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| Titel |
Seasonal and inter-annual variability of energy exchange above a boreal Scots pine forest |
| VerfasserIn |
S. Launiainen |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 7, no. 12 ; Nr. 7, no. 12 (2010-12-01), S.3921-3940 |
| Datensatznummer |
250005100
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| Publikation (Nr.) |
 copernicus.org/bg-7-3921-2010.pdf |
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| Zusammenfassung |
| Twelve-years of eddy-covariance measurements conducted above a boreal Scots
pine forest in Hyytiälä, Southern Finland, were analyzed to assess
the seasonal and inter-annual variability of surface conductance (gs)
and energy partitioning. The gs had distinct annual course, driven by
the seasonal cycle of the Scots pine. Low gs (2–3 mm s−1 in
April) cause the sensible heat flux to peak in May–June while
evapotranspiration takes over later in July–August when gs is typically
5–7 mm s−1. Hence, during normal years Bowen ratio decreases from 4–6
in April to 0.7–0.9 in August. Sensitivity of gs to ambient
vapor pressure deficit (D) was relatively constant but the reference value at
D = 1 kPa varied seasonally and between years. Only two drought episodes when
volumetric soil moisture content in upper mineral soil decreased below 0.15 m3 m−3
occurred during the period. Below this threshold value,
transpiration was strongly reduced, which promoted sensible heat exchange
increasing Bowen ratio to 3–4. Annual evapotranspiration varied between
218 and 361 mm and accounted between 50% and 90% of equilibrium
evaporation. The forest floor contributed between 16 and 25% of the
total evapotranspiration on annual scale. The fraction stayed similar over
the observed range of environmental conditions including drought periods.
The inter-annual variability of evapotranspiration could not be linked to
any mean climate variable while the summertime sensible heat flux and net
radiation were well explained by global radiation. The energy balance
closure varied annually between 0.66 and 0.95 and had a distinct seasonal
cycle with worse closure in spring when a large proportion of available
energy is partitioned into sensible heat. |
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