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| Titel |
Methane fluxes above the Hainich forest by True Eddy Accumulation and Eddy Covariance |
| VerfasserIn |
Lukas Siebicke, Lydia Gentsch, Alexander Knohl |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250135702
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| Publikation (Nr.) |
 EGU/EGU2016-16601.pdf |
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| Zusammenfassung |
| Understanding the role of forests for the global methane cycle requires quantifying
vegetation-atmosphere exchange of methane, however observations of turbulent methane
fluxes remain scarce.
Here we measured turbulent fluxes of methane (CH4) above a beech-dominated
old-growth forest in the Hainich National Park, Germany, and validated three different
measurement approaches: True Eddy Accumulation (TEA, closed-path laser spectroscopy),
and eddy covariance (EC, open-path and closed-path laser spectroscopy, respectively). The
Hainich flux tower is a long-term Fluxnet and ICOS site with turbulent fluxes and ecosystem
observations spanning more than 15 years. The current study is likely the first application of
True Eddy Accumulation (TEA) for the measurement of turbulent exchange of methane and
one of the very few studies comparing open-path and closed-path eddy covariance (EC)
setups side-by-side.
We observed uptake of methane by the forest during the day (a methane sink with a
maximum rate of 0.03 μmol m−2 s−1 at noon) and no or small fluxes of methane from the
forest to the atmosphere at night (a methane source of typically less than 0.01 μmol m−2
s−1) based on continuous True Eddy Accumulation measurements in September
2015.
First results comparing TEA to EC CO2 fluxes suggest that True Eddy Accumulation is a
valid option for turbulent flux quantifications using slow response gas analysers (here CRDS
laser spectroscopy, other potential techniques include mass spectroscopy). The TEA system
was one order of magnitude more energy efficient compared to closed-path eddy covariance.
The open-path eddy covariance setup required the least amount of user interaction but is often
constrained by low signal-to-noise ratios obtained when measuring methane fluxes over
forests. Closed-path eddy covariance showed good signal-to-noise ratios in the lab, however
in the field it required significant amounts of user intervention in addition to a high power
consumption.
We conclude, based on preliminary evidence, that the Hainich forest acted as a moderate
net sink for methane during the investigation. This supports earlier findings from chamber
measurements at the Hainich forest site and is similar to findings from other forest
sites.
Our observations will be continued through 2016 and beyond to provide longer-term
methane flux time series spanning entire seasons. However, the current data set already
provides a basis for further consolidating methods of measurements and analysis of turbulent
methane fluxes using eddy covariance and true eddy accumulation. |
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