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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Nov 2018

Research article | 05 Nov 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Hydrology and Earth System Sciences (HESS).

Seasonal origins of soil water used by trees

Scott T. Allen1,2, James W. Kirchner1,3,4, Sabine Braun5, Rolf T. W. Siegwolf2,3, and Gregory R. Goldsmith2,6 Scott T. Allen et al.
  • 1Department of Environmental 5 Systems Science, ETH Zurich, Zurich, 8092, Switzerland
  • 2Ecosystem Fluxes Group, Laboratory for Atmospheric Chemistry, Paul Scherrer Institute, Villigen, 5232, Switzerland
  • 3Swiss Federal Research Institute WSL, Birmensdorf, 8903, Switzerland
  • 4Department of Earth and Planetary Science, University of California, Berkeley, California, USA
  • 5Institute for Applied Plant Biology, Witterswil, 4108, Switzerland
  • 6Schmid College of Science and Technology, Chapman University, Orange CA, 92866, USA

Abstract. Rain recharges soil water storages and either percolates downward into aquifers and streams, or is returned to the atmosphere through evapotranspiration. Although it is commonly assumed that summer rainfall recharges plant-available water during the growing season, the seasonal origins of water used by plants have not been systematically explored. We characterize the seasonal origins of waters in soils and trees by comparing their mid-summer isotopic signatures (δ2H) to seasonal isotopic cycles in precipitation, using a new seasonal origin index. Across 182 Swiss forest sites, xylem water isotopic signatures show that summer rain was not the predominant water source for mid-summer transpiration in any of the three sampled tree species. Beech and oak mostly used winter precipitation, whereas spruce used water of more diverse seasonal origins. Even in the same plots, beech consistently used more winter precipitation than spruce, demonstrating consistent niche partitioning in the rhizosphere. All three species' xylem water isotopes indicate that trees used more winter precipitation in drier regions, potentially mitigating their vulnerability to summer droughts. The widespread occurrence of winter isotopic signatures in mid-summer xylem implies that growing-season rainfall may have minimally recharged the soil water storages that supply tree growth, even across diverse humid climates (690–2068-mm annual precipitation). Beyond these ecological and hydrological implications, our findings also imply that stable isotopes of δ18O and δ2H in plant tissues, which are often used in climate reconstructions, may not reflect water from growing-season climates. More broadly, these results conflict with common assumptions on tree water use and provide empirical support for developing more realistic concepts of how water flows through soils and is accessed by roots.

Scott T. Allen et al.
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Status: final response (author comments only)
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Scott T. Allen et al.
Scott T. Allen et al.
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Short summary
We used stable isotopes of xylem water to study differences in the seasonal origin of water in more than 900 individual trees from three dominant species in 182 Swiss forested sites. We discovered that, surprisingly, mid-summer transpiration was mostly supplied by winter precipitation across diverse humid climates. Our findings provide new insights into tree vulnerability to droughts, transport of water (and thus solutes) in soils, and the climatic information conveyed by plant-tissue isotopes.
We used stable isotopes of xylem water to study differences in the seasonal origin of water in...