Water potential gradients drive water fluxes, while plant hydraulic traits modulate them, shaping ecosystem function. Yet, traditional methods for measuring plant water potential (e.g., pressure chambers) are destructive and labour-intensive, yielding sparse data with low temporal resolution. This contrasts with high-resolution environmental data (e.g., soil moisture, climate), creating a mismatch that limits our understanding of plant water dynamics. We address this gap by evaluating a novel microtensiometer for continuous stem water potential (y stem) monitoring. This collaborative study utilized data from 21 microtensiometers across three deciduous species ( Fagus sylvatica, Fraxinus excelsior, Carpinus betulus) and 31 soil matric potential sensors over the course of several months during the vegetation period of 2023. We demonstrate that microtensiometers deliver reliable results for the tested deciduous tree species. The microtensiometer measurements agree with the classical pressure chamber method in Fagus sylvatica and Carpinus betulus. By integrating high-frequency soil matric potential (y soil) data at multiple soil depths, meteorological variables, we further assessed the primary drivers of y stem across sites using boosted regression trees. The key driver of hourly y stem in this climatologically average summer, was species and y soil with upper soil layers exerting greater influence during the day and deeper layers becoming more important in the evening. These findings emphasize the potential of microtensiometers to advance plant hydraulics research by offering a continuous, cost-effective, and minimally invasive approach to monitoring water potential dynamics in forest ecosystems.

Water potential is a crucial parameter for assessing tree water status and hydraulic strategies. However, methods for measuring water potential, such as the Scholander pressure chamber, are destructive and discontinuous, and difficult to perform in tall forests. Consequently, important dynamics in water potentials, particularly during short-term drought, are difficult to capture. Recent advancements have introduced low-maintenance sensors capable of measuring continuous, high-resolution stem water potentials. If applicable to forest trees, such sensors hold the potential to significantly enhance our understanding of tree water relations. We evaluated these sensors in a temperate, diffuse-porous tree species ( Carpinus betulus) over a growing season marked by dry-down periods and heat. Concurrent measurements of branch water potential, sap flow, and environmental factors (air temperature, vapor pressure deficit, and soil water content) were conducted. Midday stem water potentials of C. betulus reached minimum values of -3.39 ± 0.10 MPa and exhibited pronounced seasonal fluctuations, mirroring changes in environmental conditions and sap flow. Comparison of stem water potentials with Scholander-type measurements revealed a very good correlation with predawn (R 2 = 0.98) and a general agreement with midday measurements (R 2 = 0.71). Diurnal variations in stem water potentials and sap flow exhibited a hysteresis, consistent with other plant parameters. In this first assessment, the agreement with Scholander-type measurements, sap flow, and environmental parameters suggests the tested water potential sensors yield reliable data. If applicable to other tree species, including conifers, these sensors could significantly advance our understanding of tree water relations and their role in forest drought responses.

Knowledge of plant hydraulic strategies (isohydric vs anisohydric) is crucial to predict the response of plants to changing environmental conditions, such as climate-change induced extreme drought. Several abiotic factors, such as evaporative demand, have been shown to seasonally modify the isohydricity of plants, however, the impact of biotic factors, such as plant-plant interactions on hydraulic strategies has seldom been explored. Here, we investigated adaptations in hydraulic strategies for two woody species in response to seasonal abiotic conditions, experimental drought and plant invasion in a Mediterranean cork oak (Quercus suber) ecosystem with a combined shrub invasion (Cistus ladanifer) and rain exclusion experiment. From summer to winter, the degree of isohydricity shifted from partial isohydric to anisohydric in Q. suber and inversely from strict anisohydric to partial isohydric for C. ladanifer. During drought, plant invasion significantly modified the hydraulic strategy of invaded Q. suber to a higher degree of anisohydricity with severe negative consequences for tree functioning, implying progressive leaf and xylem damage. The rain exclusion alone led to a non-significant increase in anisohydricity for both species. We demonstrate that the degree of isohydricity of plants is dynamically determined by the interplay of species-specific hydraulic traits and their abiotic and biotic environment.