Comparative Water Use Strategies of European beech and Douglas fir: Influence of Climate, Soil Water Availability and Canopy Traits in a Temperate Forest Stand in Central Germany

Ongoing climate change is leading to more frequent and severe droughts in Central Europe, making it vital to understand how different tree species control water consumption. Given the increased susceptibility of native timber species in central Europe, several non-native species are under trial. One among them is Douglas fir (Pseudotsuga menziesii Mirb.), which is often considered a more drought resilient species compared to other native species in central Europe. However, we still lack a clear understanding of species water-use strategies and how they respond to dry air (vapor pressure deficit), drying soils, and tree structure. To address this, our study directly quantified sap flux density (Js) in non-native Douglas fir and native European beech (Fagus sylvatica L.) and analyzed their responses to key environmental, soil and structural drivers in a clayey loamy soil in Central Germany. During the 2025 growing season in a temperate Central German forest, sap flow velocity (Vh) was measured continuously at two sapwood depths (0.5 and 1.5 cm) across 18 mature trees (10 beech, 8 Douglas fir) using the dual method approach (DMA). Daily sapflux density was analyzed against local microclimate variables, continuous soil moisture profiles at 5, 20, 50 and 100 cm depths, and tree structural traits such as crown volume, exposed crown area, tree height, and stem diameter. Across the season, sap flux density was significantly higher in European beech than in Douglas fir (p<0.001). Beech continued to increase sap flux as vapour pressure deficit rose, even up to 5.0 kPa, while Douglas fir imposed a control cap on water loss, restricting flow once VPD exceeded 3.0 kPa. In both species, the inner sapwood at 1.5 cm depth carried greater sap flux. Beech also showed a much longer time lag between changes in VPD and the sap-flow response (median 45 minutes) compared to Douglas fir (median 10 minutes). When relating sap flux to soil moisture, matric potential emerged as a better predictor than volumetric water content in both species. In beech, sap flux density scaled with tree size (DBH and height each explaining ~50% of between-tree variance), whereas in Douglas-fir no structural trait explained more than 10%; yet both species showed high tree-to-tree variability (CV 34-38%), driven by size in beech but not in Douglas-fir. The two species, however, drew water from different parts of the soil profile: beech extracted moisture fairly evenly across depths, whereas Douglas fir tapped predominantly into deeper layers. Taken all together, these patterns highlight contrasting hydraulic strategies, a more anisohydric, capacitance-buffered system in the broadleaf and a tightly regulated, leaning towards more isohydric behaviour. Beech increases sap flux density under favorable conditions through efficient hydraulic conductance and capacitance. However, its dependence on water throughout the soil column makes it more susceptible to extended drought. Douglas fir, by contrast, with lower flux density and its strict regulation with increasing atmospheric dryness complements its isohydric water use regulation by strong stomatal control and access to water at deep profiles. These contrasting strategies show fundamental ecohydrological trade-offs that should guide species selection for forest management. In contrast to a study from sandy soil, the present work in loamy soil suggests contrasting results with beech showing higher flux density compared to Douglas fir. Further analysis is essential to also compare the results in a more systematic manner by upscaling the water use at stand level and analyze how stand characteristics such as soil type, stand density and tree structure influence water use. Overall, the present work highlights the importance of studies at a given site condition and management level which is important to tailor site specific forest management.