Considering scale when assessing wetland methane emissions: wetland forest soils versus wetland forests

Gauci, Vincent and Pangala, Sunitha R. (2012). Considering scale when assessing wetland methane emissions: wetland forest soils versus wetland forests. In: 9th INTECOL International Wetlands Conference: Wetlands In A Complex World, 03-08 Jun 2012, Orlando, Florida, p. 156.



Up to 60% of Earth's wetlands are forested and yet these ecosystems are quite poorly understood with respect to emissions of methane (CH4). It is generally thought that CH4 produced in waterlogged soils is emitted by a combination of three processes: 1) by diffusion through water-filled pores, 2) by abrupt release of bubbles, and 3) through internal spaces in the stems of herbaceous graminoids which are adapted to live in waterlogged soils. The capacity for trees to also emit CH4 has received only limited attention, principally through investigations on seedlings or saplings grown under artificial conditions, where they are shown to emit significant quantities of CH4 that is produced in the soil. It is hypothesized that there are two ways by which CH4 produced in wet soils may be transported and emitted through trees: i) as a gas through air-filled tissue in trees that has formed as an adaptation to enable transfer of oxygen from the atmosphere to the tree's roots which are growing in oxygen-poor waterlogged soil, and ii) dissolved in sap and then liberated to the atmosphere when tree water is lost by transpiration through pores in tree stems and leaves.

Trees offer large surfaces of stem and leaf in wetland forest ecosystems and as such, even small fluxes relative to those measured at the soil surface may be important at the ecosystem scale. This presents a challenge to both accurately quantifying emissions that are routinely ignored in measurement campaigns and also in attributing fluxes measured at the large scale to separate emissions pathways in order to understand their relative importance. We present a range of approaches to quantifying CH4 emissions from trees and soils in situ in both temperate and tropical forested wetlands. These include static and dynamic chamber approaches for measurement asynchronously (chamber samples retrieved and processed in the laboratory) and in real-time using cavity ring-down laser spectroscopy. We compare emissions at the wetland surface emitted via well understood pathways with those measured from trees and estimate their relative contributions to ecosystem flux.

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