Drought reduces water uptake in beech from the drying topsoil, but no compensatory uptake occurs from deeper soil layers.

Arthur Gessler,Stefan Seeger,John Marshall,Kerstin Treydte,Christian Hug,Roman Zweifel,Matthias Saurer,Marcus Schaub,Markus Weiler,Katrin Meusburger,Andreas Rigling,Lukas Bächli,Matthias Haeni,Frank Hagedorn,Elham Rouholahnejad Freund

doi:10.1111/nph.17767

Abstract

Summary The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial.We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2‐h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use.Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre‐drought levels within 3 wk after rewetting.These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Highlights

Water is one of the central limiting resources for plant growth and ecosystem functioning (Churkina & Running, 1998)
The regression line was close to the 1 : 1 line, and there were no clear outliers, neither for the soil nor for xylem samples. These findings points to the fact that spectral interferences with plant-produced volatile organic compounds (VOCs), which are known to affect d18O measurements with isotope ratio infrared spectroscopy (IRIS) (e.g. Volkmann et al, 2016b) did not play a role in the xylem water of beech trees
We applied no further postprocessing corrections (such as the ones, e.g. described by Martın Gomez et al (2015)). These results indicate that the water extracted by cryogenic distillation, which is assumed to include both mobile and tightly bound soil water, is not different in its d18O as compared to the soil water that is in equilibrium with the water vapour trapped by the in situ IRIS system

Summary

Introduction

Water is one of the central limiting resources for plant growth and ecosystem functioning (Churkina & Running, 1998). Trees take up the water from different layers of the soil via their fine roots and partially via mycorrhizal hyphen (Allen, 2007) and a match between how the available water is vertically distributed and where the roots are located is important for tree water use. Trees may adjust their rooting system to changing soil moisture conditions (Poorter et al, 2012), but it remains uncertain how tree water use copes with extreme drought events during which the upper soil layers, where fine roots are most abundant, dry out quickly

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