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 (T_EL), gas exchange, and plant water status.

We observed clear site- and treatment-dependent differences in thermal tolerance. Overall, control trees exhibited higher T_EL 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 T_EL than shaded leaves in Spain, but in France, shaded leaves had higher T_EL than sun-exposed leaves. In the control treatments, shaded leaves consistently had higher T_EL 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.