trees are known to transport and release soil-produced methane to the atmosphere through woody stem surfaces, yet the magnitude and controls of tree-mediated methane emissions remain unknown for mature forests. Although 60% of global wetlands are forested, and many tropical forests are either permanently or seasonally flooded, the ecosystem level contribution of tree-mediated methane flux relative to other gas transport pathways (e.g., ebullition, pore-water diffusion and via aerenchyma of herbaceous plants) has received limited attention. The role of trees as a conduit for methane export from soil to the atmosphere was assessed in situ in a temperate forested wetland (Flitwick Moor, UK) and tropical forested wetlands in Borneo, Indonesia and Amazonia, Brazil. Mesocosm experiments also were conducted in the temperate region to characterise emission characteristics of Alnus glutinosa saplings subjected to different water-table treatments. Methane emissions from trees were compared to fluxes from the soil surface in both the in situ and mesocosm studies. Temperate and tropical tree species both released significant quantities of methane from stem surfaces. Emission rates for young trees exceeded that of mature trees by several orders of magnitude on a stem surface area basis. Key factors controlling rates of tree-mediated flux were tree physiology (e.g., wood specific density, stem lenticel density), abiotic conditions (e.g., soil temperature) and methane gas transport mechanisms (e.g., passive diffusion, convective transport). Tree-mediated methane emissions contributed 6 to 87% of total ecosystem methane flux with the largest relative contribution from trees occurring in tropical wetlands. Recent data from Amazonian wetlands demonstrate very high rates of tree-mediated methane emission relative to other types of forested wetlands. These results indicate that exclusion of tree-mediated methane fluxes from measurement campaigns conducted in forested wetlands may result in a significant underestimate of total methane flux from such ecosystems.

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