Dominant controls of transpiration along a hillslope transect inferred from ecohydrological measurements and thermodynamic limits
We combine ecohydrological observations of sap flow and soil moisture with thermodynamically constrained estimates of atmospheric evaporative demand to infer the dominant controls of forest transpiration in complex terrain. We hypothesize that daily variations in transpiration are dominated by varia...
Main Authors: | , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-05-01
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Series: | Hydrology and Earth System Sciences |
Online Access: | http://www.hydrol-earth-syst-sci.net/20/2063/2016/hess-20-2063-2016.pdf |
Summary: | We combine ecohydrological observations of sap flow and soil
moisture with thermodynamically constrained estimates of atmospheric
evaporative demand to infer the dominant controls of forest transpiration in
complex terrain. We hypothesize that daily variations in transpiration are
dominated by variations in atmospheric demand, while site-specific controls,
including limiting soil moisture, act on longer timescales.
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We test these hypotheses with data of a measurement setup consisting of five
sites along a valley cross section in Luxembourg. Both hillslopes are covered
by forest dominated by European beech (<i>Fagus sylvatica</i> L.). Two independent
measurements are used to estimate stand transpiration: (i) sap flow and (ii) diurnal
variations in soil moisture, which were used to estimate the daily
root water uptake. Atmospheric evaporative demand is estimated through
thermodynamically constrained evaporation, which only requires absorbed solar
radiation and temperature as input data without any empirical parameters.
Both transpiration estimates are strongly correlated to atmospheric demand at
the daily timescale. We find that neither vapor pressure deficit nor wind
speed add to the explained variance, supporting the idea that they are
dependent variables on land–atmosphere exchange and the surface energy
budget. Estimated stand transpiration was in a similar range at the
north-facing
and the south-facing hillslopes despite the different aspect and the largely
different stand composition. We identified an inverse relationship between
sap flux density and the site-average sapwood area per tree as estimated by
the site forest inventories. This suggests that tree hydraulic adaptation can
compensate for heterogeneous conditions. However, during dry summer periods
differences in topographic factors and stand structure can cause spatially
variable transpiration rates. We conclude that absorption of solar radiation
at the surface forms a dominant control for turbulent heat and mass exchange
and that vegetation across the hillslope adjusts to this constraint at the
tree and stand level. These findings should help to improve the description
of land-surface–atmosphere exchange at regional scales. |
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ISSN: | 1027-5606 1607-7938 |