Root water uptake depth in temperate forest trees: species‐specific patterns shaped by neighbourhood and environment

Root water uptake regulates plant transpiration and internal water supply. Forests in Central Europe increasingly face water limitations, highlighting the need to understand tree water sources and consumption. However, knowledge about water uptake depth and how it varies with tree species identity, neighbourhood, site, and environmental conditions remains scarce. We used stable water isotopes to study water uptake depth in pure and mixed stands of European beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii), and Norway spruce (Picea abies). Investigations included natural abundance sampling at four sites, and a weekly subsoil tracer experiment (1 m mineral soil depth) at one of these sites. European beech and Douglas fir accessed relatively deeper water than Norway spruce. Beech used deeper water in mixtures with both conifers, while spruce shifted to shallower layers in mixture with beech. Douglas fir showed no significant difference between pure and mixed stand. Deepest uptake occurred on well-drained, sandy soil, while shallowest uptake was on an occasionally stagnic soil. The tracer experiment revealed a relatively low contribution from subsoil water (>1 m, ca. 9%); the organic layer contributed similar magnitudes, but varied more dynamically with tree water consumption. Our results emphasize the importance of species-specific traits and mixture effects in forest water cycles, and how they are mediated by site and environmental conditions. Douglas fir appears more drought resistant than Norway spruce, by accessing deeper water sources. We conclude that beech and Douglas fir may equally coexist, while beech presence exacerbates the drought exposure of spruce.