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.
<p>Tree species differ in their ability to utilize existing soil water pools due to their root architecture, but also due to their capacity to react on spatiotemporal variations of the supply. The interplay between variations of water availability and species-specific utilization plays a crucial role in determining the water balance and cycle of ecosystems. Despite a large number of studies on the various aspects of ecosystem water relations, there exists still uncertainty regarding the plasticity of tree roots to take up water from different soil depths in relation to the mechanisms and patterns of water infiltration into the root zone.</p><p>We will present results from a holistic tracer irrigation experiment in the L&#246;tschental, Swiss Alps. A subalpine forest plot (150 m<sup>2</sup>) of Larix decidua and Picea abies was irrigated with, relative to natural soil abundance <sup>18</sup>O and <sup>2</sup>H depleted glacier water during 10 subsequent days in summer 2019. Water was taken from a nearby glacier river. Irrigation was conducted through a dripping system installed on the ground to increase and keep soil water content at field capacity during the experiment. Throughout the irrigation, soil moisture at three locations in the experimental as well as in a control plot was monitored in 15-minutes intervals in two soil depths. Four larch and four spruce trees per plot were selected and equipped with continuously measuring sapflow sensors. Sampling of soil and tree tissues took place on a daily basis always before noon: Soil samples were taken in close distance to the soil moisture sensors from at least three soil depths, needles and twigs from all experimental trees were sampled in the canopy of the sun-exposed crowns. Every third day xylem samples were taken from the tree stems with a 5mm increment corer. All samples were immediately cooled until the isotopic analysis. In parallel to the soil and tree sampling, physiological measurements were performed on the same trees with a Licor. In addition, also pre-dawn leaf water potentials were measured every third day. Finally, also micro cores were taken several times before, during and after the experiment for monitoring of xylem cell growth as a basis for high-resolution tree-ring isotope analysis at a later project phase. From all soil, needle, twig and stem core samples water was extracted by cryogenic vacuum distillation and d<sup>18</sup>O and d<sup>2</sup>H measured.</p><p>The data of this experiment together with mechanistic modelling will elucidate the spatiotemporal pattern of soil water dynamics, water uptake by roots and tree-water relations of two species that have ecologically different life forms but are both highly representative for subalpine regions. Understanding their ability to react and capitalize on soil rewetting after dry periods will be crucial for the estimation of their survival potential and competitiveness under future dry and wet extreme events.</p>
<p>Water uptake under variable soil water supply is highly critical for the functioning of trees and the services provided by forests. Current climate projections predict an increasing variability of precipitation and thus a higher frequency of droughts alternating with extreme precipitation events. Reduced water availability is the most critical driver for tree mortality and impairment of trees&#8217; functions. Under variable water supply, both the ability of a plant species to utilize remaining water under drought and to immediately capitalize on soil rewetting from subsequent rainfall events will be crucial for its survival and competitiveness. High uncertainty still exists regarding the ecohydrological belowground interactions at the soil&#8211;root interface on short to seasonal time scales.</p><p>To overcome previous limitations, we carried out high-resolution <em>in situ</em> observations of &#948;<sup>18</sup>O&#160;in soil and xylem water to track the water uptake of beech trees based on the approaches of Volkmann et al. (2016a & b) in the hot dry summer 2018. We set up a laser isotope system to continuously probe the &#948;<sup>18</sup>O signature in the water vapor in equilibrium with the soil water at different soil depths and with the xylem of beech trees in a forest in Switzerland and applied a Bayesian isotope mixing model (BIMM) to resolve the origin of the water taken up. Moreover, we installed xylem flow sensors, dendrometers and soil moisture sensors in the trees.</p><p>Mid of June the drought period started with extended phases of high temperature and only infrequent precipitation. At the same time, soil water content sharply decreased, especially in the upper soil layers and transpiration as well as radial growth started to decline, and this pattern became more pronounced until the end of August. In the soil water, strong <sup>18</sup>O enrichment in the upper 5 cm and slighter enrichment in 15 cm developed during this period. The BIMM results indicated that tree xylem water was made up by > 80% of shallow soil water (0-15 cm) at the onset of the drought and that this contribution continuously dropped to < 20% by the end of August, when deeper soil water and groundwater became more important. End of August, intensive rainfall events along with decreasing temperatures terminated the drought period when shallow soil water pools became partially replenished, and transpiration increased again. Within days, the contribution of shallow soil water to tree xylem water increased and reached a share of > 70% a couple of weeks after the end of the drought. &#160;With the<em> in situ</em> method applied here, real-time information of the plasticity of soil water use becomes available and we can l trace the effect of drought and drought release on root activity of trees in different soil depths.</p><p>&#160;</p><p><strong>Volkmann THM, Haberer K, Gessler A, Weiler M. 2016a.</strong>High-resolution isotope measurements resolve rapid ecohydrological dynamics at the soil&#8211;plant interface. The New phytologist<strong>210</strong>: 839-849.</p><p><strong>Volkmann THM, K&#252;hnhammer K, Herbstritt B, Gessler A, Weiler M. 2016b.</strong>A method for in situ monitoring of the isotope composition of tree xylem water using laser spectroscopy. Plant, Cell and Environment<strong>9</strong>: 2055&#8211;2063.</p>
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