Continuous Stem Water Potential Measurements with Microtensiometry Reveal Species Identity and Soil Matric Potential Control of Stem Water Potential in Temperate Forests

Water potential gradients govern water fluxes, and plants respond with species-specific hydraulic traits that influence ecosystem function. While understanding these traits is key to predicting vegetation responses to climate change, traditional methods like the pressure chamber limit temporal resolution and continuity. Additionally, suitable methods for continuous monitoring remain limited. We addressed this gap by evaluating the potential of microtensiometers for continuous stem water potential (psi stem) monitoring in mature forest trees across three temperate sites. We collected psi stem data over the 2023-2024 vegetation periods from 21 microtensiometers installed in Fagus sylvatica, Fraxinus excelsior and Carpinus betulus, alongside high-frequency soil matric potential (psi soil) measurements and meteorological data. Microtensiometers provided realistic daily cycles and good agreement with pressure chamber-derived leaf water potential (psi leaf). Crucially, our results show that psi soil, next to species identity, is one dominant driver of stem water potential across sites. Diurnal patterns showed that psi stem is driven by shallow soil layers during the day, when strong transpirational pull creates more negative psi stem, enabling access to these drier upper soils. At night, when transpiration ceases and psi stem becomes less negative, deeper, wetter soil layers predominantly supply water, reflecting the physical constraints of water flow within the root zone. This soil control was consistent across all species and sites under moderately dry to normal conditions. We further show that psi stem data can support broader physiological analyses, including refining tree water deficit-water potential relations and highlighting spatial variability in root water uptake. Our findings indicate that horizontal and vertical psi soil heterogeneity must be accounted for to avoid oversimplified interpretations of plant water use. This study demonstrates that microtensiometers can provide continuous, nondestructive measurements of plant water status and, when carefully installed and continuously monitored, can reveal meaningful ecological patterns while also requiring attention to methodological constraints. Most notably, we show that species identity and soil moisture regulate stem water potential, underscoring the central role of soil-plant interactions in shaping forest water dynamics under changing climatic conditions.