In this session, we broadly explore the roles of temperature extremes, evaporative demand, and soil moisture in carbon, water, and energy relations, along with plant mortality across various spatial and temporal scales. We encourage submissions dealing with novel approaches for measuring and modeling plant and soil water status, responses to extreme conditions, and their impacts on ecosystem function. We invite contributions on these topics at scales ranging from individual plant tissues to entire ecosystems, applying experimental, observational, or modeling approaches and dealing with diverse disciplines such as plant physiology, community ecology, ecosystem ecology, land management, and biogeochemistry.
Orals: Tue, 5 May, 08:30–12:25 | Room 2.23
Hydraulic failure has been causally linked to cellular damage at the leaf level, yet the functional pathway connecting xylem dysfunction to whole-tree mortality remains unresolved. One recent hypothesis is that tree death ultimately depends on the loss of meristem vitality. However, because meristems are difficult to access and assess, their response to drought has rarely been investigated. Here, we aimed to identify whether reduced water supply resulting from hydraulic dysfunction directly compromises cambial integrity and determines tree survival. We subjected potted saplings of a gymnosperm species, Abies concolor, and an angiosperm species, Fagus sylvatica L. to severe drought by withholding water, generating different levels of loss of hydraulic functioning ranging from 30% to complete loss of conductivity (PLC 100). Prior to rewatering, water potential, percentage of embolism, relative water content, and level of cellular damage were quantified at the stem level, while cambial cell integrity was assessed using transmission electron microscopy. Sapling survival was monitored for one year following drought release.
Surprisingly, saplings of both species could display similar water potentials, relative water content status or level of hydraulic failure at the time of rewatering but exhibited different survival capacities. Saplings that maintained structurally intact cambial cells recovered, whereas those showing cambial damage died, regardless of their level of hydraulic status. Thus, our results provide direct evidence that cambium integrity represents a critical bottleneck linking hydraulic failure to tree mortality. They also evinced that the mechanisms behind loss of cambial cell integrity are mainly explained by the consequences of tree dehydration after hydraulic failure. Focusing on the water relocation towards cambial cells during a drought event could help understand the mechanisms associated with cambial cell death, and identify potential thresholds for improving the precisions of the mechanistic models aiming at predicting tree mortality.
How to cite: Mantova, M., Steven, J., Cochard, H., Szczepaniak, C., Brunel-Michac, N., Delzon, S., King, A., and Torres-Ruiz, J. M.: Die or survive from drought? The role of cambium integrity in saplings resilience , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17986, https://doi.org/10.5194/egusphere-egu26-17986, 2026.
The changing climate is increasing the frequency and intensity of hurricanes and drought, which cause tree crown damage and widespread tree mortality. Forests undergoing post-hurricane succession may be particularly vulnerable to drought because crown damage alters tree carbon allocation, which could be exacerbated by drought. Hurricane canopy damage also results in elevated light in the understory, leading to the recruitment of fast-growing early succession species that tend to be more vulnerable to drought. Yet, our understanding of the effects of drought on post-hurricane forest recovery is extremely limited. Hurricane María is the strongest hurricane to make direct landfall in Puerto Rico since 1928. We are leveraging the disturbance caused by Hurricane María in 2017 to gain important new knowledge on the compound effect of hurricanes and droughts by conducting a large-scale throughfall exclusion experiment. We have established three 20 x 20 m experimental plots and three 20 x 20 m control plots in the Luquillo Experimental Forest, in northeastern Puerto Rico. The forest is an ever-wet tropical forest at approximately 300 m above sea level and has a mean annual temperature of 24 °C and a mean annual precipitation of 3500 mm. We tagged and identified to species the trees in the six plots. We selected 62 target individuals from the five dominant tree species and one dominant palm on which we installed point dendrometers and are measuring predawn (PDΨ) and midday leaf water potential (MDΨ) and sap flow three times per year (during the driest, wettest, and highest solar irradiation period of the year). We have collected one year of pre-treatment data, have finished installing the throughfall exclusion roofs in the three experimental plots, and have begun post-treatment sampling of tree responses. We have also started collecting leaf samples and sapwood cores to extract nonstructural carbohydrates and histological sections for imaging of stored starch distribution and depletion. Our pre-treatment data show similar mean PDΨ (-0.2 to -0.4 MPa) and MDΨ (-0.5 to -0.6 MPa) among the three pre-treatment campaigns, meaning that there was no significant drought stress. Sap flow was higher during the highest solar irradiation (mean species-level pick whole tree sap flow 1,500 to 8,000 cm3 h-1), whereas during the wettest and cloudiest time of year, there was ~ 50% reduction in sap flow (1,000 to 4,000 cm3 h-1). On the same dominant species, we measured xylem vulnerability to embolism (P50), leaf turgor loss point (TLP), and calculated stomatal safety margins (SSM = TLP-P50). Species fell along a range of P50 from drought-vulnerable (P50 = -1MPa; SSM = 0 MPa) to relatively drought-tolerant (P50 = -3 MPa; SSM = 1 MPa). Given the differences in trait values among the dominant tree species, we expect very different species-level responses to the imposed drought that will likely change seasonally and throughout time as the experimental drought progresses.
How to cite: Smith-Martin, C., Boeschoten, L., Esbri, L., Montgomery, R., Nytch, C., Picon, M., Wood, T., Xu, X., Zimmerman, J., and Uriarte, M.: Effects of experimental drought on post-hurricane recovery of an ever-wet tropical forest, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2008, https://doi.org/10.5194/egusphere-egu26-2008, 2026.
Thermal infrared remote sensing is widely used to estimate transpiration rates and drought stress in crops (e.g. the Crop Water Stress Index, CWSI). However, interpretation of surface temperature data in forests is more difficult due to more complex canopy structure and uncertainty in the canopy aerodynamic resistance. To better understand the utility of canopy temperature data for drought detection in a deciduous forest, we mounted three thermal infrared (TIR) sensors on a tower, pointing onto the crowns of three individual beech trees, which were at the same time equipped with sap flow sensors and dendrometers used to record reductions in sapflow and increase in tree water deficit during dry periods. The tower was also equipped with sensors for air temperature, relative humidity, horizontal wind speed and net radiation, and the site was equipped with a rain gauge and soil moisture sensors at different depths down to 1 m depth.
Observed crown temperatures were put into relation with simulated temperature variations of two single 3 cm wide leaves, one with 0 stomatal conductance and one with infinite stomatal conductance, representing the extreme cases of a non-transpiring dry leaf, and a wet leaf, respectively.
Simulated dry leaf temperatures exceeded critical temperatures of 50 oC on several summer days in 2023 and 2024, indicating that evaporative cooling is needed to avoid permanent heat damage. At the same time, measured canopy temperatures deviated upwards from the simulated wet leaf temperatures with declining soil moisture and increasing tree water deficit as deduced from high-resolution dendrometer data. This illustrates the crucial effect of combined heat and drought stress, when evaporative cooling is most needed, but hampered by inadequate water supply.
The striking consistency between observed crown temperatures and simulated single-leaf temperatures of dry and wet leaves suggests that meteorological conditions at the top of the canopy (net radiation, air temperature and humidity, wind speed) are decisive for the energy balance of the majority of the leaves seen by the TIR sensors and opens the path to spatially resolved assessments of tree drought and heat stress. Hereby, characteristic leaf sizes play an important role for the interpretation of canopy temperature data, and for the vulnerability of plants to thermal stress during heat and drought waves. This presentation highlights these roles quantitatively and points to common pitfalls and knowledge gaps when modelling and interpreting leaf and canopy temperature data.
How to cite: Schymanski, S., Keim, R., Schlerf, M., and Gerhards, M.: Leaf-scale physics and thermal infrared sensing provide a glimpse into drought and heat stress of beech trees, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16939, https://doi.org/10.5194/egusphere-egu26-16939, 2026.
A common effect of drought is the reduction of the hydraulic capacity as xylem is embolised. Furthermore, trees can prematurely shed leaves during more severe droughts, presumably to protect xylem from excessive embolisms. Both of these effectively change the ratio of water supply and demand (i.e. the ratio of sapwood to leaf area; Huber value). While changes in this ratio during drought should therefore affect a trees’ water household, this has never been experimentally tested at the branch level.
We asked: How does experimentally changing the branch Huber value affect the branch water household in deciduous and evergreen trees? To address this question, we experimentally changed the Huber value of trembling aspen (Populus tremuloides) and subalpine fir (Abies lasiocarpa) branches and measured regular stomatal conductance and water potentials. We did this with a fully factorial experiment either removing half of the leaf area, cutting through half of the xylem area, or both on branches in situ. At the end of the experiment, we conducted native and max hydraulic conductivity and dye perfusion measurements. We hypothesized that reducing leaf area leads to increased area-specific stomatal conductance, resulting in constant whole-branch transpiration. We also hypothesized that reducing sapwood area leads to a decrease in water potentials, stomatal conductance and hydraulic conductivity.
We found that after leaf removal there was a small increase of stomatal conductance in aspen but not enough to keep whole-branch transpiration constant, otherwise we did not see any effects. Surprisingly, after cutting through the xylem area there was no difference in any measured traits. This implies that removing leaf area, at least in aspen, has a greater effect on the water household than removing xylem area. We found that in aspen, the xylem was transporting much less water than its potential, implying high xylem redundance. This pattern was not as strong in subalpine fir, as they were operating closer to their potential. These different responses between the species may be explained by their different anatomical types, as fir xylem is more resistant and less conductive than aspen wood. The high degree of branch xylem redundancy found here shows that the water household of trees can be buffered against substantial changes in the Huber value, indicating that drought-related seasonal changes in xylem or leaf area may not affect water relations as much as hitherto assumed.
How to cite: Fickle, J. C., Zahnd, C., Wells, I., and Anderegg, W. R. L.: High xylem redundance in the branches buffers the water household of trees against changes in the Huber Value, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10163, https://doi.org/10.5194/egusphere-egu26-10163, 2026.
Increasing temperature and drought conditions can result in leaf dehydration and summer defoliation even in drought-adapted tree species such as the Mediterranean evergreen oak Quercus ilex. This phenomenon was widespread in forests of Southern France in 2022-2023 when record-breaking temperatures of 2022 were associated with two consecutive years of low precipitation, severe drought episodes and recurrent heatwaves.
Using the 20-year data series (2001-2021) of eddy-covariance carbon and water fluxes measured at the ICOS Mediterranean forest site FR-Pue (Puéchabon) prior to this event, we assess the impacts of these two exceptional years on the carbon budget of the forest. While the Puéchabon forest always behaved as a net carbon sink between 2001 and 2021, with an annual net ecosystem exchange (NEE) ranging between -450 and -137 gC m-2 y-1, the carbon balance was reversed to a net annual carbon source of +14 and +65 gC m-2 y-1 in 2022 and 2023, respectively. This anomaly is caused by a deficit of photosynthetic carbon uptake, as leaf physiology was severely impacted by both water stress and heat stress. Significantly lower photosynthetic rates than in the previous years were, however, not restricted to the most stressful conditions of heat or soil water deficit but manifested under most meteorological conditions even outside the summer period. This observation suggests that neither heat nor drought alone can explain the photosynthesis limitation in 2022 and 2023 but that the two acted in synergy. It also demonstrates that such extreme meteorological events have long lasting effects on tree physiology, mediated by cell physiological damage, leaf hydraulic failure and canopy dieback that limit photosynthetic recovery when favorable temperature and soil moisture conditions return.
Interestingly, these negative effects on photosynthesis were not observed during the following year 2024 when a complete recovery of photosynthetic rates was achieved with the production of new leaves, highlighting a strong resilience of Quercus ilex to drought and heat. Nevertheless, the annual carbon budget in 2024 was also particularly low because of an excess of total ecosystem respiration compared to the long-term mean. The higher respiration rates in 2024 could be caused by the decomposition of dead trees and organs after the extreme years 2022-2023, and by the reallocation of trees carbon reserves to the production of short-lived organs such as new leaves and seeds.
This study is, yet, a rare example of an inversion of a forest carbon balance driven merely by meteorological conditions and it highlights the value of long-term observations to better understand and interpret the consequences of extreme events on ecosystem functioning.
How to cite: Limousin, J.-M., Kempf, J., Poughon, J., Rambal, S., and Ourcival, J.-M.: When drought, heat and canopy dieback turn a Mediterranean forest into a net annual carbon source for two consecutive years, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3954, https://doi.org/10.5194/egusphere-egu26-3954, 2026.
Climate change is increasing drought frequency, duration, and severity in large parts of the world, thereby reducing soil water availability and increasing risks for forest decline and mortality. The drought response of a tree can be described by the tree’s 'drought sensitivity', indicating reversible processes such as stomatal responses to soil drying, and 'drought vulnerability', indicating irreversible damages, such as the risk of hydraulic failure. Different tree species differ substantially in their drought sensitivity and vulnerability. Yet, the underlying physiological and morphological mechanisms remain poorly understood. We tested whether species-specific differences in root water uptake depth (RWUD) can explain differences in drought sensitivity and vulnerability of mature trees belonging to nine temperate European tree species. Using a unique six-year dataset (2018–2024) from the Swiss Canopy Crane II site, we quantified drought sensitivity from the response of daily maximum sap flux density to soil drying. We quantified drought vulnerability by calculating hydraulic safety margins of trees relative to species-specific critical xylem hydraulic thresholds. RWUD was estimated from stable water isotopes and analyzed against sensitivity and vulnerability traits.
We show that species differ markedly in both sensitivity and vulnerability. We discuss that these differences are largely determined by variation in the tree's maximum RWUD: shallow-rooted species closed stomata early and rapidly approached hydraulic thresholds during drought, while deep-rooted species sustained transpiration and maintained wide hydraulic safety margins. RWUD alone explained more than 65 % of the interspecific variation in both drought sensitivity and vulnerability. Our results demonstrate that RWUD is a key morphological trait linking belowground water access to aboveground drought physiology. By quantifying this connection in mature trees, our study identifies RWUD as a strong predictor of forest drought resilience and a critical parameter for integrating rooting traits into ecosystem and Earth system models to improve forecasts of forest–climate feedbacks under intensifying drought regimes.
How to cite: Steger, D., Peters, R. L., Zhorzel, T., Dups, R., Hoch, G., Nelson, D. B., Zahnd, C., Basler, D., Meusburger, K., Bernhard, F., Arend, M., Schuldt, B., and Kahmen, A.: Root water uptake depth explains drought response of temperate tree species, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6547, https://doi.org/10.5194/egusphere-egu26-6547, 2026.
Increasing atmospheric evaporative demand is a key driver of drought stress in European forests. Yet, it remains unclear whether high vapor pressure deficit (VPD) arising from elevated temperature or, in contrast, from reduced atmospheric humidity exerts a stronger constraint on tree growth. European beech (Fagus sylvatica L.), a dominant species in Central Europe, is particularly sensitive to drought-induced growth reductions, making it an ideal model to disentangle these mechanisms.
We conducted a unique controlled phytochamber experiment at the TUMmesa facility to isolate the effects of contrasting VPD drivers on intra-annual growth dynamics of beech trees. Six climate chambers simulated (i) control conditions with low VPD (max. ~1.3 kPa), (ii) high-VPD conditions induced by elevated temperature under control relative humidity (“hot air”), and (iii) high-VPD conditions induced by low relative humidity under control temperature (“dry air”). Both atmospheric drought treatments reached the same maximum VPD levels (~2.3 kPa), allowing direct comparison of temperature- versus humidity-driven VPD effects.
Tree growth was continuously monitored using high-resolution dendrometers, providing sub-hourly insights into stem growth. Atmospheric treatments were combined with contrasting soil textures and progressive soil drying to assess whether growth responses to VPD depend on soil hydraulic context.
By disentangling the growth effects of hot versus dry air under equivalent VPD, this study advances mechanistic understanding of how atmospheric drought shapes tree growth under climate change and improves predictions of forest productivity responses to increasing evaporative demand. Moreover, this experiment provides the basis for us developing advanced mechanistic growth models which can incorporate the impact of atmospheric and soil droughts.
How to cite: Schmied, G., Köhler, T., Hilmers, T., Li, Y., Hegarty, P., Ahmed, M., Chen, C., Jákli, B., Grams, T., Meier, R., Rühr, N., and Peters, R.: Dry or hot air? Unraveling the growth stressors of European beech during drought periods, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11890, https://doi.org/10.5194/egusphere-egu26-11890, 2026.
Drought and heat events can impose high evapotranspiration demands, pushing plants to close their stomata to prevent excessive water loss. Yet, plant leaves are not perfectly hermetic and water losses continue through leaky stomata and the cuticle. Post stomatal closure residual water losses, known as minimum conductance (gmin), are relevant since they indicate the water depletion rates under severe stress. We present the first standardized dataset of gmin values for 101 species spanning high phylogenetic and ecological diversities, from ferns to flowering plants. Our sampling also included different growth forms and life cycles from annual herbs to longevous trees. We show that minimum water vapor conductance is highly variable across species, and gmin shows a weak phylogenetic signal across vascular plants. Residual water lossesdiverged across growth forms and phenologies with greater water losses in annual herbaceous as compared to woody plants. Moreover, deciduous species showed higher water lossrates as compared to evergreen species, highlighting the integration of gmin in leaf economics, where long-lived leaves show higher capabilities to retain water under stress. We used stomatal measurements to model the other side of the conductance spectrum and estimated the maximum (gth max) leaf conductance capabilities. In the sampled vascular plants gmin was dissociated with gth max, revealing the lack of a clear tradeoff between maximum potential conductance efficiency and water retention. Unlike gmin, stomata-driven gth max has a high phylogenetic signal indicating that related species have similar maximal capacities of water conductance. Leaf conductance rates are negatively correlated with climate variables such as mean annual temperature and precipitation seasonality, revealing economies in water expenses in more seasonal, and hotter environments. As major drought events are coupled with significant heat stress, we further explored the relationship of gmin and photosynthetic thermotolerance (Tcrit, T50) in the diverse genus Quercus, to investigate potential interactions of thermal and drought sensitivities. Altogether, this presentation will provide recent advances on our understanding of the evolutionary physiology of water loss dynamics under heat- and drought-stress which will be cardinal to predict the fate of vegetation under global climatic changes.
How to cite: Trueba, S., Burlett, R., Forget, G., Larter, M., N’Do, D., Ziegler, C., Middleby, K., Ángeles, G., Toledo-Aceves, T., Madero-Vega, C., and Delzon, S.: Drought-induced water losses after stomatal closure across vascular plants, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11379, https://doi.org/10.5194/egusphere-egu26-11379, 2026.
Drought events of increasing frequency, intensity, and duration are driving widespread forest dieback and mortality worldwide. While mixed-species forests are promoted as a strategy to enhance resistance and resilience to drought, increasing species richness alone does not consistently improve tree growth responses. Moreover, the tree diversity effects under unprecedented multi-year droughts remain poorly understood.
Here, we used a network of planted tree diversity experiments to assess how neighborhood tree diversity and species-specific hydraulic traits influence drought-induced growth responses. We analysed tree-rings on 948 trees from 19 species across nine experiments spanning Europe’s major climate zones. All sites experienced recent severe droughts, including the record-breaking 2018–2020 multi-year drought. Experimental gradients in tree species richness (1–6 species) allowed us to disentangle tree diversity effects while controlling for environmental heterogeneity.
We quantified radial biomass growth using X-ray computed tomography, and assessed the physiological drought stress using the carbon isotope signal of tree rings in dry and wet years (∆δ13Cdry-wet). We used a functional trait framework to evaluate diversity effects at neighbourhood scale, using hydraulic safety margin (HSMTLP) to characterise the species’ drought tolerance.
Tree growth responses were driven by drought characteristics, species drought tolerance, and neighborhood functional diversity, but not by neighborhood species richness per se. Increasing drought duration within a growing season shifted neighborhood diversity effects on growth from beneficial to negative. Under consecutive drought years, diversity effects on growth responses depended on site context, but strengthened in sites showing positive or negative effects. Neighborhood diversity reduced physiological drought stress (lower ∆δ13Cdry-wet) for drought-susceptible species (low HSMTLP) growing alongside drought-tolerant neighbors. Yet such changes in carbon isotopic composition were not directly coupled to growth responses during the same drought year.
Our results demonstrate that functional trait diversity—rather than species richness—determines how trees respond to extreme and prolonged drought. While mixing species with contrasting hydraulic strategies can alleviate physiological drought stress, increasing tree diversity does not always enhance growth resilience. The effectiveness of tree mixing is highly context dependent at neighborhood scale, and can shift with increasing drought duration and intensity. This underscores the need for trait-based approaches and locally-adapted solutions to make our forests more resilient to longer and harsher droughts.
Keywords: tree diversity, mixed plantation trials, TreeDivNet, multi-year drought, drought tolerance, functional traits, tree rings, X-ray computed tomography, 13C isotopic composition
How to cite: Serrano-León, H., Blondeel, H., Bonal, D., Guillemot, J., Martin-StPaul, N., Grossiord, C., Schnabel, F., Scherer-Lorenzen, M., Skiadaresis, G., Van den Bulcke, J., Verheyen, K., Baeten, L., and Bauhus, J. and the TreeDivNet-MixForChange team: Drought characteristics alters neighbourhood diversity effects on tree growth responses during extreme events , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17693, https://doi.org/10.5194/egusphere-egu26-17693, 2026.
A conceptual understanding on how the vegetation’s carbon (C) balance is determined by source activity and sink demand is important to predict its C uptake and sequestration potential now and in the future. We have gathered trajectories of photosynthesis and growth as a function of environmental conditions described in the literature and compared them with current concepts of source and sink control. There is no clear evidence for pure source or sink control of the C balance, which contradicts recent hypotheses. Using model scenarios, we show how legacy effects via structural and functional traits and antecedent environmental conditions can alter the plant’s carbon balance. We, thus, combined the concept of short-term source–sink coordination with long-term environmentally driven legacy effects that dynamically acclimate structural and functional traits over time. These acclimated traits feedback on the sensitivity of source and sink activity and thus change the plant physiological responses to environmental conditions. We postulate a whole plant C-coordination system that is primarily driven by stomatal optimization of growth to avoid a C source–sink mismatch. Therefore, we anticipate that C sequestration of forest ecosystems under future climate conditions will largely follow optimality principles that balance water and carbon resources to maximize growth in the long term.
How to cite: Geßler, A. and Roman, Z.: Beyond source and sink control – toward an integrated approach to understand the carbon balance in plants, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2543, https://doi.org/10.5194/egusphere-egu26-2543, 2026.
Photosynthesis fuels the biosphere and is a key regulator of Earth’s climate. As Earth warms, heat stress threatens to irreversibly impair the molecular machinery of photosynthesis, potentially pushing ecosystem productivity and carbon sequestration beyond a tipping point. Current approaches to quantifying photosynthetic heat tolerance often focus solely on temperature, overlooking exposure time, or rely on temperature-time correlations that do not identify causal mechanisms, limiting inference and prediction. Here we develop a mechanistic theory for heat inactivation of photosynthesis based on principles of chemical kinetics, and test it using data for photosystem II (PSII), the first step in the photosynthetic apparatus. Our framework links the effects of both temperature and exposure time, and enables direct tests of competing hypotheses for how heat impairs photosynthesis. Data from diverse plant species suggest that protein (not lipid membrane) denaturation is the primary mechanism of heat-induced inactivation of PSII. The theory also predicts a general upper temperature limit of 55-60 °C for acclimation of photosynthetic heat tolerance, a prediction supported by global PSII data. This quantitative, mechanistic framework can be incorporated into global change models to improve forecasts of how vegetation and the biosphere will respond to future climate change.
How to cite: Michaletz, S. and Bison, N.: The kinetic basis of photosynthetic heat tolerance, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3173, https://doi.org/10.5194/egusphere-egu26-3173, 2026.
Hot droughts are becoming increasingly frequent worldwide, causing widespread and abrupt leaf discoloration in temperate forests. Because changes in leaf colour are commonly associated with autumnal senescence, such abrupt discoloration is often interpreted as premature or stress-induced senescence. Nonetheless, under hot droughts, excessive heating and/or hydraulic failure may cause leaf tissue damage, leading to leaf scorching. This process produces visual symptoms similar to senescence but arises from fundamentally different physiological processes. Despite its potential importance, leaf scorching remains poorly studied.
Using climate chambers, we exposed three species (Fagus sylvatica, Quercus Pubescens, and Prunus mahaleb) to four temperature treatments (25°C, 35°C, 40°C, and 45°C) under severe water limitation. Through regular physiological (predawn and midday water potential, chlorophyll content, stomatal conductance, maximum potential quantum efficiency of Photosystem II, leaf embolism) and continuous leaf colour measurements, we aimed to identify the physiological tipping points of leaf scorching and to provide a clearer distinction between the leaf discoloration processes.
Leaf scorching occurred only under the highest temperature treatments (40°C and 45°C), with its extent varying among species according to their inherent thermotolerance. Notably, in Fagus sylvatica, leaf tissue damage appear to develop prior to leaf embolism, indicating that temperature excess rather than hydraulic dysfunction was the primary trigger of scorching under extreme heat.
How to cite: Bergström, M., Milano, A., Juillard, T., Jacquat, L., Hoch, G., Khamen, A., and Vitasse, Y.: Scorched, not senescent: When hot droughts burn leaves instead of aging them, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1623, https://doi.org/10.5194/egusphere-egu26-1623, 2026.
Extreme heat is intensifying across the tropics, often coinciding with high atmospheric drought, forcing trees to balance evaporative cooling against the risk of hydraulic failure. A possible response that challenges major climate–vegetation stomatal model assumptions is the decoupling of photosynthesis and stomatal conductance (gs), where gs stays high while photosynthesis declines.
We aimed to quantify how widespread photosynthesis–gs decoupling is across tropical tree species from contrasting climates and test whether it trades off with sensitivity to heat, drought, or vapour pressure deficit (VPD). We measured in-situ temperature responses (22-49°C, VPD=2.5kPa) of photosynthesis, gs, and g1 in 80 mature individuals encompassing 16 species along an elevation gradient in Panama, as well as leaf-level turgor loss point and VPD sensitivity.
All individuals showed an exponential rise in g1 with temperature, indicating widespread decoupling between photosynthesis and stomatal conductance. Although both photosynthesis and gs declined above their thermal optima, stomatal re-opening at extreme temperatures (~45°C) occurred in 55% of curves. Notably, the temperature at which gs increased again was higher in lowland than upland individuals, potentially indicating greater heat tolerance in trees from hotter environments. Contrary to expectations, there was no coordination between stomatal sensitivity to extreme heat, stomatal sensitivity to vapour pressure deficit, and turgor loss point, indicating that heat avoidance and hydraulic drought tolerance represent largely independent axes of variation in the tropical trees studied.
These results provide rare field-based evidence that tropical trees exhibit diverse temperature-dependent stomatal strategies that may shape forest resilience under future heatwaves. Future research must prioritise in situ measurements at extreme leaf temperatures (>45 °C), where tropical trees approach critical thermal thresholds and where the physiological mechanisms governing survival under heatwaves remain largely unresolved.
How to cite: Middleby, K., Rojas-Gonzalez, A., and Slot, M.: In situ evidence for a critical temperature threshold driving stomatal re-opening and widespread photosynthesis–conductance decoupling in tropical trees, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3846, https://doi.org/10.5194/egusphere-egu26-3846, 2026.
Chronically rising air temperatures and increasing soil drought threaten terrestrial ecosystems by pushing plants closer to their physiological limits, thereby altering carbon uptake, growth, and survival. However, how vegetation responds to current and future warm and dry conditions remains poorly understood, especially in high-risk Mediterranean shrublands characterized by hot, dry summers.
In this study, we combine extensive field measurements and process-based modeling to assess shrubland responses to current and future climate conditions. During summer 2024, we collected leaf and wood traits related to plant economic spectra, thermal and drought tolerances, and photosynthesis. Data were obtained for six dominant Mediterranean shrub species (Amelanchier ovalis, Arbutus unedo, Pistacia lentiscus, Rhamnus alaternus, Buxus sempervirens, and Salvia rosmarinus) across six sites along a climatic gradient in Catalonia (North-East Spain). Additional climatic data were compiled from national meteorological station networks. These datasets were used to parameterize the trait-enabled ecosystem model MEDFATE 2.9.3 to simulate daily individual-level photosynthesis, net carbon uptake, respiration, transpiration rates, and energy balance. Originally developed for forest ecosystems, MEDFATE was adapted here to represent shrubland structure and function. Simulations were conducted under current climate conditions and future scenarios of increased temperature and reduced soil water availability based on IPCC projections. To maintain model tractability, simulations focused on the summer period, when climatic stress is highest.
By comparing interspecific differences in physiological responses across current and projected climate scenarios, this research aims to advance understanding of future vegetation dynamics in Mediterranean shrublands exposed to increasing heat and drought stress. Overall, this work helps bridge key knowledge gaps in plant ecophysiological responses to climate extremes and provides valuable insights for predicting shrubland vulnerability and informing future management strategies.
How to cite: Didion-Gency, M., de Càceres, M., Mencuccini, M., and Martinez-Vilalta, J.: Predictions of the energy and carbon balance of Mediterranean shrub species under future climate scenarios, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6715, https://doi.org/10.5194/egusphere-egu26-6715, 2026.
Human activities are increasing temperatures and reducing water availability, intensifying climate variability and extremes including heat waves and droughts, which threaten forest ecosystems. Here, we characterize and model intraspecific variability in physiological traits related to resistance to hotter droughts in loblolly pine (P. taeda). For this, we use complementary approaches to evaluate trait variation and potential differences between two populations that have been geographically and spatially separated since the last glacial maximum (21,000–5,000 years before present). Adaptive variation was investigated by phenotyping provenances originating east and west of the Mississippi River Valley, where long-term geographic separation has resulted in distinct population genetic structure. We used an integrated indicator, time to hydraulic failure (THF), predicted by a mechanistic hydraulic model, SurEau, to assess how trait combinations contribute to tree resistance to hotter droughts. Measured physiological traits included xylem vulnerability to cavitation, leaf and bark residual conductance, and leaf turgor loss point, each of which is known to be essential for tree drought resistance. Surprisingly, results indicate a tendency for THF to be lower in western provenances compared to eastern ones. Time to Hydraulic Failure was negatively correlated with residual stomatal conductance and leaf mass per area. This pattern suggests a physiological differentiation between populations, although it is not only determined by traits associated with drought resistance. Ongoing work aims to leverage this intraspecific variation to guide selection within and among tree species for more drought-resistant forests under continued climate change.
How to cite: Ilinca, E., Hammond, W. M., Cinquini, A. R., Torres-Ruiz, J. M., Cochard, H., and Mantova, M.: Physiology of Pinus taeda: an Ice Age legacy? Intraspecific variability in drought-related physiological traits among provenances from east and west of the Mississippi River Valley, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11854, https://doi.org/10.5194/egusphere-egu26-11854, 2026.
Global climate change imposes growing challenges to vegetation thermoregulation through rising temperatures and increasing drought frequency. Understanding plant thermal limits (e.g., Tcrit and T50) and their associated temperature safety margins is essential to evaluate canopy resistance to thermal stress. Despite intensified heatwave events in temperate regions, research on plant temperature thresholds has predominantly focused on tropical ecosystems, and methodological inconsistencies have limited cross-study comparability.
In this study, we address these knowledge gaps by: (1) quantifying thermal thresholds (Tcrit, T50) for temperate plant species through field sampling, (2) compiling published datasets standardized under a homogenized methodology, (3) analyzing the global drivers of T50 and the inter- and intraspecific variability linked to temperature, phenology, genetics, and methodological factors, and (4) mapping temperature safety margins by integrating field data, upscaling models, and satellite-derived land surface temperatures. Finally, we project future temperature safety margins for temperate vegetation under anticipated climate scenarios. Our findings provide a comprehensive framework to assess and predict the thermal resilience of temperate plant species under ongoing and future climatic stress.
How to cite: Vallicrosa, H., Simon, A., Juillard, T., Sánchez-Martínez, P., Waldner, P., and Holbrook, N.: Mapping temperature thresholds and safety margins of temperate plant species under global change, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-96, https://doi.org/10.5194/egusphere-egu26-96, 2026.
Climate change is increasing the compounding of droughts and heatwaves, causing widespread tree growth decline and mortality. To improve predictions of forest vulnerability, understanding current tree water-use strategies is critical. However, few data exist that contrast more than a handful of species’ responses to the same growth conditions.
We investigate water-use in 38 temperate tree species (27 angiosperms, 11 gymnosperms) at a research arboretum in Germany (ARBOfun, Großpösna). In the summer of 2024, we measured stomatal conductance (gs; which regulates carbon assimilation and transpiration) in three individuals per species repeatedly over diurnal cycles. The summer was hot, yet soil water availability remained sufficient. This allowed us to isolate stomatal responses to vapor pressure deficit (VPD), a key component of tree water-use strategies under atmospheric drought, and determine proxies of stomatal sensitivity to increasing atmospheric aridity, such as the inflection point of the gs-VPD curve. Species showed a wide variation in stomatal sensitivity to VPD, ranging from early-closing to high-VPD-tolerant strategies. Ongoing analyses relate these species-specific sensitivity proxies to leaf traits and growth responses, advancing our understanding of water-use diversity under climate change.
How to cite: Sachsenmaier, L., Ahner, C., Kretz, L., Richter, R., Staude, I., Sabot, M., and Wirth, C.: The stomatal regulation of 38 tree species as a window into water-use strategies under climate change, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14027, https://doi.org/10.5194/egusphere-egu26-14027, 2026.
Earth is currently undergoing global warming and “atmospheric drying” as a result of the increase in atmospheric water vapor pressure deficit (VPD). Rising heat and VPD are exposing plants to two problems: water and temperature stress. Through closing, stomata prevent excessive water loss when the VPD is high, thereby protecting their hydraulic integrity. Conversely, through opening stomata, plants avoid overheating. As high temperatures increase VPD, an emergent trade-off arises between water saving and latent cooling. In nature, VPD inherently covaries with both temperature and relative humidity. The resulting net effect on the degree of stomatal openness and its variability in relation to pedoclimatic conditions remains elusive.
To close this knowledge gap, we grew European beech (Fagus sylvatica L.) in controlled climate chambers leveraging lysimeters filled with loam or sand to simulate contrasting soil hydraulic environments.. Trees were subjected to irrigated and drought-stressed conditions. Three VPD treatments were imposed: (1) low VPD (1.3 kPa) via decreasing relative humidity (RH) and increasing temperature, (2) elevated VPD (2.3 kPa) via decreasing RH at stable temperature, and (3) elevated VPD (2.3 kPa) via increasing temperature at stable RH. We measured the following parameters: soil water content and potential, transpiration via custom-made sensors (TransP), gas exchange using LI-6800, and leaf water potential via optical dendrometers calibrated against Scholander Bomb point measurements.
When elevated VPD was driven by increasing temperature, plants transpired linearly with rising VPD until higher thresholds in wet soil compared to humidity-driven elevated VPD, and consequently exhibited a more pronounced sensitivity to soil drying across both textures, i.e., reductions in transpiration rate and leaf water potential in wetter soil conditions. In the temperature increase treatment, trees also demonstrated enhanced thermal tolerance in both soil textures, as indicated by a higher temperature at which the plant's photosynthetic efficiency drops by 50% (T50). No VPD treatment-induced differences emerged in above- and belowground morphology (e.g., root and leaf area), whole-plant hydraulic conductance, or pre-dawn stomatal conductance, suggesting primarily physiological rather than structural-hydraulic acclimation. Soil texture modulated response strength but not direction.
These results demonstrate that different drivers of increasing VPD profoundly alter plant water-use regulation: warming-induced rises in evaporative demand allow for sustained transpiration until higher VPD in wet soil but increases water use sensitivity to soil drying. Our results indicate the need for disentangling temperature- from humidity-mediated VPD, as VPD ≠ VPD.
How to cite: Li, Y., Philip, H., Maharjan, D., Schmied, G., Zeppan, J., Rühr, N., Jakli, B., Meier, R., Hilmers, T., Peters, R., Ahmed, M., and Koehler, T.: VPD ≠ VPD: Heat-driven increases in evaporative demand amplify water use sensitivity to soil drying in European beech (Fagus sylvatica L.), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21212, https://doi.org/10.5194/egusphere-egu26-21212, 2026.
The Kranzberg Forest Roof Experiment (KROOF: https://www.lss.ls.tum.de/en/lsai/kroof/) is a long-term study investigating the impact of drought on about 100 mature European beech and Norway spruce trees in both pure and mixed stands. From 2014 to mid-2019, the trees were exposed to severe experimental drought conditions, after which they underwent a five-year recovery period. The main focus of the current study is to investigate how drought-naïve and drought-legacy trees respond to renewed droughts. Did the preceding drought, which caused a significant decline in physiological and morphological parameters such as photosynthesis, water consumption, growth and leaf area, weaken or strengthen the trees? Does growth in mixtures affect the response to renewed droughts?
Here, we present the results of two datasets. The first study made use of the natural summer drought in 2022. Drought-naïve trees exhibited strong negative effects of drought, such as reduced stomatal conductance and xylem sap flow, as well as reduced growth. Conversely, legacy trees recovering from the preceeding drought period showed significantly reduced drought stress. This was due to the lower water consumption of spruce trees, caused by their reduced whole-tree leaf area. Three years after the drought treatment, the leaf area of legacy spruce trees was still 30% lower than that of drought-naïve trees. Interestingly, legacy beech trees also benefited from the previous drought treatment despite not showing significant reductions in leaf area. It seems that beech trees benefited from the water saving of neighboring spruce trees, as their roots reach far into the soil under spruce.
The second study started in spring 2025 as the third phase of the KROOF experiment. Here we compare drought-naïve and legacy trees under experimentally induced drought conditions. In this phase of the KROOF experiment, the trees are exposed to extreme, potentially lethal drought conditions, with full exclusion of precipitation throughfall and stem runoff, over the whole year. Initial data support the hypothesis that legacy trees have acclimatised to the previous drought period. For instance, we observed a delayed reduction in predawn twig water potential in legacy trees compared to drought-naïve trees. While all trees have survived the extreme drought treatment thus far, we anticipate the first trees to die within the next two years, likely beginning with spruce. In the following years, we will study whether there are differences in mortality patterns between drought-naïve and legacy trees, and between growth in pure and mixed stands.
How to cite: Grams, T.: Come back stronger? The response of mature beech and spruce trees to renewed drought in a long-term throughfall exclusion experiment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21722, https://doi.org/10.5194/egusphere-egu26-21722, 2026.
Central Europe experienced extremely dry conditions in spring 2025, with weather forecasts predicting another extreme summer drought similar to that of 2018. To evaluate the impact of extreme climatic and edaphic drought on Central European tree species, coordinated midday leaf water potential measurement campaigns were carried out at 51 forest sites across Europe, covering a total of 12 common native tree species (four conifers, four diffuse- and four ring-porous broadleaves). Fortunately, the summer turned out to be rather moist, contrary to the early summer weather forecasts, while Northern and Southern Europe experienced extremely hot and dry conditions, setting several negative records. Nevertheless, the members of this initiative performed monthly measurements campaigns between June and October 2025 at their sites as baseline measurements, awaiting the next extreme summer drought to continue the measurement campaigns.
Here, we introduce the European Psi-leaf network, which is open to everyone as long as common protocols are followed. These include water potential measurements using classic pressure chambers, as well as additional information at tree and site levels. We also present species-specific results on leaf water status regulation during a non-drought year, revealing clear patterns across wood porosity types.
How to cite: Schuldt, B., Hafner, B., and network, P.: The European Psi-leaf network for monitoring tree water status during extreme drought, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20659, https://doi.org/10.5194/egusphere-egu26-20659, 2026.
Posters on site: Tue, 5 May, 16:15–18:00 | Hall X1
Arctic sea ice decline is known to influence mid-latitude climate, yet its impacts on terrestrial vegetation productivity and agriculture production remain insufficiently understood. Using four decades of satellite observations, agricultural statistics, and earth system model simulations, we show that variations in Barents Sea ice area (BSIA) exert a strong control on spring vegetation gross primary productivity (GPP) across Europe. BSIA loss enhanced spring GPP in eastern Europe but suppresses it in the western Europe, driving a pronounced increasing trend in of GPP in eastern Europe. Wheat yields respond similarly, with low-ice years producing up to +16.51% higher national yields and more than 20% increases at the pixel scales. These impacts are dominated by temperature: reduced BSIA induces large-scale circulation anomalies that warm eastern Europe through cyclonic conditions, enhanced horizontal temperature advection, and increased shortwave radiation, collectively alleviating frost risk and promoting photosynthesis. Current ESMs capture the sea-ice–temperature linkage but systematically underestimate the GPP response, primarily due to weak GPP–temperature sensitivities. Our results highlight BSIA decline as a major but underrepresented driver of spring ecosystem productivity in mid-latitude Europe, and indicate that existing models may substantially underestimate future productivity changes in a rapidly warming Arctic.
How to cite: Huang, Z., Wang, J., and Ju, W.: Barents Sea ice loss substantially enhances spring vegetation growth and wheat yields in Eastern Europe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3219, https://doi.org/10.5194/egusphere-egu26-3219, 2026.
Starch is the ubiquitous carbon reserve in trees that is stored decentralized in sapwood parenchyma of branches, stems and roots. As a transitory carbon pool between photosynthesis and carbon sinks (growth, respiration,...), tissue concentrations of starch are assumed to mirror carbon source-sink-relations, with concentrations positively correlating with the net balance between gross primary productivity and the sum of all carbon sink activities of a tree. In this study, we investigated if starch concentrations in branch sapwood of mature trees are suitable indicators for drought induced changes of the trees’ carbon source-sink activities.
Taking advantage of the Swiss Canopy Crane II facility, we studied mature trees of 6 common European broadleaved species over five consecutive growing seasons that varied significantly in terms of temperature and precipitations. Despite the very different climatic conditions, we found surprisingly small variations of end-of-season starch concentrations in terminal branches for most years and species. This is in stark contrast to leaf gas-exchange and growth that both declined significantly in all species in years with extended drought periods. Further, among all investigated species, deviations from the species-specific average starch concentrations in some years were not consistently correlated with climatic anomalies (e.g., exceptionally dry seasons were not uniformly associated with decreased branch starch concentrations). Overall, these findings suggest that starch formation in branch sapwood possesses a high priority, and the fast refilling of starch reserves in wood parenchyma of younger branches after spring bud break occurs largely independent of the total tree annual carbon balance.
How to cite: Hoch, G., Fröhlicher, S., and Kahmen, A.: Starch concentrations in branches are not consistently changing with drought stress in mature temperate tree species, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3789, https://doi.org/10.5194/egusphere-egu26-3789, 2026.
Grasslands and forests play a central role in regulating terrestrial water, energy, and carbon (WEC) cycles. Vegetation–atmosphere interactions shape and respond to hydroclimatic extremes such as heatwaves, droughts and compound dry–hot events. Under climate change, the influence of these extremes on ecosystem functioning is intensifying, altering the extent to which vegetation modulates WEC fluxes and, in some regions, driving ecosystems toward turning points or functional role reversals. The CausalHeat project aims to characterize the causal dynamics underlying dry–hot extremes, their impacts on ecosystem resilience and vulnerability, and the subsequent ecosystem–climate feedbacks influencing the development and persistence of these events. Using entropy-based information-theoretic causality methods, we quantify the dominant drivers of dry–hot episodes and assess how these drivers propagate through ecohydrologic process networks to influence vegetation structure and function across biomes.
Observational evidence reveals a pronounced biome-dependent divergence in ecosystem responses — more resilient or vulnerable. Forested ecosystems and croplands exhibit strengthened ecohydrologic process coupling and increased network organization, consistent with adaptive reorganization under recurrent drought exposure. However, enhanced vapor pressure deficit (VPD) coupling to forest function and structure yields episodic shocks that can push systems into transiently vulnerable states. In contrast, grassland-, savanna-, shrubland-, and wetland-dominated ecosystems show progressive decoupling of ecohydrologic processes, indicative of potential declining resilience. Grassland ecosystems emerge as particularly sensitive to aridification, with vulnerability driven by the synergistic amplification of atmospheric water demand and declining soil moisture, rather than by the dominance of either factor alone. Together, these results highlight how hot extremes reorganize ecosystem process networks in biome-specific ways, with important implications for terrestrial WEC partitioning, ecosystem stability and ecosystem–atmosphere feedbacks. CausalHeat provides a framework for improving the prediction of dry–hot extremes and assessing ecosystem responses relevant to food and water security under climate change.
How to cite: Hagan, D. F. T., Siddique, F., Zhou, F., Liu, M., Geirinhas, J. M., and Miralles, D. G.: The CausalHeat Project: Land–climate feedbacks shaping ecosystem vulnerability to dry–hot extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7475, https://doi.org/10.5194/egusphere-egu26-7475, 2026.
Hydrological extremes are continuing to intensify under climate change. However, the responses of vegetation to dry and wet soil moisture extremes, and the dominant drivers of these responses, have not yet been analyzed consistently. In this study, we utilize long-term observations of Normalized Difference Vegetation Index (NDVI) as a proxy of vegetation responses to soil moisture extremes. We then analyze related drivers with a machine-learning attribution approach to assess the role of pre-extreme vegetation conditions, characteristics of extremes, and of the environmental background. Vegetation generally loses greenness during dry extremes, indicated by widespread and consistent negative NDVI anomalies. This is mainly modulated by pre-extreme vegetation conditions and the characteristics of the extreme (especially seasonal timing) which reflect varying vegetation vulnerability. In contrast, wet extremes lead to more heterogeneous responses, including both positive and negative NDVI anomalies. This is modulated by multiple aspects including pre-extreme vegetation conditions, the characteristics of the extreme (especially seasonal timing) as well as environmental background variables such as climate (e.g., long-term mean air temperature, aridity) and topography (topographic variability). This illustrates that vegetation response to wet extremes is complex and potentially influenced by different processes. Further, regions with negative NDVI anomalies during extremes that are strongly modulated by environmental background indicate localized vulnerability arising from adverse climatic, soil or topographic conditions, such that vegetation stress can occur even under extremes with less severity. These results highlight the roles of seasonal timing and of environmental background conditions for impacts of soil moisture extremes on vegetation. This clarifies the predictability of ecosystem responses to hydrological extremes, and serves as a basis for related management planning.
How to cite: Cheng, X., Zhan, C., De Kauwe, M., Hildebrandt, A., and Orth, R.: Global vegetation responses to wet and dry soil moisture extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5488, https://doi.org/10.5194/egusphere-egu26-5488, 2026.
Forests worldwide are increasingly exposed to compound climate extremes, yet the physiological and ecological pathways through which concurrent heat and drought damage vegetation remain poorly understood. These compound stresses pose significant risks to ecosystem resilience, but their interactive effects have rarely been quantified across large landscapes. The unprecedented June 2021 Pacific Northwest “Heat Dome” provided a unique natural experiment to address this gap. Using high‑resolution satellite imagery and spectral–temporal diagnostics, we mapped leaf scorch across 93,420 ha with 87% accuracy and quantified the relative contributions of abiotic drivers and species identity. Unexpectedly, water-stress variables, particularly rapid atmospheric drought captured by vapor pressure deficit anomaly, dominated spatial variation in canopy injury (35.5%), slightly exceeding the contribution from heat‑associated stress (33.1%). The synergistic effect of hydraulic stress and heat stress further amplified canopy injury. Species identity accounted for 19.1%, with divergent sensitivities: Thuja plicata was disproportionately vulnerable to water deficit, whereas Abies amabilis was most sensitive to elevated heat. Trait‑based analysis linked these vulnerabilities to distinct functional syndromes, enabling predictive insight into species‑specific responses. By disentangling damage drivers at the landscape scale, our findings advance understanding of forest responses to compound climate extremes, trait based predictive frameworks and provides actional insights for adaptive management under accelerating climate change.
How to cite: Yu, C., Still, C., Detto, M., Feng, Y., Guo, Z., Zhao, Y., Peng, J., Sibley, A., Albert, L., and Wu, J.: Drought and Heat Jointly Drive Forest Canopy Injury During Compound Climate Extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11192, https://doi.org/10.5194/egusphere-egu26-11192, 2026.
The combined occurrence of drought and heatwaves, known as hot droughts, poses a major threat to forest ecosystems by disrupting plant physiological processes and increasing tree mortality. Yet, the key physiological mechanisms underlying tree acclimation to hot droughts remain poorly understood, particularly where potential trade-offs between drought tolerance and thermotolerance may constrain acclimation. In addition, canopy microclimate, especially differences between sun-exposed and shaded leaves, may strongly modulate these responses but is rarely explicitly considered.
We investigated how acclimation to long-term precipitation exclusion (>20 years) in mature Quercus ilex trees affects their drought and heat tolerance. During the summer of 2025 (June, July, and August), we assessed physiological responses under control and drought treatments in sun-exposed and shaded leaves. Key measurements included thermal tolerance of photosynthesis and cell integrity (TEL), gas exchange, and plant water status.
We observed clear site- and treatment-dependent differences in thermal tolerance. Overall, control trees exhibited higher TEL than droughted trees, although the magnitude and direction of this effect varied between sites. In contrast, plant water potential showed limited treatment effects, potentially indicating hydraulic acclimation to long-term drought. Across both sites, sun-exposed and shaded leaves differed markedly in thermal tolerance, underscoring the role of microclimate. In droughted trees, sun-exposed leaves had higher TEL than shaded leaves in Spain, but in France, shaded leaves had higher TEL than sun-exposed leaves. In the control treatments, shaded leaves consistently had higher TEL at both sites.
Our results suggest that long-term drought acclimation alters physiological responses to heat stress in a canopy-position-dependent manner. While canopy microclimate strongly shapes thermotolerance, the extent to which drought and heat tolerance are linked by physiological trade-offs remains unclear. Understanding these interactions is critical for predicting forest resilience under future climate change.
How to cite: Heintzelman, C., Bachofen, C., Urban, L., Vallicrosa, H., Milano, A., Limousin, J., and Ogaya, R.: Balancing heat and drought tolerance: evidence for physiological trade-offs in Quercus ilex, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12560, https://doi.org/10.5194/egusphere-egu26-12560, 2026.
The biodiverse northern Western Ghats, historically characterized by tropical evergreen forests, have undergone a pronounced shift toward deciduous vegetation following long-term anthropogenic disturbance, such as logging, plantations, and fire. This vegetation transition has led to declining biodiversity, altered ecosystem processes, and reduced hydrological stability. In the context of rising temperatures, prolonged dry periods, and increasingly variable moisture regimes, climate-smart restoration approaches are urgently needed to reestablish the native evergreen forest structure through strategic, trait-informed species selection.
To inform species choice for restoration programs, we established a replicated juvenile pot experiment comprising 16 species spanning a slow–fast growth spectrum. We measured functional traits associated with drought and heat resilience, including diurnal water-use patterns (stomatal conductance), embolism resistance, dehydration tolerance, and thermal tolerance.
Our preliminary analysis reveals trait covariation describing distinct hydraulic strategies among coexisting species. Clear differences emerge in diurnal water use patterns and dehydration tolerance among the studied species. Such differences in hydraulic strategies might improve ecosystem performance compared to monocultures under climate change. We are further investigating whether species with comparatively higher dehydration tolerance, and putative embolism and heat resistance can better withstand climatic stressors and exhibit improved growth performance, or trade-offs exist. Field monitoring of planted saplings will be essential to validate these preliminary insights and guide climate-resilient restoration in the northern Western Ghats.
How to cite: Paligi, S., Yadav, A. K., Singh, M., Patil, M., Najeeb, N., Sahu, S., Deshpande, S., Gaude, S., Jangra, S., Sadekar, V., Mishra, T., Naniwadekar, R., and Joshi, J.: Restoring evergreen forests: trait-based evaluation and field monitoring for climate-resilient restoration, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15298, https://doi.org/10.5194/egusphere-egu26-15298, 2026.
Since its spread across Central Europe, the Oak lace bug (Corythucha arcuata, Say 1832), an invasive species rapidly spreading since 2012, has caused persistent and spatially extensive canopy stress in oak-dominated forests, clearly detectable in satellite-derived vegetation indices. Between 2016 and 2024, characteristic NDVI declines associated with repeated infestations were consistently observed during the late summer, indicating chronic biotic stress superimposed on background climatic variability. During this period, the oak lace bug after establishment in an area showed only a weak interannual variability, but no signs of retreating.
Unexpectedly, in 2025 these biotic stress signals were largely absent across the affected regions. NDVI time series in 2025 showed substantially reduced late summer declines compared to previous years both in Hungary and Croatia, suggesting a sudden weakening of the oak lace bug-related canopy impacts. This abrupt change raises the question of which climatic mechanisms may have contributed to the apparent collapse of remotely sensed infestation signals. Two non-exclusive hypotheses are considered: (i) legacy effects of the extreme heat and drought conditions in August 2024, which caused widespread deterioration of oak canopy condition and may have disrupted host–insect interactions, and (ii) adverse winter conditions following the 2024 growing season, potentially affecting overwintering survival of the insect.
Using multi-year satellite time series (Harmonised Landsat-Sentinel-2, MODIS, VIIRS) and meteorological data (FORESEE), we investigated the changes in canopy greenness dynamics in relation to the preceding thermal, hydrological and seasonal weather extremes. Our analysis reveals a striking shift in the detectability of biotic stress signals and discusses possible climate-related controls on their persistence. The results demonstrate the value of satellite-based monitoring for capturing not only the emergence and spread of forest pests, but also their sudden decline, emphasizing the importance of considering compound and lagged climate effects when interpreting vegetation stress signals.
Keywords: Space-borne remote sensing, Vegetation indices, MODIS, Harmonized Landsat-Sentinel-2 dataset, invasive pest detection, extreme weather
Funding: The research has been supported by the Hungarian Scientific Research Fund (NKFIH FK-146600). This work has been implemented by the National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014) project within the framework of Hungary's National Recovery and Resilience Plan, supported by the Recovery and Resilience Facility of the European Union. The study was supported by the EU NextGenerationEU through the Recovery and Resilience Plan for Croatia under the project Dendro-Carbon (No. 400-01/23-01/6-2).
How to cite: Kern, A. and Marjanović, H.: Reduction of oak lace bug-related NDVI signals in 2025 following nearly a decade of persistence: legacy effects of extreme heat & drought in 2024 or subsequent winter conditions?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20008, https://doi.org/10.5194/egusphere-egu26-20008, 2026.
The summer of 2022 was marked by unprecedented heatwaves and droughts across Europe and the Yangtze River Basin (YRB) in China, triggering record-breaking negative anomalies in gross primary productivity (GPP) since 2000. To elucidate the drivers of these shifts, we employed a machine-learning-based factorial experimental design using FluxSat GPP data to quantify the contributions of concurrent climatic drivers and short-term legacy effects—specifically biotic vegetation growth carryover (VGC) and abiotic lagged climatic effects (LCE). Our results demonstrate that legacy effects are the primary drivers of GPP fluctuations, with the preceding month exerting the strongest influence. Attribution analysis further reveals that during the peak of these compound hot-dry events, vapor pressure deficit (VPD) was the dominant driver of GPP anomalies. However, VGC from the previous month subsequently emerged as the leading factor, with its relative contribution intensifying as the events progressed.
How to cite: Wang, J. and Yan, R.: Elucidating the Mechanisms of GPP Decline Triggered by Compound Drought-Heatwave extremes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13061, https://doi.org/10.5194/egusphere-egu26-13061, 2026.
Climate change is not only increasing mean temperatures but also the frequency and intensity of climatic extremes, including heat waves and freezing events. Such extremes directly affect plant physiological performance and increase stress and mortality risk in terrestrial ecosystems. Combined with land-use change such as shifts in agricultural management practices, increasing climate variability poses a growing threat to the semi-natural coastal heathlands of Europe, where the dwarf shrub Calluna vulgaris functions as a keystone species, influencing ecosystem structure and resilience.
Responses of Calluna to environmental stressors are known to vary across successional stages, which are determined by fire regimes which promote grazing, yet empirical data on how short-term temperature extremes affect physiological tolerance across life stages remain scarce. In addition, most studies address heat or cold tolerance in isolation, limiting our understanding of plant responses to the full range of thermal stress encountered throughout the seasons and under increasingly variable climatic conditions.
To address this knowledge gap, this experimental study investigates seasonal and ontogenic variation in leaf-level thermal tolerance limits of Calluna in the red-listed Norwegian coastal heathlands. The work is a contribution to an ongoing multi-season research effort at the semi-managed heathlands on Lygra in western Norway and includes four post-fire successional stages (pioneer, building, mature, and degenerative). Here, we focus on data collected so far during two key seasonal phases (autumn and winter), capturing contrasting physiological states relevant to thermal acclimation. Across these seasons, Calluna individuals are sampled from each successional stage and exposed to controlled short-term heat and freezing treatments designed to simulate extreme temperature events.
Thermal tolerance is quantified at leaf level using chlorophyll fluorescence to determine the temperature at which photosynthetic efficiency declines by 50% (T₅₀). Heat tolerance is assessed using water bath exposures across a temperature range of 20–56 °C, while freezing tolerance is measured using controlled freezing treatments down to −20 °C. In parallel, leaf functional traits are measured to examine links between seasonal shifts in key traits such as leaf area, thickness, and mass with the physiological temperature limits.
By identifying when and which successional stages are most vulnerable to thermal extremes, this work will improve our understanding of shrub-dominated ecosystem sensitivity and inform predictions of heathland resilience under an increasingly variable climate and increasing land abandonment.
How to cite: Sagabraaten, B., Skogstad, E., Vandvik, V., and Geange, S.: Seasonal variation in tolerance to short-term heat and freezing extremes across land-use-driven successional stages in Calluna vulgaris, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17558, https://doi.org/10.5194/egusphere-egu26-17558, 2026.
Climate change is increasing the frequency, intensity and duration of drought events. Globally, the observed area in drought was on average 27% for the period 2018-2022, up 74% compared to the period 1981-2017 (S. H. Gebrechorkos et. al. 2025). This global amplification of droughts has contributed to widespread declines in forest productivity and increased drought-induced mortality.
In line with global trends, the United Kingdom has experienced increasingly dry and hot summers, with record temperatures in excess of 40 °C measured in the summer of 2022. As this trend continues under climate change, predicting the immediate and legacy impact of droughts on forests is increasingly important for forecasting ecosystem functioning and resilience, and for constraining how carbon uptake modulates carbon-climate feedbacks.
Land surface models, including the UK land surface model (LSM) JULES (the Joint UK Land Environment Simulator), tend to overestimate the direct effects of water stress on gross primary productivity (GPP) and latent energy (LE) fluxes, whilst lacking a mechanistic representation of post-drought legacy effects. In this study, we implement two stomatal optimisation models into the JULES improving predictions of GPP and LE under drought conditions. We extend the hydraulic components of these models to capture the legacy impact of drought-induced hydraulic conductance loss. The first approach treats conductance loss as instantaneous, depending solely on the historic maximum water stress under drought. The second approach treats conductance loss as cumulative, depending on both the magnitude and length of the drought event.
Focussing on the 2022 UK drought at the Alice Holt eddy covariance site in Southern England, we find that our models predict reductions in GPP and LE of -6% and -20% respectively when comparing 2022 to non-drought years. Flux tower observations indicate a -20[+1,-11]% reduction in LE. While the instantaneous and cumulative hydraulic legacy models predict permanent conductance losses of 11.56[+0.02,-0.3]% and 7[±1]%, respectively, neither captures a notable decline in GPP or LE in the year following the 2022 drought.
We then asked what change in drought extremes would be required to induce hydraulic failure in this UK Oak Woodland. To test this, we repeated the simulated experiment, intensifying the drought by removing spring rainfall (broadly consistent with spring 2025) and progressively reducing 2022 rainfall, first by half and then to a quarter of its original amount. The instantaneous and cumulative hydraulic legacy models, when applied to the half (quarter) rainfall, predict permanent conductance losses of 16.6[+0.1,-0.4]% (34[+0,-2]%) and 39[+2,-0]% (65.2[+0.6,-0.2]%) respectively. The larger permanent conductance losses under the increased drought conditions were sufficient to induce significant reductions in GPP and LE in the year following the drought.
Our results present an important advance in our ability to forecast the long-term impact of drought on tree productivity and resilience within LSMs. By mechanistically capturing both immediate and legacy hydraulic responses, these predictions provide a robust evidence base for decision-making related to forest management, the resilience of restoration plantings, and the role of forests in achieving net-zero emission strategies.
How to cite: Baguley, C. and De Kauwe, M.: Predicting the imediate and legacy impact of the 2022 UK extreme summer on forest productivity., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21906, https://doi.org/10.5194/egusphere-egu26-21906, 2026.
Beech is one of the most ecologically and economically important tree species in Europe and in the context of climate change it is expected to expand towards higher and cooler elevation in the Alpswhile facing stress and potential decline at its lower, warmer limits due to increased drought and heat.
Studying this species ecology is therefore essential to understanding its ability to cope with shifts of climatic regimes towards warmer mean temperatures and more frequent climatic anomalies.
This study evaluated the xylematic sap flux response of a beech stand in the eastern Alps at an elevation of 1250 m in the area of Cembra (Trentino, Italy), in relation to the climate anomalies occurred between 2019 and 2024, determined from a 20+ years archive of standard meteorological observations.
Experimental data were collected using Tree Talker (TT+) devices, which allow continuous monitoring of sap flux density (SFD) at hourly step by application of the thermal dissipation technique; probes were installed on 32 trees of similar size and hierarchy divided into three plots.
Raw flux data from individual trees were routinely subjected to data quality check, including the scrutiny of temperature probes correct functioning, data transmission errors, anomalous differences of temperature between heated and reference temperature probes produced by below canopy solar irradiance gradients
We first characterized the temporal variability of SFD from hourly to seasonal scale as well as the spatial one at individual tree scale and across plots.
The functional resistance of individual trees and of the stand during the identified anomalies in air temperature and VPD were analyzed by quantifying the variation in the daily mean and maximumSFD observed at the peak of the climate anomalies compared to pre-event conditions. Similarly, functional resilience was retrieved considering post-event conditions.
SFD differences among plants was large (up tp a factor 5), and to some extent explained by factors such as tree density and topography (slope, aspect). No significative relationships with tree diameter or height were found.
The seasonal and monthly pattern of sap flux resulted in being driven by two fundamental variables: total solar radiation and VPD, the former triggering the flux while the latter modulating its intensity.
Beech trees appeared to be able to maintain stable SFD values during moderate droughts and heat waves but showed a significant reduction (-45%) under more intense anomalies combining drought and heat waves, as in July 2022. Nevertheless, even after even such cases the monitored trees were able to restore pre-anomaly sap flux rates, exhibiting good resilience.
How to cite: Eccher, M., Anfodillo, T., and Belelli Marchesini, L.: Impacts of climate anomalies on sap flow rates in a montane beech forest in the Italian eastern Alps, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22742, https://doi.org/10.5194/egusphere-egu26-22742, 2026.
- Atmospheric warming and high vapour pressure deficit (VPD) often co-occur and threaten forage production by constraining photosynthetic capacity and stomatal regulation. Yet their relative effects on plant productivity remain poorly resolved, necessitating a mechanistic understanding of how plants respond to heat and atmospheric dryness.
- We experimentally isolated the effects of warming and VPD on growth and physiology of the perennial C3 pasture grass Dactylis glomerata by growing plants in controlled-environment chambers at 26 °C and 30 °C under low (1 kPa) and high (2.4 kPa) VPD.
- High VPD reduced productivity more strongly at ambient than at elevated temperature, driven by a higher respiration-to-photosynthesis ratio, revealing an antagonistic interaction between warming and VPD. At ambient temperature, high VPD induced conservative water-use strategies that restricted stomatal conductance and suppressed photosynthesis. Under warming, however, thermal acclimation enhanced carbon assimilation and partially offset the negative effects of high VPD.
- Our results demonstrate that rising VPD poses a major threat to forage productivity primarily through stomatal limitation. Although reduced stomatal sensitivity under high VPD curbed water loss, sustained stomatal closure constrained carbon assimilation and growth. Warming partially mitigated these effects, indicating that atmospheric dryness—not temperature alone—may dominate future constraints on plant production.
How to cite: Chandregowda, M., Tjoelker, M., Pendall, E., and Power, S.: Warming offsets productivity losses from high evaporative demand in a widespread C3 pasture grass, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8431, https://doi.org/10.5194/egusphere-egu26-8431, 2026.
Megadroughts are extreme drought events defined by exceptional severity, duration, and spatial extent, potentially causing irreversible impacts on regional hydrological conditions and terrestrial ecosystems. With climate change driving more frequent global droughts, the likelihood of megadroughts has increased. Yet, their spatiotemporal patterns, evolutionary trends, and ecological impacts over the past century remain underexplored. By using a state-of-the-art clustering method, we identified 50 megadrought events between 1901 to 2020, with geographical hotspots concentrated in the western United States, southern Africa, and the Mediterranean region. Both drought duration and spatial coverage have increased markedly alongside global warming. Analysis of tree-ring chronologies from 4,595 sites worldwide using mixed-effects models reveals that the spatial extent of droughts exhibit stronger negative impact on radial growth than drought duration. Extensive droughts are likely associated with enhanced atmospheric aridity and increased risks of insect outbreaks facilitated by regional-scale migration, thereby amplifying growth reductions. Our findings challenge the long-standing emphasis on drought duration as the primary determinant of ecosystem functioning.
How to cite: Chen, H., Zhang, Y., and Zhang, H.: Stronger impacts of spatial extent than duration on tree growth during megadroughts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10923, https://doi.org/10.5194/egusphere-egu26-10923, 2026.
Under climate warming, compound drought–heat events have become increasingly frequent and intense, posing growing threats to terrestrial ecosystem productivity. However, the spatiotemporal patterns of atmospheric and soil drought–heat stresses under different compound occurrence modes, and their impacts on vegetation productivity, remain poorly understood. In particular, atmospheric drought–heat events (ACDHE), soil drought–heat events (SCDHE), and simultaneous atmospheric and soil drought–heat events are often not explicitly distinguished, limiting clear assessments of ecosystem responses to compound climate extremes. Here, we identify ACDHE and SCDHE during 1982–2020 using ERA5 reanalysis data and the daily Standardized Precipitation Evapotranspiration Index (SPEI). ACDHE was detected based on maximum air temperature (Tmax) and SPEI, while SCDHE was identified using soil temperature and soil moisture. Based on their temporal concurrence, compound events are classified into three occurrence modes, including independently occurring ACDHE events (Indep_ACDHE), independently occurring SCDHE (Indep_SCDHE), and simultaneous atmospheric–soil compound events (Simultaneous). We quantified long-term changes in event frequency, duration, and intensity across the three modes, and further assess vegetation productivity losses using FluxSat gross primary productivity (GPP) data. Results show that during 1982–2020, all three compound drought–heat modes exhibit significant increasing trends in event frequency, duration, and intensity (p < 0.001). Indep_SCDHE shows the fastest increase in occurrence frequency (+0.14 events decade⁻¹), whereas simultaneous events display the strongest increase in duration (+0.51 days decade⁻¹). Indep_ACDHE exhibits comparatively smaller increases across all event characteristics. Analyses of vegetation responses indicate that simultaneous events are associated with more prolonged and severe vegetation impacts than independent events. Specifically, simultaneous events are associated with longer decline and recovery times than independent events, with decline and recovery times extended by about 1–2 days. In addition, simultaneous events exhibit greater productivity losses, with maximum GPP loss (Z-score) and cumulative GPP loss exceeding those of Indep_ACDHE by 0.09 and 4.60, and those of Indep_SCDHE by 0.01 and 0.99, respectively. This study explicitly distinguishes Indep_ACDHE, Indep_SCDHE, and simultaneous events, enabling a clearer quantification of vegetation productivity responses across compound drought–heat occurrence modes and highlighting the disproportionate impacts of simultaneous atmospheric–soil drought–heat events on ecosystem productivity under climate extremes. Building on these results, we are further investigating the relative roles of atmospheric, soil, and ecosystem-related drivers in shaping vegetation productivity responses across different compound drought–heat occurrence modes.
How to cite: Yuan, B., Lee, S.-C., Peng, J., and Du, P.: Stronger vegetation productivity responses to simultaneous atmospheric and soil compound drought–heat events, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11930, https://doi.org/10.5194/egusphere-egu26-11930, 2026.
Snow is critical for many ecosystems across the Northern Hemisphere, as snowmelt provides a reliable water supply during spring. However, the increasing frequency of snow droughts, defined by reduced or absent winter snow accumulation, poses a growing threat to soil moisture recharge and consequently, to terrestrial ecosystem functioning. Snow droughts are commonly quantified by reductions in snow water equivalent (SWE), which represent the amount of water stored in the snowpack. Reduced SWE limits soil water recharge during melt, leading to soil moisture deficits that can persist into the growing season, diminish late-season water availability and ultimately reducing plant productivity.
Despite their importance, the impacts of snow droughts on the terrestrial carbon cycle remain poorly understood, particularly with respect to gross primary productivity (GPP). Moreover, interactions between snow droughts, soil moisture and precipitation during the growing season, and their potential to amplify or offset ecosystem impacts are largely unknown.
Here, we investigate how and where snow droughts have adversely impacted GPP across various ecoregions of the Northern Hemisphere over the past decades. Using the LPJmL dynamic global vegetation model and observational, gridded snow and GPP products, we attribute summer GPP anomalies to snow droughts and identify a wide range of ecoregions where snow droughts both directly reduce GPP and amplify productivity losses when occurring in combination with low spring precipitation.
This research advances understanding the feedback mechanism across seasons between snow, soil moisture, and vegetation productivity, providing new insights into ecosystem vulnerability under a changing climate.
How to cite: Varela, M., Koch, F., and Gampe, D.: Snow Drought Impacts on GPP Anomalies Across Ecoregions of the Northern Hemisphere , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14524, https://doi.org/10.5194/egusphere-egu26-14524, 2026.
Abstract:
Increased frequency and duration of extreme heat and drought events threaten European forests. The mechanisms that trees, and oaks in particular, have evolved to cope with such events are not fully understood. In August 2024, forests across Central and Southeastern Europe experienced an unprecedented episode of premature leaf senescence (PLS). The most widespread PLS occurred in forests of northern Croatia, as well as in Hungary, Bosnia and Herzegovina, Serbia, Romania, and Bulgaria. The aim of our work was to investigate the extent, timing, and species-specific characteristics of PLS and, based on recovery in the following year, to which degree the observed senescence reflected a controlled physiological response or tree mortality.
We used meteorological data (FORESEE, ERA5-Land) in combination with satellite-derived spectral reflectances and vegetation indices (VI), to detect the event at the regional scale with Terra/MODIS (2000–2025) and place it in a multi-decadal context. At the local scale, we used 30 m resolution Harmonized Landsat–Sentinel-2 (HLS, 2017–2025) data for precise characterization of the onset and spectral signature of PLS. Forest management maps and soil data were used to investigate PLS occurrence and intensity with the respect to tree species and location in Croatia.
GRVI and red reflectance proved superior to NDVI for discriminating heat- and drought-induced PLS from the gradual damage of oak lace bug (Corythucha arcuata, Say), an invasive species affecting oaks and complicating remote-sensing monitoring of phenology events. Species-specific analyses revealed that although extreme meteorological conditions reduced photosynthetic activity across multiple forest types, premature leaf senescence predominantly affected sessile oak (Quercus petraea, (Matt.) Liebl.), while European beech (Fagus sylvatica) remained mostly unaffected. In central Croatia in 2024 approximately 67% of sessile oak experienced PLS, occurring on average 54 days earlier than the long-term mean timing of autumn senescence.
Spring 2025 green-up confirmed that PLS in sessile oak was a reversible stress response rather than widespread mortality. Our results highlight the capacity of sessile oak for controlled premature senescence as an adaptive strategy under compound climate extremes, with implications for forest resilience, carbon cycling, and management.
Keywords:
Premature leaf senescence, Sessile oak, Space-borne remote sensing, GRVI, extreme weather, HLS, MODIS.
Funding:
The study was supported by the EU NextGenerationEU through the Recovery and Resilience Plan for Croatia under the project Dendro-Carbon (No. 400-01/23-01/6-2), the Hungarian Scientific Research Fund (OTKA FK-146600), National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014) project within the framework of Hungary's National Recovery and Resilience Plan, supported by the Recovery and Resilience Facility of the European Union.
How to cite: Marjanović, H. and Kern, A.: Extreme heat and drought induced large-scale leaf senescence in sessile oak in summer 2024 with near-full recovery in the following year, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19269, https://doi.org/10.5194/egusphere-egu26-19269, 2026.
High temperature, and atmospheric vapor pressure deficit and soil water stress increasingly threaten tree functions that regulate forest ecosystem productivity. Tree growth reflects the dynamic balance between carbon acquisition and allocation, and is highly sensitive to water availability, yet the mechanisms linking short-term water status to radial growth remain poorly understood, particularly in tropical forests experiencing intensifying dry seasons. Here, we explored the environmental and functional drivers of tree growth and water status in the Congo basin rainforest, the second largest tropical forest on Earth.
Specifically, in the Dja faunal reserve of eastern Cameroon, we quantified radial growth (RG) and tree water deficit (TWD) over an entire year and assessed their drivers, with a particular focus on understanding tree responses during dry periods. High-frequency (15-minute resolution) automatic dendrometer data were used to quantify dynamics of growth and stem shrinkage in 100 individuals of 16 tree species along a water availability gradient (wet vs dry conditions). Key functional traits related to resource use strategies and drought response syndromes were also measured on the same trees, including specific leaf area (SLA), wood density, water storage capacity, capacitance, turgor loss point, minimal conductance and variations in leaf water potential and relative water content over the dry season, enabling analyses of individual-level growth and water status, response to edaphoclimatic drivers, and functional trait syndromes.
In line with the view that species with rapid carbon acquisition capitalize on short favorable periods but remain highly sensitive to water limitations, we hypothesized that trees with acquisitive resource-use traits (e.g., high SLA, low wood density) and larger tree size would grow faster, but that trees with higher drought tolerance traits would show smaller growth reductions during dry periods and sustain functions under higher TWD. We found that prolonged dry seasons extended the duration of stem shrinkage, delayed post-drought growth recovery, and reduced the proportion of dry-season growth relative to annual growth, particularly in drought-sensitive species. By jointly analyzing individual growth, water status and functional traits, this study revealed how contrasting strategies of resource acquisition and drought tolerance regulate growth-water trade-offs and shape tropical forest resilience under increasing climatic stress, with implications for ecosystems functioning under future climate extremes.
How to cite: Adet, L., Poyatos, R., Binks, O. J., Mencuccini, M., Dauby, G., Fortunel, C., Maréchaux, I., Ploton, P., Trueba, S., Djamnou, G. N., Ndjock, F. Y., Mfout, A. V., Pélissier, R., and Martínez-Vilalta, J.: Environmental and functional drivers of tree radial growth and water status in the Congo basin rainforest, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3502, https://doi.org/10.5194/egusphere-egu26-3502, 2026.
Escalating droughts are posing unprecedented challenges to environmental stability and sustainable development. In particular, meteorological drought can propagate to soil and ecological droughts, triggering cascading disruptions in ecosystem functioning. However, whether the ecohydrological damage—the sum of standardized vegetation greenness and soil moisture losses—is disproportionately amplified through drought propagation has not been systematically assessed, which severely limits our ability to anticipate catastrophic drought cascades and implement timely adaptation strategies. Using global remote sensing data, we found that ecohydrological damage reached 162% to 310% of the initial meteorological drought intensity, due to prolonged drought duration and increased peak intensity. Once meteorological drought intensity exceeded the standardized threshold of 2.18, ecohydrological damage escalated nonlinearly. Externally, soil and ecological droughts were more sensitive to meteorological droughts driven by precipitation deficits and potential evapotranspiration surpluses, respectively, but the former propagated more efficiently. Internally, vegetation–soil feedbacks promoted the propagation from soil to ecological drought, while dampened the reverse process, resulting in the greatest ecohydrological damage when meteorological drought first triggered soil drought and then ecological drought. Declining ecosystem resilience and increasing climate variability may exacerbate future drought propagation and its damage. These insights are critical for advancing early warning systems and mitigating cascading drought losses.
How to cite: Qu, Z., Li, X., and Peñuelas, J.: Drought Propagation as a Nonlinear Amplifier of Ecohydrological Damage, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4079, https://doi.org/10.5194/egusphere-egu26-4079, 2026.
The shifts in temperature, vapor pressure deficit, and soil moisture associated with anthropogenic climate change are causing new extremes for trees. Understanding their impact on tree growth, however, remains challenging given limited large-scale soil moisture data, the complex interaction between multiple climate drivers, and size-dependent tree growth. We developed a new modeling framework that integrates individual-level growth trends with species-specific climate sensitivities. This hierarchical Bayesian model can accommodate different sampling regimes and is specifically designed to capture extreme growth responses across trees, species and ecosystems. Using new soil moisture data from WLDAS, we apply the model to 1.6 million observations of tree-ring width across Western North America. We identify the significant drought period of the 2000-2007 as causing exceptional reductions in tree growth. These reductions are well beyond those predicted from direct responses to temperature, vapor pressure deficit and soil moisture or their interactions, suggesting gaps in our fundamental understanding of tree growth responses to climate. Ultimately, these results demonstrate that tree growth is a critical indicator of drought, and that many current models may underestimate growth declines associated with extreme drought events.
How to cite: Van der Meersch, V., Cook, B., Betancourt, M., and Wolkovich, E.: Tree growth responses to extreme drought events are not well predicted by climate, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14062, https://doi.org/10.5194/egusphere-egu26-14062, 2026.
Understanding how tree species access and use water resources under conditions of more frequent and intense droughts is important for predicting the resilience of forest ecosystems to climate change. This knowledge is particularly needed in the light of recent droughts, which have led to unprecedented rates of tree mortality in European forests. The southern Rhine valley region of Rhineland-Palatinate and Hesse (Germany) was one of the most affected forest areas where massive crown dieback and tree mortality have been observed. The species-rich Lenneberg Forest in the centre of this region was selected for setting up an intensive monitoring plot for studying water use strategies in native broadleaved European tree species. The hydrological regime of this old-grown, species-rich forest is exclusively shaped by atmospheric precipitation and a deep reaching soil profile and thus offers ideal conditions for studying effects of drought on soil-plant water relations.
The research aim was to investigate the differences in soil water uptake depths among six co-occurring tree species (Fagus sylvatica, Quercus robur, Tillia cordata, Acer platanoides, Fraxinus excelsior, Prunus avium). Four sampling campaigns were carried out from 2023 to 2025 to collect soil water samples from various depths within the rooting zone of 57 selected trees. Thermal dissipation sap flow sensors were installed on all trees and the calculated sap flow velocities were used to estimate the water uptake time. Based on this, twigs for xylem water extraction were collected by tree climbers from the upper canopies. Stable isotope ratios of hydrogen (δ2H) and oxygen (δ18O) were measured in both soil and xylem water. Bayesian stable isotope mixing models were employed to estimate the relative contribution of each Root Water Uptake depth to the xylem water mixture on a species and individual-tree level. Electrical resistivity tomography was additionally used to visualize the spatial distribution of humidity across the soil profile.
During wet conditions the dominant source of xylem water was the topsoil layer (0-10 cm) across all species. However, with drying of the soil profile we found three different responses: (i) a pronounced downward shift in water uptake depth (Quercus robur, Fagus sylvatica, Fraxinus excelsior), (ii) slight shift toward deeper sources while maintaining primary reliance on the topsoil layer (Acer platanoides, Prunus avium), and (iii) continuous uptake from the topsoil (Tillia cordata). None of the species showed substantial contributions of deeper soil to the xylem water (20-30, 30-70 cm). This observation is consistent with a sustained depletion of water reserves in deeper soil layers during the vegetation period. Our findings contradict previous reports that trees continuously shift water uptake deeper into the soil profile with increasing drought. The consistent reliance on shallow soil water observed in this study highlights potential vulnerabilities of certain species to prolonged drought and underscores the need to integrate site-specific rooting and soil hydraulic constraints into forest management and climate adaptation strategies.
How to cite: Ognjenovic, M., Greve, M., Wambsganß, J., Reiter, P., Beyer, F., and Arend, M.: Soil water uptake strategies of European tree species: a comparative study in a drought-prone forest ecosystem, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19418, https://doi.org/10.5194/egusphere-egu26-19418, 2026.
