Advances in in situ measurement methods, remote sensing and growing monitoring networks provide unprecedented information on the drivers of drought and plant responses. The automation and increasing affordability of new measurement methods are generating a wealth of new data, presenting a unique opportunity for modellers to improve simulations of water transport across the soil–plant–atmosphere continuum. In particular, this has the potential to improve the representation of plant-water interactions across scales–from the leaf-level to entire terrestrial ecosystems–along with more accurate carbon, water, and energy fluxes on land.
Despite these recent advances, transferring knowledge across disciplines (e.g. climate science, ecology, agronomy, hydrology and soil science) and scales (from individual leaves, plants, and up to the ecosystem and regional scales) remains a major challenge that hinders progress in drought research. This session aims to provide a platform to connect and exchange knowledge and establish links across scales and disciplines. We encourage interdisciplinary contributions that bring together a wide range of perspectives and, in particular, contributions led by students and early career researchers.
Soil water availability is a critical factor for plant transpiration and photosynthesis. As soils dry, their hydraulic conductivity declines, limiting water supply to the roots and ultimately constraining the whole water flow through the soil–plant-atmosphere continuum. To investigate where and when this limitation arises, we combined several experiments across scales and degrees of complexity.
Neutron radiography, a non-invasive technique that directly visualizes water distribution in soils, applied to young maize plants revealed sharp water potential gradients forming in the rhizosphere during drying, showing local depletion around roots. These rhizosphere-scale dynamics are tightly coupled to reductions in transpiration. The onset and severity of this hydraulic bottleneck depend strongly on soil texture: in sandy soils, weak capillary forces lead to early hydraulic breakdown at comparatively high water potentials (relatively wet conditions), whereas loamy soils sustain water supply over a wider drying range.
Plants can transiently buffer this process through the release of extracellular polymeric substances that enhance root–soil contact and displace depletion zones away from the root surface. However, this buffering delays rather than eliminates hydraulic disconnection. Analogous thresholds are observed in trees under field conditions, where individuals growing in sandy soils close stomata at higher soil and leaf water potentials than those in finer-textured soils.
Together, these converging observations point to universal, texture-dependent thresholds controlled by rhizosphere processes. By linking pore-scale hydraulics to whole-plant responses, this work positions the rhizosphere as a central regulator of plant water use and a key, yet often overlooked, determinant of ecosystem drought sensitivity.
How to cite: Di Bert, S.: Where Does Drought Begin? Linking Rhizosphere Processes and Forest Hydrology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12573, https://doi.org/10.5194/egusphere-egu26-12573, 2026.
Plant adaptive traits that regulate water transport from soil to leaf are essential for maintaining gas exchange and productivity, especially under drought conditions. Yet, how such traits respond to contrasting soil textures remains unclear. Here we grow maize (Zea mays) and sunflower (Helianthus annuus) in contrasting soil textures, namely, sand and loam. We measured transpiration rate, soil-plant hydraulic conductance and abscisic acid (ABA) concentration during soil drying. At the end of the experiment, root systems were extracted, scanned and analyzed for morphological traits. We showed that, during soil drying, maize and sunflower adopt distinct root strategies to regulate root water influx under two contrasting soil textures (sand vs. loam). In sand, maize increased root diameter by 60% without altering root length, while sunflower increased root length by threefold compared to loam. These changes moderate the flux of water into root per unit surface area, buffering soil–plant hydraulics across soil textures. Interestingly, ABA concentration decreased with increasing root length in sunflower, with higher levels in loam (shorter roots) and lower levels in sand (longer roots), whereas maize showed no substantial variation in ABA levels between soil textures. Notably sunflower exhibited three times higher transpiration, highlighting the need to adapt to soil hydraulic limitations, particularly in sand, where hydraulic conductivity declines steeply upon drying. These observed species-specific patterns underscore that root trait plasticity might be complemented by hormonal regulation of stomatal conductance in maintaining water balance under soil drying. Taken together, our findings demonstrate that contrasting root morphological adjustments can achieve functional vantage maintaining plant water balance across soil textures, highlighting the importance of root plasticity for coping with edaphic drought.
How to cite: Abdalla, M., Christmann, A., Gigl, M., Dawid, C., and Ahmed, M.: Contrasting belowground strategies between maize and sunflower to adapt to soil texture, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10913, https://doi.org/10.5194/egusphere-egu26-10913, 2026.
Understanding of water transport within soil plant atmosphere continuum (SPAC) is essential for predicting tree functioning under changing environmental conditions. Xylem water potential (Ψxylem) reflects the energy state of water in plants, yet continuous monitoring has long been technically challenging, limiting insights into tree response to drivers such as vapor pressure deficit (VPD), soil water availability, and stem water storage. In a throughfall exclusion experiment (KROOF site, Germany) we continuously measured water fluxes and Ψxylem in mature beech trees along the entire SPAC. Our objectives were to quantify influence of soil water potential (Ψsoil), stem water storage and VPD on tree water uptake and Ψxylem over diurnal cycles, and to test whether stem water storage predicts the hysteresis relationship between Ψxylem and sapflow (J). We installed soil water potential and water content sensors in four different soil depths and took soil samples for natural abundance δ2H and δ18O isotopes to assess water uptake depths. Xylem water potential was measured continuously with microtensiometers (FloraPulse) at breast height and at the lower end of the crown, where we also installed sapflow sensors and point-dendrometers. We used 24 XGBoost models, separated by hour and calculated SHAP values to provide information about the importance of soil water potential, stem water storage and VPD on Ψxylem generally and over a diurnal cycle. We determined stem water storage using detrended (daily centered) and scaled dendrometer data (SDV) and calculated a mixed model to investigate its relationship of min and max Ψxylem values combined with Ψsoil. Finally, we computed XGBoost models to predict Ψxylem hysteresis with J, J + SDV as well as J + SDV + Ψsoil. Our models show a strong impact of SDV and VPD on Ψxylem while the impact of Ψsoil was marginal. Water uptake occurred mainly from upper soil layers (0-30 cm depth) but Ψsoil of depth 30 and 50 showed the largest impact on Ψxylem. Diurnally SDV and VPD had the biggest impact, while there was no shift in importance of different soil depths on Ψxylem. We observed a linear relationship between min and max Ψxylem and SDV. At breast height, we found a significant interaction with Ψsoil, while this was not observed in the lower crown. Sapflow as a single predictor for Ψxylem showed a direct relationship while SDV in addition was able to predict the daily hysteresis of Ψxylem. Water uptake was only weakly depended on Ψsoil, possibly because the observed trees were not limited by water supply. SDV, which can be seen as a proxy for stem water storage, seemed to be a main factor predicting Ψxylem. The influence of Ψsoil on SDV at breast height and its absence in the lower crown could show that storage status may vary within the tree. SDV, in addition to sapflow, is able to provide a second axis of information to also predict hysteresis curves between daily extremes.
How to cite: Humpert, J., Hafner, B. D., Wilms, F., Peters, R. L., Grams, T. E. E., and Zare, M.: Investigation of high resolution stem water potential in mature beech trees and relationships to water supply, demand and storage, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20583, https://doi.org/10.5194/egusphere-egu26-20583, 2026.
Stomatal conductance plays a key role in the exchange of carbon and water between vegetation and the atmosphere, by controlling photosynthesis and transpiration in plants. However, its representation in land surface models (LSMs) is still considered a major source of uncertainty. The Medlyn model is widely used in LSMs to describe how strongly stomata and the model parameter g1 respond to carbon uptake and atmospheric dryness. This parameter is often introduced as a fixed value in global LSMs for broad vegetation types despite evidence that stomatal behaviour varies with plant water availability, particularly in water limited ecosystems such as those found in large parts of Australia.
We estimated g1 for Australian vegetation using observational data obtained from three intrinsic water use efficiency techniques: leaf gas exchange, stable carbon isotope discrimination, and eddy covariance. These approaches together can provide a comprehensive knowledge on the estimation of g1 across a range of spatial and temporal scales, from leaf to ecosystem and from short to long term responses. To account for water stress, we relate g1 to soil moisture for both leaf-scale gas exchange and eddy covariance datasets, where direct plant water status measurements are rarely available. For the stable isotope dataset, water stress is represented using an aridity index that reflects longer-term water limitation experienced by plants over the period of carbon assimilation.
We compared g1 estimates from these datasets along with soil moisture data to observe the shifts in the sensitivity of stomata under dry conditions and to determine consistency between scales. We found that g1 varied systematically across Australian plant functional types (PFTs), with lower values in xeric shrubs and C4 grasses and higher values in savanna and rainforest trees. Relative differences among PFTs were consistent across methods, but isotope-derived g1 values were generally higher than leaf gas exchange estimates. Eddy covariance data from Australian flux-tower sites showed a clear increasing trend in g1 with increasing soil moisture, and isotope-derived g1 decreased with increasing aridity, indicating more conservative stomatal behaviour under dry conditions.
These findings will be used to generate representative values of g1 for Australian PFTs that can be implemented in land surface models (e.g., The Joint UK Land Environment Simulator JULES) for evaluation at flux-tower sites and the continental scale.
How to cite: Al-Naseri, R., Medlyn, B., Stephens, C., Tian, S., Williams, L., and Marchionni, V.: Estimating the stomatal slope (g1) parameter from the Medlyn model across Australian vegetation using multiple observational datasets, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8726, https://doi.org/10.5194/egusphere-egu26-8726, 2026.
Droughts have emerged as the primary driver of forest disturbances across Europe in the 21st century, significantly impacting both tree growth dynamics and mortality rates. Tree species are differently affected under drought, and these differences are related to species-specific plant hydraulic traits that govern water storage, hydraulic conductivity, and stomatal regulation. However, quantifying variability in these hydraulic traits across sites, species, and time remains challenging, as site measurements have historically rarely been comprehensive enough to assess the evolution of plant hydraulic behavior under drought stress. New continuous, high temporal resolution observational plant hydraulic data paired with process-based plant hydraulic modelling opens an opportunity to address this gap, by providing a framework to test and quantify theories based on first principles across species and sites.
In this study, we apply the terrestrial biosphere model QUINCY, augmented by a recently developed plant hydraulic architecture module, across three eddy covariance sites in Germany covering broadleaved forest species (Aplern, Hainich, and Hartheim). The model is parameterized for three common temperate tree species present at the aforementioned sites. We constrain QUINCY across these species and sites using 30-minute resolution stem water potential measurements collected during the summer and autumn of 2023. Our results show that two groups of model parameters explain most of the simulated plant water potentials: parameters controlling plant water uptake from soil (plant ability to extract water from soil and the root distribution), and parameters regulating stomatal sensitivity to pre-dawn leaf water potential. Across species, we find ash to be more drought resistant than beech and hornbeam, as it closes its stomata earlier than other species under similar levels of drought stress, and it is characterised by a higher hydraulic capacitance per unit stem volume. Our study demonstrates how integrating the new generation of in situ plant hydraulic observations into vegetation models can facilitate the quantification of species-specific hydraulic parameters, effectively reducing uncertainty in, and providing robust constraints on, modelled responses to drought.
How to cite: Duanmu, Z., Papastefanou, P., Sabot, M., Hildebrandt, A., Magh, R., Haberstroh, S., Werner, C., Nelson, J., and Zaehle, S.: Characterizing plant hydraulic behaviour under drought stress using vegetation modelling , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21061, https://doi.org/10.5194/egusphere-egu26-21061, 2026.
Droughts threaten ecosystems worldwide and are projected to occur more frequently and with greater intensity in the future. Accurately projecting ecosystem responses to these climate extremes relies on vegetation models. While process-based models are evolving fast and many models now represent plant hydraulic processes, including these mechanisms often comes at the cost of an increase in the number of difficult-to-constraint model parameters. Concurrently, recent experimental advances open a new avenue for parameter constraining, by providing high-temporal resolution data on plant hydraulic variables, for example continuous in situ measurements of water potential and sap flux. However, much of this novel data has not yet been considered by vegetation models.
Here, we utilize a rare, comprehensive time-series of data obtained in the Hainich experimental forest, specifically high-temporal resolution datasets of (1) sap flux, (2) stem water potential, (3) Net Ecosystem Exchange (NEE), and (4) Evapotranspiration (ET). With this data spanning both the water and carbon axes of plant function, we constrain the terrestrial biosphere model QUINCY and the latest development of its plant hydraulic architecture. We find that integrating such complementary experimental data yields three key outcomes. First, it evaluates the physical representation of plant hydraulic theory within the model. Second, it results in tighter constraints on plant-hydraulic parameters. High-temporal resolution water potential and sap flow data are vital here, as they resolve the diurnal lags necessary to identify capacitance parameters that remain unidentifiable under daily or weekly sampling. By capturing these fast-response dynamics, the model not only narrows parameter uncertainty but also reveals critical functional interdependencies and correlations that define plant hydraulic strategy. Third, these constraints yield more robust projections by significantly reducing the variability in simulated stocks and fluxes under future climate scenarios. We conclude that the growing availability of continuous data from novel physiological sensors is essential to constrain and build trust in increasingly complex vegetation models, as demonstrated here for plant hydraulics.
How to cite: Papastefanou, P., Donfack, L., Klosterhalfen, A., Knohl, A., Magh, R.-K., Paligi, S. S., Sabot, M., Schellenberg, K., and Zaehle, S.: Improving model robustness to drought stress by constraining plant hydraulics with complementary in situ measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18010, https://doi.org/10.5194/egusphere-egu26-18010, 2026.
Forests provide critical ecosystem services, including timber production and climate and water regulation, but these are increasingly threatened by climate-driven disturbances such as drought. The 2018 Swedish drought exemplified this risk, causing extensive wildfires, a severe spruce bark beetle outbreak, and reduced forest productivity. Projections for Nordic countries indicate warmer conditions and more frequent and intense droughts, highlighting the need for tools that can accurately predict such impacts to support adaptive forest management. We evaluated recent LPJ-GUESS developments for simulating drought impacts in Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) in Sweden. We evaluated two LPJ-GUESS versions: a European-optimized version with updated trait and structural parameters for major European tree species, and a second version that adds a mechanistic plant hydraulic scheme to this same parameterization, enabling representation of contrasting isohydric and anisohydric stomatal strategies. Model outputs were evaluated against high-resolution carbon and water flux data from three Swedish ICOS sites and against National Forest Inventory growth records. Preliminary results show that the combined version better captures the 2018 drought signal observed in carbon flux data but does not necessary yield improvements in annual fluxes of gross primary production and evapotranspiration. We conclude with an outlook for steps to improve simulations of drought stress in Nordic forests.
How to cite: Gomes de Almeida, F., Brinkhoff, R., Akselsson, C., Kljun, N., and Pugh, T.: Evaluating LPJ-GUESS for Simulating Drought Responses in Swedish Forest, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1194, https://doi.org/10.5194/egusphere-egu26-1194, 2026.
Protecting and restoring peatlands requires quantitative measures of ecosystem health, function, and resilience, across vast, remote areas and long time periods. Peatlands are experiencing higher temperatures and more extreme patterns of rainfall, making monitoring their vegetation health increasingly urgent. Remote sensing provides a uniquely scalable tool for monitoring peatland health and restoration at national scales, but to unlock its full potential we must understand the biological processes behind the wealth of data.
We address this challenge through controlled ground-truthing experiments that investigate how heat and water stress affect the thermal, reflectance, and fluorescence signals of peatland vegetation over time. By integrating physiological measurements with optical remote sensing and emerging high‑resolution thermal imaging technologies, we aim to establish mechanistic links between peatland vegetation stress responses and remotely sensed signals.This project focuses on the Sphagnum genus, a keystone genus in peatland formation and persistence. Understanding how thermal and optical signals across different Sphagnum species respond under heat and drought stress is critical for developing operational methods of remotely sensing peatland health in a changing climate.
By linking physiological responses to stress with combined thermal and optical remote sensing signals, our research will enhance our ability to harness Earth observation and machine learning advances to monitor, protect, and restore peatlands as critical ecosystems for climate mitigation.
How to cite: Geldbach Jones, A., Royles, J., and Kromdijk, J.: Ground‑Truthing Remote Sensing of Peatland Vegetation Under Heat and Drought Stress, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22511, https://doi.org/10.5194/egusphere-egu26-22511, 2026.
Energy and water availability are essential controls on terrestrial ecosystem functions. Recent studies suggest widespread shifts from energy- to water-limited conditions under global warming. We demonstrate that incorporating a thermodynamically appropriate energy indicator fundamentally changes this projection. Surface energy availability for evapotranspiration is primarily determined by net radiation rather than downwelling shortwave radiation or air temperature, as supported by both theory and observations. Using this improved framework, we find no projected net increase in terrestrial ecosystem water limitation under greenhouse warming. Instead, projected bidirectional transitions between water- and energy-limited conditions exhibit comparable magnitudes, with a slight net reduction in the water-limited regime in 1.4% to 2.9% of global warm land areas. These findings are consistent with patterns reported in other ecohydrologically based studies and are supported by empirical evidence of reduced vegetation sensitivity to dry conditions under elevated CO2. Our study bridges ecological and physical theories to improve ecosystem water-energy limitation analysis and provide a clear mechanistic understanding of future ecosystem dynamics.
How to cite: Xu, J., Fan, Y., McColl, K. A., Berg, A., Liang, Y., and Yang, J.: Global ecosystem water limitation under warming driven by energy constraints and physiological CO2 effects, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4972, https://doi.org/10.5194/egusphere-egu26-4972, 2026.
The increase in severity, frequency, and global footprint of droughts highlights the urgent need to improve our understanding of plant responses to changes in water availability. This requires better integration and synergy of knowledge from different scientific communities in the field of plant-water interactions, primarily: land-climate modelers, ecophysiologists, and soil hydrologists. Each community describes the soil–plant–atmosphere continuum using different primary variables, focusing on different spatial and temporal scales, invoking different key assumptions, and using different concepts and model structures. A separation between disciplines hinders exchange and thus limits scientific progress. If, on the other hand, dialogue can be established at these interfaces, then this methodological diversity could allow key processes such as stomatal and hydraulic regulation of plants, water storage below the surface and in vegetation, and material flows in the ecosystem to be brought together and deciphered holistically.
In this presentation, we synthesize evidence from different angles and scales including leaf water status, soil moisture and ecosystem-scale observations to identify missing links and consistent thresholds that can connect plant-water relations across disciplines.
How to cite: Giardina, F., Deluigi, J., Martinetti, S., and Carminati, A. and the Monte Verità Team: Connecting plant-water interactions across scales and disciplines, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22728, https://doi.org/10.5194/egusphere-egu26-22728, 2026.
Flash drought (faster-developing drought) has been pervasively intensified, posing detrimental constraints on vegetation productivity. However, the divergence in the underlying drivers governing vegetation productivity responses to flash and slow droughts (slower-developing droughts) remains unknown. We quantified the dominant drivers underlying vegetation productivity resilience (the departure of post-drought productivity anomalies to the long-term mean) to both flash and slow droughts. There exhibited significantly lower productivity resilience to flash drought at flash drought hotspots than non-hotspots. Carbon dioxide fertilization effect exerted the greatest positive effect on productivity resilience to both flash and slow droughts, although that effect was smaller under flash droughts. The productivity resilience to flash drought was more sensitive to reduction in productivity anomaly and intensified climate stress than slow drought at flash drought hotspots. This study highlights the increasing risk of flash drought spread on global ecosystem productivity resilience.
How to cite: Guo, R., Wu, X., Wang, P., Chen, T., Chen, X., Cai, J., Wang, X., Zhang, Z., Meng, Z., and Liu, Y.: Increased Spread of Global Flash Droughts Threatens Vegetation Productivity Resilience, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2323, https://doi.org/10.5194/egusphere-egu26-2323, 2026.
During drought periods, the survival of trees and forest ecosystems strongly depends on soil water reserves. Isotopic studies show that during droughts, trees can use deep soil water resources (Carrière et al, 2020). However, the quantification of accessible water reserves to trees has so far mostly considered surface layers. A quantification of deep water resources accessible to and used by trees is missing.
In this work, we considered the total amount of water trees can access and use, defined as “TAW” (Total Available Water). TAW includes both root access to water and the ability of trees to take up this water. Our objective was to quantify TAW and to analyse its variability across four French forest sites: two temperate sites (Barbeau and Hesse), dominated respectively by Sessile Oak with Hornbeam and by Beech, and two Mediterranean sites (Puéchabon and Font-Blanche), dominated respectively by Holm oak and Aleppo Pine.
We used complementary approaches to quantify TAW: (i) the application of pedotransfer functions (Szabó et al., 2021) on soil core analyses; (ii) the analysis of soil moisture profiles obtained from sensors installed at different depths (Maysonnave et al. 2022) ; and (iii) the calculation of cumulative water deficit based on evapotranspiration measurements from flux towers (Giardina et al., 2023).
Our results show that TAW is higher in temperate forests than in Mediterranean forests. This difference is strongly linked to soil depth and to the proportion of stones in the soil. Other factors also play a role, especially the leaf water potential at the wilting point, which is lower in Mediterranean forest species. This allows these species to absorb water more efficiently during drought and increases their effective water availability. A strong intra-site variability of TAW was also observed, with coefficient of variation ranging from 8% to 60% depending on the site (for method (iii)).
Each of the three methods used in this work has its own limitations for estimating TAW. By using a combination of these three methods, we obtained complementary information and a more robust estimation of TAW and deep water reserves accessible to trees at the study sites. These approaches can contribute to mapping soil water stocks in France and to modelling the future of forests under climate change.
How to cite: Grasset, M., Limousin, J.-M., Kempf, J., Joetzjer, E., Cuntz, M., Courtois, P., Naiken, A., Simioni, G., Marloie, O., Bervillier, D., Girardin, C., Morfin, A., and Delpierre, N.: Quantification of Total Available Water in French temperate and mediterranean forests by three different methods., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6941, https://doi.org/10.5194/egusphere-egu26-6941, 2026.
The semi-arid to semi-humid transition zone in northern China exhibits strong interannual climate variability and frequent drought events, making it a critical ecological transition zone where tree growth is highly sensitive to drought stress. Analysis of long-term climate records from multiple sites indicates that annual precipitation in the study area has shown a significant decline over recent decades, accompanied by a marked decrease in the regional average SPEI. As annual mean temperature and vapor pressure deficit (VPD) increase, the vessel diameter and hydraulically weighted diameter of non-porous wood species have significantly decreased. This indicates that under intensified atmospheric drought conditions, non-porous wood species become more sensitive to climatic stress through adjustments in their hydraulic structure. Tree growth exhibits the most pronounced response to medium-to-long-term drought signals. Notably, SPEI12 during the growing season shows a significant positive correlation with RWI, indicating that water deficit has become the dominant climatic factor limiting tree growth in the study area. Sliding correlation analysis further reveals that tree growth sensitivity to drought-related factors such as VPD, temperature, and solar radiation significantly increases within specific interannual time windows, highlighting the time-nonstationarity of the climate-growth relationship. At the regional scale, tree radial growth exhibited widespread negative anomalies during drought periods, with growth declines exceeding 10% in some years. This aligns with signals of reduced vegetation productivity, indicating that drought stress impacts on growth extend from the individual to the regional scale. Further analysis of growth variability revealed that the coefficient of variation in tree growth significantly decreased in the semi-humid zone, while no significant trend was observed in the semi-arid zone. This indicates that the modulating effect of inter-individual resource competition on growth heterogeneity under drought conditions exhibits significant regional differences. In summary, this study reveals the mechanisms by which climatic drought stress, hydraulic restructuring, and biological interactions jointly drive tree growth changes in the semi-arid-semi-humid transition zone, providing multi-scale evidence for understanding the responses of transitional forest ecosystems under intensifying drought conditions.
How to cite: Liu, Y., Wu, X., Wang, P., Guo, R., Zhang, Z., Cao, W., Liu, J., and Yuan, Y.: Multi-scale Responses of Tree Growth to Climate Change and Their Hydraulic Mechanisms in Semi-Arid to Semi-Humid Regions of Northern China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7149, https://doi.org/10.5194/egusphere-egu26-7149, 2026.
The resilience of terrestrial ecosystems to drought and environmental stress is critical for the future of the terrestrial carbon balance. Vegetation water dynamics play a central role in these ecosystems, as they are closely coupled to carbon assimilation at the plant stomata. Sub-daily variations in plant water status are expected to reflect both abiotic and biotic stress responses. Improving our understanding of these short-term dynamics could enable us to detect early signs of vegetation health decline and provide new metrics to quantify ecosystem resilience and tipping points.
However, sub-daily vegetation water content (VWC) dynamics remain poorly understood and are weakly represented in terrestrial biosphere models. This knowledge gap is largely driven by the scarcity of sub-daily observations, as VWC is difficult to measure both in-situ and via satellite remote sensing. To address this observation gap, the SLAINTE mission concept was proposed as one of ESA’s New Earth Observation Mission Ideas in response to the 12th Call for Earth Explorers. The goal of the mission was to capture sub-daily variations in vegetation water storage, including vegetation optical depth, VWC, plant water potential, and surface soil moisture.
A critical aspect of the continued development of this mission concept is consolidation of the observation and measurement requirements. Therefore, this study focuses on simulating sub-daily time series of forest radar backscatter using a radiative transfer (RT) model. The simulations are driven by continuous, non-destructive ground-based measurements of forest transmissivity collected at several forested sites across Europe. The resulting synthetic backscatter time series allows us to characterize and quantify the influence of sub-daily variations in plant water dynamics, vegetation structure, and biogeophysical properties on the radar backscattering coefficient.
We present initial results from simulations at two forest sites and discuss their implications for strengthening the science case of the SLAINTE mission. We also highlight key limitations encountered during the modeling effort. These include the high sensitivity of RT simulations to forest structural parameters and the limited availability of sub-daily validation data. Accurate model parameterization requires detailed information on forest geometry (such as foliage density), which is difficult to obtain even by field measurements. We attempt to quantify forest architecture using terrestrial laser scanning. Validation remains challenging due to limited availability of sub-daily observations of VWC, vegetation dielectric properties, and radar backscatter, particularly when interpreting short-term fluctuations. Additionally, separating the effects of internal vegetation water dynamics from surface canopy water associated with interception and precipitation remains a significant challenge at sub-daily timescales. Addressing these issues will require continued interdisciplinary collaboration combining field observations, modeling and remote sensing.
How to cite: Neyer, A. S., Van Der Borght, N., Tronquo, E., Świątek, P., Villarroya Carpio, A., Guerriero, L., and Steele-Dunne, S.: Simulating Sub-daily Forest Backscatter to track Water, Carbon and Health in Forested Ecosystems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9312, https://doi.org/10.5194/egusphere-egu26-9312, 2026.
Stress signals propagate through ecosystems via interactions between atmospheric aridity and soil water depletion, resulting in physiological stress in plants, altered structural dynamics, and changes in carbon, energy, and water fluxes. Monitoring these impacts is possible using a growing suite of established and novel sensor systems. Available approaches span tree-level measurements such as stem water potential, stand-scale fluxes derived from eddy-covariance towers, and spatially continuous indicators from Earth observation satellites using optical and radar backscatter. Yet, the degree to which such measurements track complementary stress processes, and the extent to which their responses are temporally coupled, remain poorly quantified.
We address this gap through a multi-scale analysis of a distinct hot and dry period in August 2025 (August 7–19) at the temperate forest of the ICOS-associated Forest Research Site DE-Har (Hartheim, Germany), which is characterized by limited soil water storage and rapid soil drying. Using distributed sensors, we tracked the propagation of stress signals across five interacting levels. These include (1) atmospheric demand (vapour pressure deficit, air temperature), (2) soil water status (volumetric water content), (3) plant hydraulics (stem water potential, tree water deficit, sap flow), (4) canopy structure and leaf properties (leaf angle distribution via AngleCam, GNSS-T based vegetation optical depth, plant area index from permanent terrestrial laser scanning, leaf area index from hemispherical photographs, vegetation greenness, Sentinel-1 radar backscatter, Sentinel-2 optical indices), and (5) ecosystem fluxes (net ecosystem exchange, gross primary productivity, evapotranspiration).
Using cross-correlation and lag analysis at daily resolution from May to October 2025, we quantify the temporal sequence in which these measurements respond to the hot and dry period in August 2025. We determine whether certain variables act as leading indicators and to what extent time-lags emerge as stress signals propagate from the atmosphere to ecosystem fluxes. This integrated perspective can reveal which measurements track similar aspects of stress and which provide complementary information that would be missed by any single approach alone. Moreover, this analysis emphasises the potential of novel, scalable sensor techniques such as tracking leaf angle dynamics from video cameras (AngleCam) and GNSS-T-based vegetation optical depth.
Our outcomes provide a temporally resolved view of stress signal propagation in a drought-impacted temperate forest ecosystem, which can inform ecosystem modelling and the design of multi-sensor monitoring networks.
How to cite: Kremer, L., Haberstroh, S., Sulzer, M., Schellenberg, K., Stanley, V., Brede, B., Christen, A., Werner, C., and Kattenborn, T.: Temporal dynamics of stress signal propagation across ecosystem scales during a hot and dry period: A multi-sensor analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19717, https://doi.org/10.5194/egusphere-egu26-19717, 2026.
As the human population grows and droughts become more frequent and intense, identifying drought-responsive anatomical and ecophysiological traits in crops is critical to safeguard food production in a world that is becoming more demanding for plant growth. Solanum lycopersicum (tomato) is a major herbaceous crop species in which stem woodiness increases with age, especially in the basal stem regions, providing an opportunity to investigate how developmental changes in stem structure influence plant–water relations under drought. In this study, we investigated a 2-month and a 4-month old batch of two woody knockout mutant genotypes (double SOC1-like, quadruple FUL SOC1-like), as well as the wild type Solanum lycopersicum var. Moneyberg, to assess how differences in stem woodiness from genetic modification and plant age influence total plant drought tolerance. Therefore, we quantified a suite of drought-responsive anatomical traits and monitored ecophysiological traits from stems and/or leaves under well-watered and/or drought conditions. These traits included stem lignification, intervessel pit membrane thickness, stomatal traits, plant water potential dynamics, and resistance to drought-induced embolism. Overall, our results show that drought tolerance increases with plant age, primarily through enhanced resistance to drought-induced embolism in the stem, which correlates with increasing stem lignification at the basal stem. Stomata control plays a minor role, as resistance to drought-induced embolism drives major differences in the stomatal safety margin. When comparing developmental stages, variation in embolism resistance and woodiness in stems explains drought tolerance differences within genotypes, whereas intervessel pit membrane thickness is the primary driver of drought tolerance differences among genotypes. These findings demonstrate the dynamic role of drought-associated plant traits at the species level, highlighting once again the remarkable ability of plants to adapt to their environmental conditions.
How to cite: Pereira Zaldivar, A., Bortolami, G., Van den Bulcke, J., Gheyle, T., Josipovic, I., Verschuren, L., Basu, E., Bemer, M., Pandey, K., de Jong, G., Balazadeh, S., and Lens, F.: Effects of Plant Age and Wood Formation on Drought Tolerance in Tomato, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5074, https://doi.org/10.5194/egusphere-egu26-5074, 2026.
The TreePulser project investigates how short-term environmental variability and species-specific physiological strategies interact to shape intra-annual tree growth dynamics in temperate forests. It relies on an extensive monitoring network of 300 dominant trees clustered within study sites distributed across Belgium, where a coordinated, multi-layer measurement set-up captures both environmental conditions and tree functioning.
A first, growth-centred approach integrates dendrometer-derived time series with a broad suite of exogenous drivers to characterise species- and site-specific growth phenology. Environmental monitoring includes atmospheric conditions (air temperature, relative humidity, precipitation, radiation), soil conditions (soil moisture and temperature), and physical soil characterisation aimed at capturing spatial variation in soil water availability. Stand structure and local competitive environment are incorporated through neighbourhood surveys quantifying the size, distance, and spatial arrangement of surrounding trees. Together, these variables are used to analyse growth onset and cessation, seasonal growth rates, and short-term variability, as well as to quantify the relative contributions of climatic, edaphic, and competitive drivers and the temporal lags between environmental variation and growth responses.
A complementary, mechanism-centred approach focuses on identifying the physiological and functional traits underlying observed growth patterns and drought sensitivity. Repeated canopy sampling provides measurements of predawn and midday leaf water potential, capturing seasonal dynamics in tree water status and nighttime rehydration capacity. Stomatal strategies are investigated through anatomical traits, including stomatal density and size. Hydraulic functioning is characterised using pressure–volume curves and xylem vulnerability measurements to quantify drought-relevant properties related to tissue water relations and resistance to embolism. Additional traits associated with leaf and wood economics, including specific leaf area, leaf nitrogen content, and wood density, provide a broader functional context by describing contrasting resource-use strategies.
By integrating high-frequency growth monitoring with multi-dimensional site characterisation and ecophysiological measurements on dominant trees, the project aims to better represent the interconnected processes governing tree performance in natural stands, where atmosphere, soil conditions, local competition, and endogenous regulation interact across multiple temporal scales. This integrative design supports the identification of trait-based predictors of growth sensitivity to atmospheric demand and soil water availability, thereby improving the capacity to anticipate species performance under increasingly variable climatic conditions.
How to cite: Hauzeur, H.: TreePulser : Growth Phenology and Physiological Responses of Temperate Tree Species Under Environmental Drivers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5828, https://doi.org/10.5194/egusphere-egu26-5828, 2026.
Hotter droughts in European forests increasingly combine declining soil moisture with rising atmospheric demand, raising fundamental questions about how trees sustain transpiration while avoiding embolism-induced mortality under drought stress. While stomatal regulation and transpiration responses are well documented, the role of upstream, within-tree water fluxes, particularly the use and replenishment of internal stem water storage, represent an emerging research frontier.
Here, we present high-temporal resolution observations of stem water storage use and rehydration dynamics in mature Pinus sylvestris, combining sap-flow and dendrometer measurements from the VPDrought experiment at the Pfynwald research platform in the dry inner-Alpine Rhône valley of Switzerland. By independently manipulating soil moisture and vapour pressure deficit (VPD), this experiment allows us to disentangle atmospheric and soil controls on internal tree water fluxes.
We show that under drought, trees increasingly “run on savings”: the contribution of stem water storage to daily transpiration rises sharply from approximately ~5% under well-watered soil conditions to up to ~40% under dry soil conditions, when transpiration declines but storage water use persists. In parallel, the replenishment of stem storage-water reserves through water flow into the stem declines with decreasing soil water potential. Notably, even under mild soil drought, elevated VPD substantially constrains nighttime rehydration of stem storage-water reserves.
The findings we present emphasize stem water storage as a dynamic and drought-responsive component of tree-water use. Accounting for both the mobilization and rehydration of internal water reserves is essential for understanding how trees buffer hydraulic stress during drought and enhance model representations of plant-water interactions under increasingly frequent hotter droughts.
How to cite: Peters, R. L., Buras, A., Grichting, S. A., Schaub, M., Grossiord, C., Bortolami, G., Gisler, J., Trotsiuk, V., Gessler, A., Meusburger, K., Hunziker, S., Kahmen, A., Rigling, A., Walthert, L., Klesse, S., and Zweifel, R.: When trees run on savings: Stem water storage and rehydration as a missing link in drought responses, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6862, https://doi.org/10.5194/egusphere-egu26-6862, 2026.
Drought stress in forests has been increasing with climate warming, both through reduced soil water availability and increased atmospheric water demand (vapor pressure deficit). The consequences are enhancing (i.e. positive) temperature feedbacks, increased tree mortality and shifts in species composition. This is concerning because forests contribute to mitigate excessive surface temperatures and increases in atmospheric CO2 concentrations. While drought stress-related declines in photosynthesis are well established, important questions remain regarding (i) changes in respiration, (ii) compensating effects of understory vegetation, (iii) attenuating (i.e. negative) forest-atmosphere feedbacks, and (iv) small-scale processes (i.e. at soil, leaf or tree-level) that also emerge at the ecosystem scale (Wankmüller et al. 2024).
Here we present an overview on the current knowledge of drought impacts on forest biosphere-atmosphere interactions (Wolf & Paul-Limoges 2023), recent evidence for the potential of understory (i.e. below-canopy) eddy-covariance flux measurements (Wolf et al. 2024), and the results of an ongoing drought manipulation experiment using paired (i.e. drought-stressed and irrigated) eddy-covariance flux towers to measure understory biosphere-atmosphere interactions at the Pfynwald forest in Switzerland.
Finally, we will discuss the challenges and perspectives for scaling fluxes of carbon, water and energy from tree to ecosystem scale using a combination of established and novel in situ measurements.
References
Wolf S & Paul-Limoges E (2023) Drought and heat reduce forest carbon uptake. Nature Communications 14: 6217 (https://doi.org/10.1038/s41467-023-41854-x)
Wolf S, Paul-Limoges E, Sayler D, Kirchner JW (2024) Dynamics of evapotranspiration from concurrent above- and below-canopy flux measurements in a montane Sierra Nevada forest. Agricultural and Forest Meteorology 346: 109864 (https://doi.org/10.1016/j.agrformet.2023.109864)
Wankmüller FJP, Delval L, Lehmann P, Baur MJ, Cecere A, Wolf S, Or D, Javaux M, Carminati A (2024) Global influence of soil texture on ecosystem water limitation. Nature 635(8039): 631–638 (https://doi.org/10.1038/s41586-024-08089-2)
How to cite: Wolf, S., Paul-Limoges, E., Unverricht, P., and Carminati, A.: Impact of Drought on Forest Biosphere-Atmosphere Interactions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14357, https://doi.org/10.5194/egusphere-egu26-14357, 2026.
There is concern about an increase in drought induced dieback in forests as the climate becomes warmer and drier. In response, land holders are looking to new strategies in the restoration of forests. One such approach is provenancing; importing genotypes from seperate populations that may be adapted to different climates. However, in many cases, populations are facing similar pressures, and there may not be a population from a significantly ‘drier and hotter’ region. In these circumstances, provenancing may be reliant on disjunct populations that face similar climatic pressures having different adaptations to hot and dry climates. We tested drought responses, traits and strategies in provenances of two Eucalyptus species, E. melliodora and E. microcarpa, from the drier ranges of their respective distributions. The species co-occur in woodland communities in south eastern Australia, but E. microcarpa extends into regions that are drier and hotter. We measured a) chronic drought responses in a 17 week drought experiment of provenances planted in-ground in a rain exclusion shelter, b) transpiration responses to acute drought in a glasshouse experiment, and c) growth and seasonal water relations in a common garden field experiment. Both species had high within-provenance intraspecific variation in many drought traits, but similar adaptations to drought between provenances. The two species, despite co-occurring, had contrasting drought strategies. E. melliodora had a drought avoidant strategy, with much greater allocation of biomass to root growth and highly sensitive stomata. The comparatively greater root growth resulted in successfully avoiding drought and having comparatively better growth outcomes in the chronic drought experiment. In contrast, E. micropcarpa was much more drought tolerant and had greater hydraulic function at greater water deficits during acute drought. However, both species had almost identical growth outcomes over a five year period when planted in a provenance trial in the field. Despite the two species co-occurring and coming from the same section (Adnataria) within the Eucalyptus genus, they had significantly contrasting drought strategies. Therefore, understanding a species’ drought strategy may be important when considering which traits may confer an adaptive advantage to drought.
How to cite: Browning, R. and Arndt, S.: Limited evidence for provenancing in two co-occurring species with contrasting drought strategies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14888, https://doi.org/10.5194/egusphere-egu26-14888, 2026.
The importance of water and its limitations for the functioning of plants and terrestrial ecosystems have long been studied. However, science sometimes hinders its own progress by the negligence of significant findings from earlier periods.
One example is the pioneering work of W.R. Gardner (1960+), who made significant contributions to our current knowledge on limitations to plant water use. Gardner’s early work focused on the dynamic (un)availability of soil water at the small scale, which is caused by the distinct decrease in soil hydraulic conductivity around the water-absorbing roots during transpiration.
Since then, however, this soil-specific dynamic limitation through soil hydraulic conductivity has often been neglected. While this is well justified at times, for example when focusing on seasonal rather than daily drought conditions, we argue that these Gardner-like limitations to plant water use at the small scale should not be ignored, even if observations are made at much larger scales (e.g. using Eddy-Covariance or remote sensing) than where plant roots take up water.
This is particularly relevant as drought research has become more interdisciplinary. While originally a challenge for agriculture-related soil physics (e.g., W.R. Gardner and D. Hillel), plant and ecosystem water limitations have increasingly been addressed by other disciplines, such as plant hydraulics and climate science, at larger scales. Our recent work reinforces the idea that small-scale soil hydraulic conductivity limitations can be important at larger scales in a soil- and plant-specific manner.
We believe that the field of water limitation research exemplifies not only the pitfalls of generating scientific knowledge, but above all the great potential of interdisciplinary research initiatives.
How to cite: Wankmüller, F.: From W.R. Gardner to the present day: How research on water (un)availability to plants sometimes hindered its own progress, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14305, https://doi.org/10.5194/egusphere-egu26-14305, 2026.
In the 21st century droughts have become more frequent, have shown an increased duration, a greater spatial extent and are increasingly exacerbated by human water demands. Understanding the impacts of droughts on vegetation dynamics, the legacy effects and especially the recovery are essential aspects to understand the prolonged effects that meteorological droughts can have on ecosystems.
This research looks at the spatial dependence of vegetation recovery after a meteorological drought, i.e. the extent to which events co-occur at multiple locations simultaneously, explaining underlying mechanisms and patterns which could potentially support recovery forecasting in the future. To understand the spatial dependence of vegetation recovery we characterized spatiotemporal dynamics of vegetation recovery with the use event synchronization and complex networks and identified hydroclimatic and geophysical predictors of this behaviour using remote sensing and ERA5 reanalysis data.
We found that there are strong global patterns in vegetation drought synchronization, which was specicially high in Australia and southern Africa, and low in large parts of Africa and east Asia. Overall, the biggest drivers of differences in spatial dependence are temperature, aridity and precipitation variability. On a global scale high dependence is mainly occurring in regions experiencing large-scale spatially connected droughts, mostly related to strong climate signals like ENSO. Areas with a low spatial dependence are characterized by a high natural water availability, resulting in more local and vegetation type-specific resilience to drought.
Our work indicates a diverse set of features driving ecological drought occurrence, synchronization and recovery. These findings could be a useful tool to use in forecasting ecological drought response to ongoing meteorological droughts.
How to cite: Verhoeve, S., Hauswirth, S., de Jong, S., and Wanders, N.: Global spatial dependence of vegetation recovery from meteorological drought impacts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17832, https://doi.org/10.5194/egusphere-egu26-17832, 2026.
Vegetation attenuates microwave radiation that is emitted or reflected by the Earth surface. The degree of attenuation derived from passive and active microwave satellite observations is commonly referred to as vegetation optical depth (VOD). While high-frequency bands such as the Ku-, X- and C-band retrieve information from the upper-most fraction of the canopy, low frequency bands (e.g.: L-band) are thought to convey information about the entire forest profile. Thus, different frequencies could be complementary for the study of plant-water interactions along the vertical vegetation profile. However, the fraction of the canopy that is detected within a given band is itself a function of water content, which challenges the interpretation and practical use of multi-channel VOD products. Understanding the VOD signal in tropical forests and how water dynamics might affect it is crucial, as microwave observations are one of the only reliable methods that can consistently measure tropical areas frequently covered by clouds.
Hydraulic capacitance in plants – the ratio between water content and water potential – is approximately constant within a physiologically non-damaging range of water potentials. Thus, during periods of minimal flux while systems are close to hydraulic equilibrium (e.g., predawn, drought), a linear relationship between predawn water potential and the total amount of water contained in the above ground biomass is expected. The deviation from a constant ratio between VOD bands under equilibrium conditions could thus be an indicator of 1) variation in penetration depth caused by the change in water content, 2) changes in surface moisture/interception, or 3) the transition over a physiological threshold, when hydraulic capacitance changes.
Here, we aim to exploit changes in ratios between VOD bands to understand the seasonality of the vegetation water status in Central African tropical forests within the framework of the CoForFunc international project. Specifically, we hypothesize that different forest structures might lead to varying seasonal responses to water availability and distinct plant phenologies detectable from satellite measurements of passive microwave radiation. To do so, we obtained night-time monthly VOD observations from the Ku-, X-, C-, and L-band from VODCA products between 2012 and 2018, and calculated the ratios between each pair of bands. Over forested pixels in Central Africa, we explored the variability of the VOD ratios in relation to precipitation and interception estimates and other potential climatic predictors to tease out the seasonality of water availability and other confounding factors. We further tested the contribution of within-pixel land cover fractions and heterogeneity metrics on the variability of the VOD ratios.
Our results link physiological and biophysical understanding at the tissue-scale to the scale at which satellite observations provide information on water and biomass relations for land surface models. This will become particularly relevant as future missions such as the CIMR Copernicus Expansion Mission will ensure global daily multi-channel VOD products for continuous vegetation water monitoring. Understanding water-vegetation dynamics in tropical forests will further help the investigation and monitoring of such crucial but understudied areas.
How to cite: Gomarasca, U., Binks, O., Piles, M., and Duveiller, G.: Understanding the interaction of plant water status and vegetation optical depth from passive microwave satellite observations in Central African Forests, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3834, https://doi.org/10.5194/egusphere-egu26-3834, 2026.
Soil hydraulic properties regulate water movement from the soil to the leaves and thus impact plant hydraulic functions. Water moves along a pressure gradient that allows the plant to take up water from the soil. As vapor pressure deficit increases and soil water becomes scarce, tension rises in the plant vascular system and leaves progressively lose turgidity. Although the decline in both soil and plant hydraulic conductance has been extensively studied, it is under debate the sequence of the declines in hydraulic conductance along the soil-plant continuum. Precisely it is not clear whether it is the soil that loses its capacity to transport water to plants fast enough, triggering stomatal closure before substantial decline in any plant tissue conductance. Here, we propose a method to quantify and compare the soil and plant hydraulic conductance in plants undergoing soil drying. We studied wheat (T. Aestivum) grown in two contrasting soil textures and subjected to a drought treatment. We targeted conditions when water started to be limiting but before excessive soil drying – i.e. when transpiration was about half of its maximum. Together with novel rehydration techniques and high temporal resolution water potential measurements, we quantified and isolated the various compartments within the soil-plant system. Our results show that the soil hydraulic conductance in the coarser soil limits the total hydraulic conductance of the whole system at less negative soil water potentials. Although less limiting to water movement when fully wet, a coarse soil proves to be much more limiting as soon as it starts drying. These results highlight the central role of understanding soil-specific properties when evaluating plant drought resilience.
How to cite: Piovano, D. C., Bourbia, I., di Bert, S., Carminati, A., and Brodribb, T.: Quantifying the limiting role of soil hydraulic conductance on plant water relations during drought, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18662, https://doi.org/10.5194/egusphere-egu26-18662, 2026.
