Are temperate tree species able to adjust root-water uptake depth during drought?

<p>Temperate trees are facing an increasing challenge from limited water supply and increased atmospheric demand due to an extreme trend in drought events. Particularly, structural-morphological traits such as root-water uptake depth (RWUD) are key for determining the performance of a species during drought. In addition to RWUD, ultimately understanding the temporal origin of the soil water used by trees could provide important insights into a species drought vulnerability.</p> <p>The goal of this study is to determine the ability of species to adjust RWUD during drought. For this purpose, we analyzed &#8706;²H and &#8706;¹⁸O isotope data from xylem water samples collected from mature trees of nine co-occurring temperate tree species growing at the Swiss Canopy Crane II site in Switzerland, over three growing seasons (2020 &#173;&#8211; 2022). Additionally, we analyzed bi-weekly water samples from precipitation, throughfall, and soil water at different depths. Both 2020 and 2022 were relatively dry (compared to the reference of 1991-2021), while 2021 had a wet summer, allowing us to identify RWUD dynamics within a wide range of soil water supply. We applied the LWFBrook90.jl hydrological model to assess RWUD dynamics as response to variability in soil moisture and soil water potential in high temporal resolution.</p> <p>Initial results revealed distinct species-specific differences in the summer &#8706;²H and &#8706;¹⁸O isotopic signals. Xylem-water &#8706;²H and &#8706;¹⁸O values of <em>Quercus sp</em>., <em>Fraxinus excelsior</em> and <em>Sorbus torminalis</em> were the lowest and coincided with soil water values in deeper soil. These patterns were contrasted by <em>Carpinus betulus</em>, <em>Picea abies</em> and <em>Abies alba</em> showing highest &#8706;²H and &#8706;¹⁸O values and a suggested predominant RWU in the top soil. <em>Fagus sylvatica</em> and <em>Pinus sylvestris </em>showed variability within their values suggesting RWUD adjustment.</p> <p>The initial findings already indicate different rooting strategies as well as a certain level of resource partitioning within the rhizosphere. We expect the final results of this study to have significant implications for understanding the mechanisms behind drought vulnerability of temperate trees and for improving the prediction of tree species' responses to an altered hydrological regime.</p>