Soil water uptake of larch and spruce recorded by stable isotope tracing in the Swiss Alps

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.

We will present results from a holistic tracer irrigation experiment in the Lötschental, Swiss Alps. A subalpine forest plot (150 m²) of Larix decidua and Picea abies was irrigated with, relative to natural soil abundance ¹⁸O and ²H 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 d¹⁸O and d²H measured.

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.