Critical below-ground drought effects on temperate trees – insights from six years of ecophysiological monitoring and two years of rain exclusion

Climate change, including a reduction in precipitation, increased atmospheric moisture demand, and drying soils, threaten the life-supporting function in trees. In response, trees can exhibit different below-ground drought acclimation strategies, including increased root-water uptake depth and root growth to increase water supply. We initiated a long-term monitoring experiment at the Swiss Canopy Crane II (SCCII) site in Switzerland in 2018, including a rainfall exclusion of 50% during the vegetation period (April-October) since 2023. The SCCII site provides growing conditions representative of a central European mid-mountain range forest and hosts 10 co-occurring European temperate tree species. For six years, we measured the δ 2 H and δ 18 O values of samples collected from tree xylem, soil water in different depths, and precipitation, as well as a multitude of ecophysiological measurements within a great range of environmental conditions (wet and dry), including the exceptionally dry summer in 2023. The extreme conditions in 2023 caused canopy dieback and mortality in individuals of Fagus sylvatica , Picea abies , and Abies alba in the drought treatment indicating critically low soil water supply. We utilized the data to parameterize the hydrological model LWFBrook90.jl with the goal of simulating soil moisture, soil water potential and soil water isotope transport, as well as root water uptake depth under different environmental conditions. By quantifying the temporal origins of root water uptake and running scenarios of further increased drought conditions, we will quantify the access of different tree species to soil water from various depths and the soil water residence time. Moreover, we will quantify how soil water residence time and, thus, the supply of water to tree species at different soil depths varies under different climate change scenarios. First results show that depending on the severity of drought and tree water consumption, the soil water is used up almost entirely within one growing season indicating the vital role of summer precipitation and winter-time refilling. We expect the final results of this study to provide us with valuable insights on soil water retention time and the temporal dynamics of root water uptake under various drought conditions. These findings will increase our understanding critical below-ground drought effects and acclimation of temperate trees.