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| Titel |
Inferring dominant controls on transpiration across a hillslope transect from ecohydrological measurements and thermodynamic limits |
| VerfasserIn |
Maik Renner, Sibylle Hassler, Theresa Blume, Markus Weiler, Anke Hildebrandt, Marcus Guderle, Stan Schymanki, Axel Kleidon |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105764
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| Publikation (Nr.) |
 EGU/EGU2015-5326.pdf |
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| Zusammenfassung |
| We combine ecohydrological observations of sapflow and soil moisture-derived root water
uptake with a thermodynamically constrained estimate of atmospheric evaporative demand to
infer the dominant controls on forest transpiration in complex terrain. Specifically we
hypothesize that short term variability is dominated by land-atmosphere interaction,
whereas site-specific controls determine the sensitivity of transpiration to atmospheric
demand.
To explore topographic controls on forest transpiration we use data from a extensive
ecohydrological measurement setup which was established within the CAOS (Catchments As
Organized Systems) research unit. Specifically, we use data of 5 sites along a steep hillslope
transect (15 - 22°) in a deciduous beech forest in Luxembourg. Two sites are located at the
north-facing slope, three at the opposite south-facing slope. Each site was equipped with soil
moisture sensors at three depths in three profiles as well as heat-pulse sap flow sensors in four
trees per site. Meteorological observations (solar radiation, temperature, humidity) are
recorded at a nearby pasture.
Both sapflow and soil moisture-derived root water uptake allow for an independent
assessment of site-scale transpiration. Although each method has specific limitations, there is
a robust relation across sites. This relationship can be used to estimate site scale
growing season transpiration. Atmospheric evaporative demand is estimated through
thermodynamically constrained evaporation which only requires absorbed solar radiation
and temperature as input data and thus allows prediction of evaporative demand
independent of surface conditions. The joint analysis of daily data shows that sapflow is
highly linearly correlated with atmospheric demand (r2 = 0.82) independent of
location, tree size and soil moisture content. Hence the sensitivity of sapflow and root
water uptake to atmospheric demand allows for estimating long term controls on
transpiration.
Aspect of the hillslopes and stand characteristics were found to be the major long term
control: the south-facing sites with larger atmospheric demand show significantly
lower sensitivity to atmospheric demand than the north-facing sites. Interestingly,
we find that the sensitivity is decreasing with stand density, which is markedly
different between the contrasting hillslopes. Thus periodical water limitation induced
by larger absorption of solar radiation on the south-facing slopes may have led
to long term plant adaptation or forest management strategies which satisfy the
trade-off between highly variable atmospheric demand and the depletion of soil water
storage.
In summary, the strong linear link of daily transpiration to atmospheric demand as well as
the more long-term adaptation to soil moisture availability may allow for the spatially explicit
prediction of forest transpiration from solar radiation, temperature and stand characteristics. |
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