Journal cover Journal topic
Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
Hydrol. Earth Syst. Sci., 20, 2063-2083, 2016
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
25 May 2016
Dominant controls of transpiration along a hillslope transect inferred from ecohydrological measurements and thermodynamic limits
Maik Renner1, Sibylle K. Hassler2,6, Theresa Blume2, Markus Weiler3, Anke Hildebrandt4,1, Marcus Guderle4,1,7, Stanislaus J. Schymanski5, and Axel Kleidon1 1Max-Planck-Institut für Biogeochemie, Jena, Germany
2GFZ German Research Centre for Geosciences, Section Hydrology, Potsdam, Germany
3Universität Freiburg, Hydrologie, Freiburg, Germany
4Universität Jena, Ecological Modelling Group, Jena, Germany
5ETH Zürich, Department of Environmental Systems Science, Zurich, Switzerland
6Karlsruhe Institute of Technology, Institute of Water and River Basin Management, Karlsruhe, Germany
7Technische Universität München, Chair for Terrestrial Ecology, Department of Ecology and Ecosystemmanagement, Munich, Germany
Abstract. 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.

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 (Fagus sylvatica 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.

Citation: Renner, M., Hassler, S. K., Blume, T., Weiler, M., Hildebrandt, A., Guderle, M., Schymanski, S. J., and Kleidon, A.: Dominant controls of transpiration along a hillslope transect inferred from ecohydrological measurements and thermodynamic limits, Hydrol. Earth Syst. Sci., 20, 2063-2083, doi:10.5194/hess-20-2063-2016, 2016.
Publications Copernicus
Short summary
We estimated forest transpiration (European beech) along a steep valley cross section. Atmospheric demand, obtained by the thermodynamic limit of maximum power, is the dominant control of transpiration at all sites. To our surprise we find that transpiration is rather similar across sites with different aspect (north vs. south) and different stand structure due to systematically varying sap velocities. Such a compensation effect is highly relevant for modeling and upscaling of transpiration.
We estimated forest transpiration (European beech) along a steep valley cross section....