This session invites contributions that address climate change effects on forest ecosystems across scales and biomes, from observations to projections. In particular, we seek submissions on:
• Quantification of how natural and anthropogenic disturbances influence forest productivity, health, and growth.
• Multidisciplinary approaches (from ground to remote sensing) to monitoring tree vulnerability at local, regional, and global levels.
• Mapping and forecasting forest mortality and dieback under diverse climate change scenarios.
• Mechanistic and data-driven models of climate and environmental controls on forest vigor and growth, across scales and processes (e.g., wood formation, leaf phenology, shoot growth, canopy dynamics).
• Vulnerability of old-growth and senescent forests to climate change.
• Assessment of forest resilience to drought and other extreme events (e.g., frost, freeze–thaw cycles).
• Standardization of growth-monitoring techniques for forests experiencing extreme climate events (heat waves, droughts, cold spells).
• Adaptive management strategies to mitigate forest vulnerability.
• Decision-support tools for forestry and land management that integrate multiple stakeholders and multifunctional objectives.
Orals: Tue, 5 May, 14:00–18:00 | Room 2.23
Rising air temperature and atmospheric evaporative demand are fundamentally altering plant water relations, yet their long-term consequences for drought vulnerability and mortality remain poorly constrained. While short-term physiological responses to heat and high vapor pressure deficit (VPD) are increasingly well documented, far less is known about how chronic exposure reshapes plant water-use strategies, hydraulic regulation, and the mechanisms underlying mortality risk under future climates.
Here, I synthesize experimental and field evidence showing that exposure to elevated temperature and evaporative demand can induce lasting acclimation in plant water-use behaviour. Results from manipulative experiments in mature forests and controlled conditions reveal that acclimation to atmospheric conditions can modify stomatal regulation, whole-plant transpiration, crown structure, and the coordination between soil and atmospheric drought responses. In some cases, these adjustments maintain carbon gain under moderate stress but accelerate soil water depletion and shift physiological thresholds governing stomatal closure and hydraulic safety during drought. Such acclimation effects can translate into altered drought outcomes, including changes in mortality risk. Together, these findings suggest that acclimation to warmer and drier atmospheric conditions does not necessarily confer increased drought resistance, but may instead reconfigure vulnerability by modifying how and when plants restrict water loss.
By linking physiological acclimation, water use, and emerging mortality patterns, this presentation highlights the need to explicitly account for atmospheric history and acclimation processes when predicting vegetation responses to future climate scenarios. Understanding when acclimation buffers stress, and when it amplifies risk, will be critical for improving projections of forest resilience under continued warming and intensifying atmospheric drought.
How to cite: Grossiord, C.: When acclimation backfires: how chronic heat and atmospheric drought reshape plant water-use strategies and mortality risk, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1705, https://doi.org/10.5194/egusphere-egu26-1705, 2026.
Drought is a major driver of change in forest ecosystems, yet its quantification remains highly inconsistent across ecological studies. Consequently, the same period or location may be classified as under the effect of drought in one study and as near-normal in another, undermining comparability, synthesis, and inference. A systematic reanalysis of 161 drought events reported in forest ecosystem studies was conducted, to assess how drought definitions and quantification practices affect the accuracy of reported drought conditions.
Drought definitions were categorized into general descriptors (e.g., “dry,” “dry season,” or “different from normal”) and specific, quantifiable metrics (e.g., reduced precipitation, low soil moisture, standardized drought indices). We then examined how these definitions varied across forest types, drought “spheres” (atmospheric, soil, and hydrological), study approaches, and global regions. A clear pattern emerged showing that drought definitions are strongly biased toward atmospheric metrics, with soil and hydrological droughts being underrepresented, largely due to differences in data availability.
Across both experimental and observational studies, drought quantification proved to be a critical determinant of classification accuracy. General, non-quantified terms such as “dry” or “dry season” were frequently used but contradicted when benchmarked against the Standardized Precipitation–Evapotranspiration Index (SPEI). This highlights the importance of explicitly defined thresholds in ecohydrological research. Clearly stated and standardized thresholds would substantially improve global comparability, reduce subjective bias, and strengthen links among observational, experimental, and modeling studies of drought impacts on forests. Such improvements are essential for robust synthesis of drought attribution, development of mechanistic physiological understanding, and effective forest management under climate change.
How to cite: Gharun, M.: When ‘Dry’ Isn’t Dry: How drought definitions shape our understanding of forest responses to drought, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19476, https://doi.org/10.5194/egusphere-egu26-19476, 2026.
Forest disturbances and excess tree mortality are increasingly reported worldwide, yet satellite monitoring is still often limited to coarse-resolution, binary forest loss products that miss fine-scale mortality where only a few trees decline within an otherwise intact canopy. This limits our ability to quantify emerging disturbance dynamics, compare regions with consistent metrics, and identify early signals of larger forest change.
We present yearly, wall-to-wall maps (2018–2025) of fractional forest cover and fractional standing deadwood cover at 10 m resolution for Europe and beyond. We use these maps to provide a first Europe-wide quantitative overview of recent mortality patterns, summarizing the spatial distribution of elevated standing deadwood and its year-to-year dynamics from 2018 to 2025. These map products enable new analyses of disturbance dynamics at unprecedented spatial detail: tracking year-over-year mortality progression patterns; distinguishing general tree removal from trees dying standing by jointly analyzing forest and deadwood fractions; and quantifying subtle early-stage disturbance signals before they aggregate into larger forest change.
The maps link centimeter-scale aerial reference data from the crowd-sourced deadtrees.earth drone archive with multi-year Sentinel-2 reflectance time series: tree and standing-deadwood masks are derived on drone orthophotos using semantic segmentation, aggregated to sub-pixel cover fractions, and used to train a per-pixel computer vision model that translates reflectance signatures into annual forest and standing deadwood cover. As the deadtrees.earth drone archive continues to grow, its automated processing pipelines can feed regular model retraining, allowing the maps and models to be iteratively improved in space and time with each new contribution.
How to cite: Mosig, C. and the co-authors: deadtrees.earth Maps: Tree Mortality and Disturbance Mapping from Sentinel-2 Timeseries Across The Globe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18072, https://doi.org/10.5194/egusphere-egu26-18072, 2026.
Rising temperatures and droughts increasingly affect tree growth in European forests. However, there is a limited understanding of how the changing climate alters intra-annual growth dynamics and phenology, which are key drivers of tree productivity and carbon uptake. To address this knowledge gap, we investigated how changes in temperature, precipitation and vapour pressure deficit (VPD) affect tree growth dynamics as well as the timing and duration of the growing season using a network of automatic dendrometers installed on more than 2500 trees of various functional types (broadleaf, coniferous, deciduous, evergreen) in major European forest ecosystems - boreal, temperate, and Mediterranean. In this ongoing project, the dendrometers have been measuring tree growth with high precision (1 µm) and frequency (15 minutes) for over 4 years, covering both climatically average and unusually hot years 2022 and 2023. The growth and growth phenology variables derived from dendrometer data were modelled as a function of the climate variables obtained from the E-OBS database. Significant variations in intra-annual growth dynamics were observed across all forest ecosystems over the study period. The growing period was substantially shorter in hot and dry years compared to hot and wet years or years with average conditions. This reduction was primarily due to a significantly earlier growth cessation (by over a month in some years), which offset a slightly earlier growth onset after winter dormancy (by up to 7 days). The large shifts in growth cessation to earlier dates were strongly associated with lower precipitation and higher VPD during the month with maximum growth (May-July), while the earlier growth onset was related to elevated early spring temperatures. Also, higher VPD and lower precipitation were the main causes of reduced growth rates in the hot and dry years. Although the magnitude of the effect varied, the pattern of precipitation and VPD strongly influencing growth and phenology, with temperature playing a lesser role, was consistent across ecosystems and species. Because lower growth rates and the shorter growing season were strongly linked to a decline in total yearly growth in all the ecosystems, it is evident that changes in atmospheric dryness and water availability are likely to be the main drivers of climate-induced shifts in tree growth phenology, productivity and carbon uptake in European forests under a warming climate.
How to cite: Matula, R. and Plichta, R.: Unravelling the influence of climate change on tree growth patterns and phenology in European forests , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4526, https://doi.org/10.5194/egusphere-egu26-4526, 2026.
European forests have seen a rise in both harvest and natural disturbances (i.e., bark beetle and windthrow disturbances) in the last decades, with consequences for Europe’s Forest carbon sink, which is already declining in some countries. Among natural disturbances, drought-driven bark beetle outbreaks accounted for a third of unplanned canopy openings between 2015 and 2020. Bark beetles are theorised to preferentially target high-biomass forests, due to favourable breeding material, suggesting that the major outbreaks in 2018–2020 were inevitable in high-biomass spruce forests. However, direct comparison of forest biomass in pre-disturbance and undisturbed forests using remote sensing data remain unexplored despite their critical implications for future forest management.
We hypothesise that forests subject to upcoming bark beetle disturbances have a higher biomass than forests that have remained undisturbed throughout the satellite era. To test this, we combine 30m spatial scale forest disturbance data (1984 to 2023) with a 30m PlanetScope-based aboveground biomass (AGB) map for 2019 and a 10m forest genus map based on Sentinel-1/2. This approach allows us to examine forest AGB in 2019 before disturbances between 2021 and 2023 occurred, which we term “forests with upcoming disturbances.” We compared this with AGB in nearby (within 10km) forests that remained unaffected by disturbances throughout the entire period, termed “undisturbed forests”. Additionally, we included nearby forests that experienced disturbances between 1984 and 2019, referred to as “disturbed forests”.
Preliminary results show that needleleaf forests subject to upcoming unplanned disturbances have significantly higher AGB compared to nearby undisturbed forests, particularly in spruce forests, where biomass values are, on average, 30Mg/ha higher than undisturbed spruce forests. In contrast, no statistical difference was found between the biomass of spruce forests subject to upcoming harvest and undisturbed forests.
Enhancing the carbon sink in European forests is a crucial climate mitigation strategy and for achieving the European Green Deal goals. Prioritising the restoration of spatially heterogeneous forests over merely high biomass forests is therefore a crucial consideration. This strategy could help mitigate the increasing risk of bark beetle outbreaks under global warming.
How to cite: Heinrich, V., Kowalski, K., Viana Soto, A., Besnard, S., de Keersmaecker, W., Van de Kerchove, R., and Senf, C.: High Biomass Forests are more Susceptible to Bark Beetle Disturbance in Europe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3924, https://doi.org/10.5194/egusphere-egu26-3924, 2026.
In 2023/2024 Amazonia was struck by a record drought[1]. To date, only few studies have quantified the impact of this event on the Amazon rainforest. While these studies provide first insights on the response of Amazonia to the record drought, they did not include the second peak of drought in September 2024 or assess recovery trajectories succeeding this event.
To fill this gap, we here present analyses based on observations spanning 23 years of canopy-greenness. In particular, we compare the impact of the 2023/2024 drought with previous major droughts and quantify long-term trends of the enhanced vegetation index (EVI), extending the period under investigation from previous studies by more than a year. Moreover, we evaluate indicators of slowing down and reduced forest resilience, i.e. temporal autocorrelation and variance[2]. Finally, we assess how shallow water-table depth have buffered canopy-greenness decline as done in previous studies[3,4].
We observed record low EVI and declining forest resilience as indicated by a rising temporal autocorrelation and variance which remained on record levels even after the drought relaxation early in 2025. Specifically, the area featuring more than 10 % EVI decline reached a record spatial extent of 14 % early in 2025, while the spatial shares of regions featuring high temporal autocorrelation and variance were 2 and 3.4 times higher than under average conditions. Moreover, we observed shallow water tables to significantly buffer the negative drought impact on canopy greenness. Interestingly, shallow water tables appeared to be more prone to a slowing down which remains subject to further investigation. Taken together, our results point at an unprecedented decline and slowing down of canopy greenness dynamics in Amazonia up until September 2025, indicating the necessity to more closely assess direct and ongoing impacts of this event by means of ground observations, remotely-sensed indicators of productivity (e.g. SIF, VOD), and simulations from dynamic vegetation models.
[1] Ferreira V, Buras A, Zscheischler J, Mahecha M and Rammig A 2025 Evaluating the 2023–2024 record dry-hot conditions in the Amazon in the context of historical compound extremes Environ. Res. Lett. 20 084055
[2] Scheffer M, Bascompte J, Brock W A, Brovkin V, Carpenter S R, Dakos V, Held H, van Nes E H, Rietkerk M and Sugihara G 2009 Early-warning signals for critical transitions Nature 461 53–9
[3] Costa F R C, Schietti J, Stark S C and Smith M N 2023 The other side of tropical forest drought: do shallow water table regions of Amazonia act as large-scale hydrological refugia from drought? New Phytologist 237 714–33
[4] Chen S, Stark S C, Nobre A D, Cuartas L A, de Jesus Amore D, Restrepo-Coupe N, Smith M N, Chitra-Tarak R, Ko H, Nelson B W and Saleska S R 2024 Amazon forest biogeography predicts resilience and vulnerability to drought Nature 631 111–7
How to cite: Buras, A., Ferreira, V., Martins, N. P., Schnell, F., and Rammig, A.: Record decline and slowing down of canopy-greenness in Amazonia during the 2023/2024 drought, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4817, https://doi.org/10.5194/egusphere-egu26-4817, 2026.
Amazon tropical forests, the most extensive on earth, are a major but now declining sink for atmospheric CO2, due to both direct human-caused deforestation and increasing mortality in intact forests from rising temperatures and more frequent droughts. However, specific mechanisms underlying rising mortality in intact forests (and associated C sink declines), and its heterogeneous spatial distribution remain unresolved. Using MODIS-based remotely sensed observations of forest responses to the exceptionally hot 2023/2024 El-Niño drought (whose duration was unprecedented in the satellite era), we tested whether a statistical model of the biogeography of remotely sensed photosynthetic responses to droughts (2005, 2010, 2015/16 and 2023/24)---including vulnerability and resilience of canopy greenness---could also explain the biogeography of forest carbon sink vulnerability and resilience across the basin.
We found that the remote sensing-derived biogeography of canopy greenness also explained decadal C sink trends in ground-based forest plots, with the resilience of canopy greenness predicting which plots had sustainable C sinks and which had weakening C sinks over time due to increasing tree mortality. Factors predicting vulnerability to increased mortality (and declining C sequestration) included: deep water tables (where water resources are far from trees’ roots), shorter forests with shallow rooting depths (where tree access to water is limited), and especially, forests on fertile soils (which grow quickly with little investment in drought tolerance traits). Our biography of carbon sink resilience and vulnerability suggests that the distribution of ground-based monitoring plots are biased towards more vulnerable regions, and hence they over-estimate the rate of carbon sink decline. Adjusting for the distribution of biogeographic factors controlling carbon sink dynamics, we find the recent basin-wide carbon sink remains effectively stable. However, the exceptionally long 2023 drought guided identification of a critical drought-length threshold of 6 months, beyond which vulnerable forest regions expanded, suggesting that since longer droughts are becoming more common, C sinks may be destabilizing. This new approach, based on remotely derived forest sensitivity to climatic perturbations, identifies key drivers of forest demography and carbon dynamics, and reveals drought-length as a major contributor to the risk of forest tipping points and loss of carbon storage, with implications for resilience of Earth’s climate system.
How to cite: Chen, S., Terrer, C., Cuartas, L. A., Nelson, B. W., Nobre, A. D., Restrepo-Coupe, N., Stark, S. C., Tavares, J. V., Quaresma, C., Botia, S., Saleska, S., Schietti, J., and Aleixo, I.: Amazon forest carbon sinks are surprisingly resilient, but vulnerable to increasing drought length, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22382, https://doi.org/10.5194/egusphere-egu26-22382, 2026.
Widespread climate-driven increases in background tree mortality rates have the potential to reduce the carbon storage of terrestrial ecosystems, challenging their effectiveness as natural buffers against atmospheric CO2 enrichment with major consequences for the global carbon budget. However, the global extent of trends in tree mortality and their drivers remains poorly quantified. The Australian continent experiences one of the most variable climates on Earth and is host to a diverse range of forest biomes that have evolved high resistance to disturbance, providing a valuable test case for the pervasiveness of tree mortality trends. Here, we compiled an 83-year tree dynamics database (1941-2023) from > 2,700 forest plots across Australia covering tropical savanna and rainforest, and warm and cool temperate forests, to explore spatiotemporal patterns of tree mortality and the associated drivers. Over the past eight decades, we found a consistent trend of increasing tree mortality across the four forest biomes. This temporal trend persisted after accounting for stand structure and was exacerbated in forests with low moisture index or a high competition index. Species with traits associated with high growth rate – low wood density, high specific leaf area, and short maximum height – exhibited higher average mortality, but the rate of mortality increase was comparable across different functional groups. Increasing mortality was not associated with increasing growth, given that stand basal area increments either declined or remained unchanged over time, but it was associated with increasing temperature over time. Our findings suggest that ongoing climate change has driven pervasive shifts in forest dynamics beyond natural recovery in a range of forest biomes with high resilience to disturbance, threatening the enduring capacity of forests to sequester carbon under current and future climate scenarios.
How to cite: Lu, R., Williams, L., Trouvé, R., Murphy, B., Baker, P., Carle, H., Forrester, D., Green, P., Liddell, M., Marunda, C., Mannes, D., Mazanec, R., Ngugi, M., Neldner, V., Prior, L., Ruthrof, K., Suitor, S., Xia, J., and Medlyn, B.: Pervasive increase in tree mortality across the Australian continent, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4330, https://doi.org/10.5194/egusphere-egu26-4330, 2026.
Climate-driven increases in tree mortality represent a major uncertainty in projections of boreal forest carbon balance and ecosystem resilience. Detecting climate sensitivity in mortality patterns is particularly challenging in managed landscapes, where harvesting and silvicultural legacies obscure underlying ecological signals. Here, we analyse a large dataset of individual standing dead trees collected in 2023–2024 across 11 protected primary boreal forest areas along a north–south gradient in Finland, using aerial image-based detection methods. This setting provides an opportunity to assess tree mortality drivers under near-natural conditions.
The analysis covered 7,495 forest stands spanning 69,791 ha, with a total of 304,458 standing dead trees detected in the upper canopy layer (typically DBH ≥20–25 cm), paired with stand-level data on forest structure, development stage, species dominance, and habitat characteristics. Tree mortality was modelled using a two-part hurdle framework that separates the occurrence of mortality from its intensity (dead trees per hectare). Occurrence was estimated using ridge-regularized logistic regression, while mortality intensity was modelled with negative binomial generalized linear models to account for strong overdispersion. Stand area was included via offsets, and model robustness was evaluated using leave-one-area-out sensitivity analyses.
Across all model formulations and spatial subsets, stand structural attributes—most notably total standing volume—emerged as the strongest and most stable predictors of mortality intensity. A one-standard-deviation increase in log-transformed volume was associated with a 55–85% increase in expected mortality, indicating that biomass-rich stands exhibit heightened vulnerability to mortality processes. Field-measured total deadwood volume, where available, further amplified mortality signals, consistent with cumulative effects of past disturbance or chronic stress.
Forest development stage showed systematic but secondary effects. Relative to old-growth stands, younger and mid-successional development classes consistently exhibited lower mortality intensity, while differences among mature and old stands were modest once structural variation was accounted for. This suggests that apparent age-related patterns are largely mediated through biomass accumulation and stand structure rather than chronological stand age alone. Dominant tree species had comparatively weak effects: spruce-dominated and mixed species stands tended to show slightly lower mortality than pine-dominated stands, while birch-dominated stands exhibited reduced mortality in some model formulations. Overall, species effects were less stable than structural predictors.
Categorical habitat descriptors, including vegetation type and Natura 2000 habitat class, exhibited limited explanatory power after accounting for stand structure, development stage, and species dominance. Together, these results indicate that the climate sensitivity of tree mortality in boreal primary forests is primarily mediated by structural factors rather than habitat type. High-biomass stands may amplify the impact of climate-related stressors—such as drought, thermal extremes, or biotic agents—by increasing competition and physiological demand via increased evaporative area and metabolic costs.
Our findings provide a quantitative baseline for detecting climate-induced changes in boreal forest mortality and highlight the importance of structurally explicit approaches for assessing ecosystem vulnerability under ongoing climate change.
How to cite: Junttila, S., Polvivaara, A., Ahishali, M., Blomqvist, M., Chowdhury, A. I., Honkavaara, E., Vastaranta, M., Kattenborn, T., Horion, S., Brandt, M., Hammond, W., Allen, C. D., and Anderegg, W.: Tree Mortality in Boreal Primary Forests is Sensitive to Climate and Stand Structure: High-Resolution Evidence Across a Gradient of Protected Landscapes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17984, https://doi.org/10.5194/egusphere-egu26-17984, 2026.
Between 2018 and 2020, severe summer droughts caused unprecedented mortality of Norway spruce (Picea abies) in Central Europe. Large amounts of standing deadwood remained in forests, appearing grey or brown and contrasting with surrounding vegetation, making them detectable via remote sensing. Remote sensing-based monitoring of tree mortality is an important source, alongside ground-based observations, providing consistent spatial and temporal coverage over large areas.
In this study, we used satellite-derived standing deadwood data to develop a weather-driven empirical model of Norway spruce mortality and integrated it into the European forestry version of the dynamic vegetation model LPJ-GUESS. The model successfully reproduces observed patterns of tree mortality and associated biomass declines in Germany for 2010–2020. Summer solar radiation anomalies emerged as a key predictor, reflecting the combined effects of drought stress, canopy heat stress, and increased bark beetle activity. Incorporating large-scale satellite data substantially improved the model’s explanatory power, outperforming previous approaches based solely on ground-based mortality data.
Future simulations (2021–2070) under RCP2.6 and RCP8.5 scenarios indicate recurring drought-induced mortality comparable to post-2018 events, resulting in substantial reductions in forest carbon stocks and increases in calamity timber, while harvests of target-diameter timber are projected to decline in subsequent years. Without the weather-driven mortality component, LPJ-GUESS strongly underestimates drought impacts.
Our results highlight the significant risks to carbon storage in Norway spruce-dominated forests under recurring droughts in Germany. Given the central role of forests in Germany’s climate mitigation strategy, continued reliance on Norway spruce plantations poses challenges to both climate goals and the stability of the timber industry. We strongly recommend a rapid transition to diverse mixed forests across Central Europe to mitigate these risks.
How to cite: Anders, T., Hetzer, J., Tölle, M., Forrest, M., Kattenborn, T., and Hickler, T.: Combining satellite-derived forest deadwood estimates and vegetation modelling to predict drought impacts on Norway spruce forest biomass, timber harvest and carbon cycling in Central Europe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18031, https://doi.org/10.5194/egusphere-egu26-18031, 2026.
Since 2010, when Allen et al.'s highly-cited seminal paper was published in Forest Ecology and Management, a flood of studies have been published on the impact of drought on forest health and condition as well as tree physiology, greatly advancing our understanding of tree physiology and mortality processes. However, these findings have been interpreted by many as signs of a global forest decline due to the increasing frequency and severity of droughts linked to climate change. Upon closer examination of the literature, it appears that forest decline is limited to certain areas in certain regions and is not always induced by drought and associated or related disease and pest attacks, but also by other disturbances, such as windstorms or forest fires. All these disturbances are often facilitated by past changes in land use, such as afforestation in not suited sites or deforestation due to conversion of land to agricultural crops. Social pressure on land and forest appears to play a key role in forest decline, in addition to the role played by drought, as in the case of the forest decline observed in Central Europe in the 1980s, probably triggered by the drought of 1976, although it is generally believed to have been caused by atmospheric pollution.
How to cite: Cherubini, P.: Is it really only due to the drought?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7155, https://doi.org/10.5194/egusphere-egu26-7155, 2026.
In 2018, northern Europe was hit with an extreme summer drought event whose impacts on regional forest carbon cycling remain poorly understood and sparsely documented. Using southern Norway as a case study, we developed a remote sensing-based algorithm to investigate the relationship between drought severity and forest resilience, identify critical thresholds, characterize damages, and study the factors shaping them. We then trained stastical models to predict damages upon critical threshold exceedance and found that the event reduced gross primary productivity (GPP) by 4.55 Mt-CO₂ yr⁻¹ between 2018-2023, driven by loss of resistance (66%) with prolonged recovery and mortality (34%). This reduction explains 26% of the observed weakening in Norway’s forest carbon sink over the same period. Site-level features were the strongest predictors of damage. Our results highlight extreme drought's role as a major carbon cycle disturbance in northern European forests and provides a framework to inform adaptive management strategies and benchmark other, more mechanistic modeling tools.
How to cite: Bright, R., Eisner, S., Merlin, M., and Astrup, R.: Can we use critical thresholds to better inform drought damage modeling in forests?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10048, https://doi.org/10.5194/egusphere-egu26-10048, 2026.
Deadwood is an important component of the global forest carbon pool, with its decomposition regulated by both biotic and abiotic factors. Current Earth system models typically predict that global warming will accelerate microbial-mediated decomposition, however, these models often overlook the biogeographical constraints of wood-decay fungal communities. In this study, we integrated distribution and decomposition data of 19 representative wood-decay fungi into an interpretable machine-learning framework to simulate spatiotemporal patterns of fungal communities and quantify variations in deadwood carbon fluxes under climate change. We show that the fungal richness will increase rapidly in the future, but the global net carbon emission flux driven by fungal decomposition is projected to decline rather than rise under the high-emission scenario (SSP5-8.5). By 2100, net carbon emissions decrease by approximately 25.1% relative to the baseline (from 0.147 ± 0.052 Pg C to 0.110 ± 0.036 Pg C). This trend primarily stems from a community functional restructuring driven by temperature and moisture: the expansion of brown-rot fungi in boreal forests (+72.7%) leads to significantly enhanced carbon retention (+14.12%), whereas warming-induced moisture stress suppresses white-rot fungi decomposition rates, reducing carbon emissions in tropical and temperate forests by 37.4% and 11.7%, respectively. Our results reveal the "biological buffering" role of wood-decay fungal functional restructuring in the global carbon cycle, providing a foundation for improving future forest carbon sink simulations.
How to cite: Ni, C., Liu, S., and Zhu, B.: Climate-driven functional restructuring of wood-decay fungi dampens global deadwood carbon emissions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3736, https://doi.org/10.5194/egusphere-egu26-3736, 2026.
Insect outbreaks have been reported to increase worldwide in association with more frequent droughts and warming temperatures. Outbreaks of bark beetles and defoliators can cause substantial tree mortality and forest die-off, posing significant threats to carbon sequestration and forest functioning. Climate change can exacerbate such outbreaks by expanding insect habitats, reducing overwinter mortality rates, and enabling multiple generations within a single year. However, the dynamics and potential global impacts of insect outbreaks under climate change remain poorly understood due to the lack of fully coupled terrestrial biosphere models that incorporate both predictive insect population dynamics and their biogeochemical effects. Here, we propose a novel framework to simulate the population dynamics of representative insect types. The model simulates degree-day-based insect development to track transitions among life stages during the growing season, capturing climatic regulation on both phenology and outbreak emergence. This framework successfully reproduced realistic intra-annual population dynamics and temperature-triggered outbreaks at reported bark beetle and defoliator outbreak sites. Idealized future climate simulations reveal increasing outbreak frequency and potential perturbations to forest functioning and carbon storage under warming scenarios. Our work provides a novel approach for predicting insect outbreak risks under future climates and supports improved forest and pest management strategies.
How to cite: Ma, Y., Carnicer, J., Wirth, C., Wunderlich, L., Bernhard, D., Jornet Puig, A., Sönke, Z., and Bastos, A.: A Process-Based Framework for Predicting Climate-Driven Insect Outbreaks and Their Forest Biogeochemical Impacts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7359, https://doi.org/10.5194/egusphere-egu26-7359, 2026.
Increasing drought frequency and intensity under climate change is driving widespread forest dieback and tree mortality, particularly in water-limited regions such as the Mediterranean Basin. Despite extensive research on drought-induced growth decline, mechanistic understanding of how interacting hydraulic, carbon, and nutritional constraints predispose individual trees to dieback remains incomplete. Most studies have focused on single processes, limiting our ability to identify early-warning signals and robust predictors of mortality risk. Here, we apply a multi-proxy, tree-level approach that links growth dynamics, water-use patterns, and nutrient status to diagnose drought-induced canopy dieback in Pinus sylvestris, Pinus pinaster, and Pinus halepensis forests along an aridity gradient in north eastern Spain. Within each stand, dominant and co-dominant trees were selected and classified as non-declining or declining trees based on crown defoliation. For each individual, we combined dendrochronological analyses with foliar elemental and isotopic composition, morphological traits, and soil properties measurements. Growth vulnerability to drought was quantified by combining long-term growth trajectories and growth–climate relationships. Leaf carbon and oxygen isotopic composition data (δ¹³C, δ¹⁸O) were used to infer intrinsic water-use efficiency (iWUE) and time-integrated stomatal conductance. Foliar macro- and micronutrient contents, expressed per unit leaf area, were measured to evaluate nutrient imbalances associated with drought stress and senescence, and were interpreted in relation to soil pH and nutrient availability. Tree size and needle morphological traits, including leaf mass per area (LMA), were also measured. We applied multivariate analyses (principal component analysis, partial least squares regression) to integrate physiological, nutritional, and structural variables, discriminate between non-declining and declining trees, and identify key predictors of crown defoliation as a proxy for vigour decline. Our approach provided mechanistic insight into how chronic drought stress propagates through coordinated reductions in growth and stomatal conductance combined with nutrient imbalances, while revealing species-specific pathways to dieback. Overall, this study addresses key gaps in drought-induced forest mortality research and contributes to improving diagnostic and prognostic frameworks on forest dieback under ongoing climate change.
How to cite: González de Andrés, E., Gazol, A., Querejeta, J. I., Valeriano, C., Colangelo, M., Fernández-Blas, C., Rodes-Blanco, M., Ruiz-Benito, P., and Camarero, J. J.: Disentangling growth, water-use and nutritional constraints underlying forest dieback in drought-prone pine forests, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9528, https://doi.org/10.5194/egusphere-egu26-9528, 2026.
Over the past two decades, the occurrence of extreme climatic events in the Mediterranean region has increased, and this climatic pressure has contributed to the spread of vegetation dieback over several forest communities. Dieback has also affected Quercus ilex L., and since this decline has worsened over the last 15 years in many Mediterranean areas, it is crucial to develop effective tools for studying this phenomenon, combining different scales of measurement. Our study was conducted over four years (2019-2023) in declining (D) and non-declining (ND) Q. ilex stands in southern Tuscany (IT), assessing physiological and biochemical traits such as gas exchange, water relations, carbohydrate analysis in the wood, and xylem sap isotopic signal (δ18O). Dendrochronological and tree-ring δ13C analyses were combined to investigate the effects of previous droughts on tree growth and water-use efficiency.
The results of physiological analyses showed that seasonality had a strong effect on these traits, with the main stress occurring during the summer of 2020, as evidenced by the lowest gas exchange values. According to the results of δ18O analyses, holm oaks mainly took up water from deep soil sources (bottom soil or groundwater) owing to their deep-root systems, resulting in only slightly different ring-width patterns and a low responsiveness to seasonal climatic variations in both stands. By contrast, the δ13C results combined with SSR genotyping revealed a more conservative water use of the population in the ND stand, underlying the potential of combining these powerful tools for the selection of seed-bearing genotypes putatively tolerant to water deficit.
How to cite: Alderotti, F., Sillo, F., Gori, A., Centritto, M., Ferrini, F., Pasquini, D., Saurer, M., Balestrini, R., Cherubini, P., and Brunetti, C.: The widespread Quercus ilex L. dieback in Mediterranean forests: investigation of the causes at plant and ecosystem level, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13580, https://doi.org/10.5194/egusphere-egu26-13580, 2026.
Extreme weather events, including prolonged droughts and heat waves, are increasingly causing forest dieback and tree mortality across many ecosystems, particularly in the Mediterranean region. In recent years, widespread decline of Quercus ilex L. (holm oak) has been reported in Southern Europe, notably in the Iberian Peninsula and Italy. As a dominant tree species, holm oak dieback has the potential to reshape understory shrub and herbaceous communities. Furthermore, Mediterranean vegetation is a major source of Biogenic Volatile Organic Compounds (BVOCs), whose emissions are highly sensitive to environmental conditions.
This study examines seasonal variations in holm oak canopy cover, understory species richness, and alpha diversity indices (Shannon–Wiener and Pielou) in two holm oak stands located in the Maremma Regional Park (Tuscany, Italy). The stands differ in crown defoliation intensity and are classified as high defoliated (HD) and low defoliated (LD). Seasonal field surveys were conducted between 2019 and 2023 to characterize temporal changes in vegetation composition. Plant species inventories were used to assess the biological spectrum within stands and estimate habitat explanatory factors using Ellenberg's indicator values. Additionally, relationships between vegetation dynamics and ecosystem-level BVOC emissions were evaluated.
During the study period, both stands experienced a 50 % reduction in holm oak canopy cover. This canopy loss increased light availability and was associated with a temporary rise in understory species richness in 2021. In contrast, a marked decline in species richness was observed in 2022, falling below 2019 levels in both stands. This reduction is likely linked to the accumulation of dead wood on the forest floor and the extreme temperatures recorded during that year. By 2023, species richness recovered to values comparable to those observed in 2020 and 2021. Notably, the Shannon-Wiener and Pielou indices did not significantly vary in the two stands, where the biological spectrum displayed a clear Mediterranean characteristic. However, both stands exhibited a progressive increase in geophytes and therophytes, suggesting worsening water stress conditions. From October 2020, the LD stand was mainly characterized by scapose hemicryptophytes and phanerophytes and caespitose phanerophytes, whereas the HD stand was dominated by nano-phanerophytes. BVOC measurements closely mirrored vegetation changes, showing a clear reduction in monoterpene emissions associated with increasing holm oak defoliation and mortality. Elevated Ellenberg indicator values for light and temperature further confirmed the Mediterranean imprint of the vegetation and revealed signs of anthropogenic disturbance in the HD stand. This was reflected in the higher abundance of nitrophilous and medicinal herbaceous species, such as Atropa belladonna L. and Datura stramonium L., whose historical introduction and subsequent expansion may have been facilitated by canopy opening and tree decline.
Overall, although diversity indices remained statistically stable, holm oak dieback induced notable shifts in species composition, particularly in the HD stand. The pronounced canopy reduction and increasing dominance of nano-phanerophytes suggest that part of the forest is undergoing a transition toward shrubland. Additionally, the observed decline in BVOC emissions highlights the potential consequences of holm oak dieback for ecosystem functioning and atmospheric chemistry.
How to cite: Beltrami, S., Alderotti, F., Gori, A., Pollastrini, M., Centritto, M., Ferrini, F., Pasquini, D., and Brunetti, C.: Coastal Monitoring in the Mediterranean Basin: Issues and Technical Approaches, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17394, https://doi.org/10.5194/egusphere-egu26-17394, 2026.
Rising temperatures and increased drought intensity are driving accelerated tree mortality rates worldwide. We investigated how these changes affect the carrying capacity of mountain ash forests (Eucalyptus regnans), the world's tallest flowering plant and one of the most carbon-dense forests on Earth (450-819 tonnes carbon per hectare).
Using data from a large network of silvicultural experiments collected between 1947 and 2000 in southeastern Australia, we quantified temporal trends in mortality rates and carrying capacity, and their relationships to spatiotemporal climate variations. We analyzed how maximum stand density changes with tree size (self-thinning line) across different climatic conditions and over time, disentangling spatial variation among sites from temporal variation within sites.
Our results show forests growing in the warmest and driest conditions (highest vapour pressure deficit) had the lowest carrying capacity. This capacity further decreased with rising temperatures. Each one-degree Celsius increase in mean annual temperature was associated with a 9% reduction in carrying capacity. Based on these relationships, a projected three-degree Celsius increase by 2080 (CSIRO RCP8.5 scenario) could reduce tree density and carbon stocks by 24%, equivalent to losing 240,000 hectares of mature mountain ash forests or releasing 108 million tonnes of carbon.
Trees that died were 0.62 times the size of living trees (i.e., they were suppressed), with no detectable effect of climate on this ratio. These findings demonstrate that reduced carrying capacity could undermine carbon sequestration and global forest restoration efforts, particularly in seasonally dry regions where warming accelerates water limitations. We discuss implications for incorporating changing carrying capacity into forest management and carbon accounting.
How to cite: Trouvé, R., Baker, P., Ducey, M., Robinson, A., and Nitschke, C.: Global warming reduces the carrying capacity of the tallest angiosperm species (Eucalyptus regnans), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15563, https://doi.org/10.5194/egusphere-egu26-15563, 2026.
Climate change is intensifying the frequency and duration of water-limited conditions, increasing the risk of climate-induced physiological stress in trees and potentially altering their function as critical carbon sinks. Understanding species-specific responses to water stress is crucial for predicting shifts in ecosystem processes and functionality. This study investigates the photosynthetic and water-use responses of two contrasting tree species—Norway spruce (Picea abies), a conifer, in Wüstebach, and European hornbeam (Carpinus betulus), a deciduous species, in Hohes Holz—over a three-year period using sap flow measurements, in situ photosynthetically active radiation (PAR) data, and eddy covariance technique. Results show that gross primary production (GPP) declines under high vapor pressure deficit (VPD), with regulatory thresholds differing between species. Norway spruce exhibits reduced stomatal conductance and photosynthetic activity beyond a VPD threshold of 10 hPa, whereas European hornbeam maintains photosynthesis up to 16 hPa. Sap flow density measurements corroborate these thresholds, highlighting that water stress diminishes ecosystem GPP, yet conifers and deciduous trees employ distinct coping strategies. Hysteresis analysis of the relationships between sap flow and VPD, as well as sap flow and absorbed PAR (APAR), revealed significant interspecies differences. Norway spruce exhibited a directional shift in hysteresis (from counterclockwise to clockwise) in both sap flow-VPD and sap flow-APAR relationships at specific VPD thresholds, suggesting dynamic adjustments to water stress. In contrast, European hornbeam exhibited directional hysteresis changes only in sap flow-APAR relationships, implying differing physiological mechanisms underlying their water-stress responses. These findings underscore the utility of hysteresis analysis in elucidating species-specific water-stress regulation mechanisms. The study provides valuable insights into how coniferous and deciduous trees modulate stomatal conductance and sap flow under elevated atmospheric demand, shedding light on the broader implications of climate change for forest carbon dynamics. Keywords: Water stress, GPP, Sap flow, Hysteresis analysis
How to cite: Abdoli, M., Borger, R. J., Bogena, H. R., Hildebrandt, A., Pohl, F., Blume, T., Mayr, S., and Leuchner, M.: Species-Specific Water-Stress Responses in Coniferous and Deciduous Trees: Insights from Sap Flow and Hysteresis Analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17723, https://doi.org/10.5194/egusphere-egu26-17723, 2026.
Posters on site: Tue, 5 May, 08:30–10:15 | Hall X1
Rapid urbanization in Nairobi (Kenya) is rapidly degrading its urban forests, threatening the city’s long-held identity as the “green city in the sun.” The loss of tree cover in Karura, Ngong Road Forest, and Oloolua reflects a broader problem of deforestation driven by development pressure and weak policy enforcement. This issue is particularly specific because Nairobi’s unique combination of rapid urban growth, legal gaps, and intense competition for land makes forest decline occur more severely here than in many other African cities. It also represents a distinct case where national legislation such as the Forest Act (2005) and EMCA (2015) exist but their implementation within an urban setting remains inconsistent.
The data for this research included multi-temporal satellite imagery accessed through Google Earth Engine, focusing on Landsat and Sentinel datasets spanning 2000 to 2024. These datasets were used to generate vegetation indices such as NDVI to quantify forest cover change across the three major urban forests. Complementary policy documents, county urban planning records, and environmental legislation were analyzed to contextualize the observed changes.
The methodology combined remote sensing analysis and GIS mapping using platforms such as Google Earth Engine and QGIS. NDVI computation, supervised classification, and change detection techniques were applied to assess temporal and spatial forest cover decline. This geospatial work will be integrated with qualitative policy evaluation to identify the governance gaps driving the ecological trends.
The results showed a clear downward trend in forest cover over the last five decades, with sharper losses occurring during periods of accelerated urban expansion. We anticipated demonstrating misalignment between policy intentions and actual land-use outcomes, particularly were development overrides environmental protections. These results will likely reveal the need for stronger urban forest governance.
This research is important because it offers a science-based understanding of how policy failures directly shape ecological degradation in growing cities. It contributes to the broader field of global environmental change by linking remote sensing evidence with governance analysis. Ultimately, the study provides an outlook for improving urban sustainability, guiding policymakers and planners in protecting Nairobi’s remaining forests while addressing future urban growth pressures.
How to cite: Kabugi, R. and Székely, B.: Remote Sensing of Urban Forests in Nairobi City linking it to policy changes and implementation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1574, https://doi.org/10.5194/egusphere-egu26-1574, 2026.
The Amazon rainforest is a cornerstone of global climate regulation, carbon sequestration, and biodiversity conservation. However, its resilience is increasingly undermined by the combined pressures of mega-scale recurrent ENSO droughts and forest degradation. Such mega-droughts, amplified by rising temperatures and declining water, are known to suppress tropical forest functioning. Yet the basin-wide sensitivity of degraded versus intact forests to these events has remained poorly quantified. Here, we present preliminary basin-scale analyses of the unprecedented 2023–2024 Amazon drought, which was driven by a confluence of large-scale climatic anomalies, including a strong El Niño, tropical North Atlantic warming, and widespread marine heatwaves. This drought event produced record-low rainfall, sustained soil moisture deficits, and temperature anomalies across much of the basin. Leveraging satellite-derived canopy greenness (Enhanced Vegetation Index, EVI) and spatially paired comparisons between degraded and intact forest areas, we find that degradation markedly amplifies drought impacts: degraded forests exhibited an average greenness loss 2.43 times greater than intact forest during this drought event. Machine-learning attribution highlights drought duration and soil fertility as the dominant drivers of this vulnerability gap, with secondary modulation by baseline climatic conditions and canopy height loss. Predictive simulations further indicate that even moderate future degradation of currently intact forests could trigger functional impairment under recurring drought regimes, with northern white-sand zones and southern Arc of Deforestation emerging as high-risk hotspots. These preliminary results provide the basin-wide evidence that forest degradation and extreme drought act synergistically to intensify vegetation greenness decline, reframing Amazonian resilience as conditionally stable and highly sensitive to forest degradation. This work underscores the urgent need to incorporate forest degradation into climate impact assessments and conservation strategies to safeguard the Amazon’s ecological and climate-regulating functions.
How to cite: Zhao, Y., Pan, Y., Song, G., and Wu, J.: Degradation amplifies Amazon forest vulnerability to extreme drought: evidence from the 2023–2024 ENSO event, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7181, https://doi.org/10.5194/egusphere-egu26-7181, 2026.
Mediterranean agroforestry systems, dominated by holm oak and cork oak woodlands, constitute ecosystems of high ecological, productive and socio-economic value. However, these systems are currently undergoing global decline as a result of multiple factors, among which climate change and the activity of some diseases, mainly Phytophthora cinnamomi Rands stand out. This soil-borne oomycete causes necrotic lesions in roots and stems, leading to fine-root loss, reduced water uptake, progressive decline, and tree mortality. Its life cycle includes both sexual and asexual phases; during the latter, motile zoospores require free soil water for infection, enabling movement and contact with host roots. In this context, Mediterranean climatic conditions are particularly favorable for pathogen proliferation, characterized by relatively warm and wet winters and springs with frequently waterlogged soils, followed by long, dry summers that induce severe water stress in trees, exacerbate root disease symptoms, and contribute to the decline of these agroforestry systems. Therefore, it is essential to identify areas more prone to be affected by the pathogen and, when affected, their potential to be a source for pathogen spread. Despite the enormous efforts carried out for monitoring and improving the understanding of pathogen propagation, there is still a way forward to better quantify pathogen spread potential.
In this study, we adapted an existing index-based framework to assess the potential for non-point pollution (PNPI) at the watershed scale to model the spread of Phytophthora. The study has been carried out in Guadalquivir river basin, southern Spain. We assume that the spread of Phytophthora follows the same logic as the one followed by other substances previously modeled using PNPI. Hence, we reinterpret this framework to map the spatial potential for hydrologically mediated pathogen spread, assuming water as the main vector, the importance of landscape connectivity, and enhanced spread potential during wet years. The adapted index integrates three components: (i) a source indicator, accounting for land cover, landscape connectivity, inoculum pressure, and soil characteristics; (ii) a hydrological connectivity indicator, representing the potential for subsurface and downslope transport; and (iii) a runoff generation and transport capacity indicator, describing the landscape ability to mobilize and convey inoculum. Temporal variability in precipitation is incorporated to capture interannual climate control on spread potential. The main methodological challenge was redefining the source indicator to include: (a) host presence and density (oak cover, woodland fraction, fragmentation), (b) inoculum pressure (distance to known infection foci and presence of symptomatic trees), and (c) soil and site suitability (texture, drainage, water retention, and conditions favoring pathogen persistence).
We prove that using simple mathematical expressions and low data requirements, we produced annual maps of estimated Phytophthora decline potential, providing a spatially explicit screening tool to identify areas with higher potential for hydrologically driven spread, showing correspondence with known infection sources.
Acknowledgments: This research was performed within DRYAD Project, which has received funding from the European Union’s Horizon Europe research and innovation program under grant agreement 101156076. This work is part of the grant RYC2022-035320-I, funded by MCIN/AEI/10.13039/501100011033 and FSE+
How to cite: Zafra, M., Contreras, E., Gómez-Beas, R., Molina, A., González-Moreno, P., Ruíz-Gómez, F., Pimentel, R., and Andreu, A.: Accounting for the potential of oak-savanna decline caused by Phytophthora cinnamomi: conceptual framework , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11171, https://doi.org/10.5194/egusphere-egu26-11171, 2026.
Tree mortality is increasing worldwide, intensifying the need for forest monitoring systems that can detect vulnerability before irreversible damage occurs. Predicting mortality remains difficult because tree death results from interacting predisposing conditions, inciting disturbances, and contributing agents that unfold over time. These drivers interact with recovery processes in highly non-linear ways, creating cascading stress trajectories in which trees may cross tipping points beyond which recovery becomes impossible.
This review synthesizes the literature on resilience indicators and early warning signals for assessing tree mortality risk. Existing approaches span a broad range, from threshold-based indicators and model-based risk predictions to disturbance-focused recovery metrics and indicators derived from changes in time-series dynamics. Despite their conceptual diversity, most approaches share two key limitations. First, many overlook the tree-level physiological stress history, such that similar drought events may lead to minimal or catastrophic damage depending on prior stress exposure. Second, many indicators are often too late for operational resilience monitoring, relying on retrospective data or signals that typically emerge only after substantial structural damage has already occurred.
Physiological theory and empirical evidence indicate that stress responses relevant to mortality risk arise at the level of hormonal regulation and stomatal control, affecting photosynthesis, transpiration, and leaf reflectance well before structural damage occurs. Recent advances in sensor technology now enable high-frequency observation of these processes through a growing range of in-situ and remote measurements. Yet few frameworks exist to interpret such time series as early warning signals. While resilience theory offers promising concepts for understanding critical transitions, it has rarely been applied to real-time forest monitoring. We highlight this gap and emphasize the role of controlled experiments in validating which physiological signals reliably precede mortality, enabling the translation of high-frequency measurements into actionable early warning indicators.
How to cite: Schneider, P., Gessler, A., and Lever, J.: Early Warning Signals for Tree Mortality: A Review of Indicators, Limitations, and Emerging Opportunities, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4778, https://doi.org/10.5194/egusphere-egu26-4778, 2026.
Both biotic and abiotic factors are raising concerns about Mediterranean oak forests resilience to climate change, particularly Holm oak (Quercus ilex L.) forests. In the last decade, many drought events impacted this species in several Mediterranean countries, and a widespread decline of holm oak forests has been observed due to a combination of drought and the soil pathogen Phytophthora cinnamoni. In addition, climate change has induced a rise in mean winter temperatures, a seasonal shift of precipitation from summer to wintertime, and a tendency towards heavy rain and prolonged droughts, which are triggering factors for the current decline of holm oak in Mediterranean regions. The resulting reduction in holm oak forest vitality and productivity can ultimately lead to profound changes in ecosystem processes and functions. Previous studies based on tree-ring δ13C and SSR genotyping showed that different holm oak populations can differ in their water use efficiency, resulting in different drought tolerance. This study highlighted the potential of this analysis for the selection of seed-bearing genotypes aimed to preserve Mediterranean holm oak ecosystem and improving its forest management. In this context, LIFE RECLOAK project aims to restore and improve the conservation status of threatened forests by holm oak dieback using genotypes characterized by high level of drought tolerance and pathogen resistance. A step-by-step approach will allow the achievement of this ambitious goal over the five years of the project. In the first instance, drought-tolerant and pathogen-resistant genotypes will be selected through a genetic screening based on SSR genotyping and the identification of genetic markers associated with stress tolerance. A mesocosm trial will be carried out to confirm the drought tolerance and the pathogen resistance of the holm oak genotypes. Then, the selected seedlings will be planted in four pilot sites areas located in Mediterranean holm oak forests included in Natura 2000 network and affected by widespread dieback: Monti dell’Uccellina in Parco della Maremma (Tuscany, Italy); Parco della Maddalena (Sardinia, Italy); Muela de Cortes y el Caroche (Valencia province, Spain), Raso del Conejo Forest (Sierra Morena, Andalucia, Spain) and Wied il-Mielaħ u l-Inħawi tal-Madwar (Malta). After that, seasonal and multi-year monitoring for three years after the plantation will begin at each pilot site. The monitoring of pilot sites will be done by visual assessment and through the measurement of plant physiological performances by integrating gas exchange measurements with proximal sensing measures. The effects of reforestation on ecosystem functioning and climate mitigation will be investigated by measuring soil moisture, respiration, and microbial communities’ composition, as well as monitoring understory and overstory vegetation cover and biomass accumulation. Overall, this project will provide a reliable demonstration of restoring forest structure, thereby promoting forestry with conservation objectives and opening the possibility of restoring other Mediterranean areas affected by holm oak dieback.
How to cite: Brunetti, C., Alderotti, F., Beltrami, S., Balestrini, R., Sillo, F., Scanu, B., Brandano, A., Deidda, A., Vettori, C., Garosi, C., Marino, G., Fabbri, A. P. M., Centritto, M., and Gori, A.: Restore and improve the conservation status of threatened forests by holm oak dieback , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13751, https://doi.org/10.5194/egusphere-egu26-13751, 2026.
Semi-arid forests are becoming increasingly vulnerable to climate change as drier conditions are more frequent. In the coastal basins of semi-arid Chile, that are disconnected from the Andes, snow and rainfall contribute little to forest water supply. Instead, fog can provide an important moisture subsidy that helps sustain forest function and persistence, potentially increasing drought tolerance. Since 2010, however, the co-occurrence of stronger drought conditions and a decline in fog frequency have raised concern about the possibility of large-scale forest collapse.
Here we evaluate forest resilience and ask whether these forests are moving toward bifurcation-driven tipping points across a 4,067 km² coastal landscape. Using satellite time series spanning 1984–2024, we reconstructed land-surface phenology and identified post-2010 anomalies to quantify the magnitude and spatial extent of drought impacts. We then combined multiple satellite-derived indices in a multivariate framework to describe vegetation states associated with drought stress, explicitly contrasting the pre-2010 period with the subsequent drought years. From this analysis, we selected pixels showing the strongest evidence of potential state change and examined them with univariate time-series methods. We computed early warning signals (EWS) and critical slowing down (CSD) metrics, including variance, lag-1 autocorrelation (AR1), and the restoring rate (λ), using rolling ten-year windows to track changes in stability through time.
Results show that the extreme 2019 drought affected 82% of the forest area. Despite this widespread impact, forest patches were generally more resilient than adjacent shrublands: across the landscape, many forests remained stable and some showed signals consistent with recovery even under high drought severity. Univariate EWS analyses indicate that drought is increasingly constraining forest states; however, CSD metrics do not yet provide consistent evidence that the system has crossed a tipping point. Even so, continued extreme drought combined with further reductions in fog could erode buffering capacity and raise collapse risk. Overall, the relative stability of forests compared with shrublands supports the idea that fog-inundated forests persist with a higher resilience under progressive drying in this region.
How to cite: Gutierrez, A., Núñez-Hidalgo, I., Gaxiola, A., Matías, P.-E., and Roberto, C.-O.: Drought, fog and vegetation resilience: Potential tipping points of forests in semiarid coastal basins of central Chile., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14190, https://doi.org/10.5194/egusphere-egu26-14190, 2026.
Tolerance of tree species to changing water availability plays a critical role in determining forest resilience to climate change, particularly in drought prone warm temperate regions. However, the specific environmental drivers and water availability thresholds governing current distribution of many southern hemisphere tree species remain largely unknown. We investigated how climate, landscape position, and soil properties influence water availability and the distribution of karri (Eucalyptus diversicolor), a tall forest tree species endemic to southwestern Australia. While predominantly found in areas with rainfall above 800 mm/year, some outlying karri stands also occur in lower rainfall zones. Using a binomial generalised linear model (GLM) with a logit link, we modelled the probability of karri presence across the southwest range as a function of mean annual rainfall (bio12), mean temperature of the warmest quarter (bio10), and the topographical wetness index (TWI). On a regional scale, we found this simple model to predict karri occurrence with high accuracy, showing the importance of high rainfall, mild summer temperatures, and well drained upland landscapes. Second, we analysed a comprehensive reference dataset of soil physical and chemical properties across southwest Australia to better characterise local scale conditions associated with karri growth. We also investigated soil depth using passive seismic and contrasts in soil electrical resistivity between karri stands and neighbouring forest types. Our preliminary results indicate that karri preferentially occurs on deep, well drained, clay rich soils, but also persists in landscape positions where shallow soils or soil types with poor water-holding capacity (e.g. sands) maintain higher water-availability, owing to deeper water storage (e.g. karstic systems) or regular rapid recharge (e.g. runoff from adjacent rocky outcrops). Our research confirms that karri distribution is likely determined primarily by water availability, but this is moderated at different scales by interacting climatic, topographic, and soil controls. This study provides a more nuanced foundation for predicting vulnerability of particular populations to future drought stress. Ongoing studies are quantifying water use across the distribution range under varying site conditions.
How to cite: Fabry, D., Eckersley, J., Leopold, M., Bleby, T., Renton, M., and Grierson, P.: Multi-scale controls on water availability shape the distribution of karri (Eucalyptus diversicolor) in southwestern Australia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15837, https://doi.org/10.5194/egusphere-egu26-15837, 2026.
A Machine Learning Framework for Urban Drought Risk Assessment under Future Climate Scenarios: A Case Study of Gangneung City, South Korea
Yunseo Bae1, Yujin Jung1, Jiyoon Choi1, Younghun Lee2 Sangchul Lee1,2*
1 Division of Environmental Science & Ecological Engineering, Korea University, Seoul 02841, Republic of Korea
2 Department of Environmental Science & Ecological Engineering, Korea University, Seoul 02841, Republic of Korea
* Corresponding author: Sangchul Lee (slee2024@korea.ac.kr)
Abstract:
In 2025, a national disaster was declared in the Republic of Korea due to severe drought, particularly Gangneung City in Gangwon State. In urban areas, drought risk is shaped not only by meteorological conditions but also by anthropogenic factors. However, conventional drought assessments largely rely on climatic or hydrological indices and often fail to reflect these socio-infrastructure factors. Recently, machine learning (ML) is widely adopted due to its ability to capture complex, nonlinear interactions among various factors. Accordingly, this study develops a ML-based framework to reproduce historical drought and to predict future urban drought risk under climate change scenarios in Gangneung City. Drought occurrence data from 2016 to 2025 were classified into five stages (Normal, Attention, Caution, Warning, and Severe) and used as multi-class target variables. Input data included meteorological (precipitation, temperature, humidity, wind speed, and evapotranspiration), topographic (DEM-based elevation, slope, aspect, watershed characteristics, and land cover), and anthropogenic variables (water supply infrastructure, population, and tourism activity). All input variables were spatially aggregated to administrative units, ensuring consistency with the spatial resolution of the observed drought occurrence data. An AutoML approach was applied to compare multiple classification algorithms and to identify the optimal model. Model performance was evaluated using time-aware validation strategies, including a temporal train–test split and time-series cross-validation. SHAP analysis was also employed to interpret the relative importance of key drought drivers. Future drought risk was projected by applying meteorological inputs derived from SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios to the trained model, while other factors were assumed to remain. Administrative-unit-based drought occurrence probabilities were analyzed for the near (2030–2050), mid (2051–2060), and far future (2080–2100). In addition, hypothetical policy-oriented scenarios were explored by modifying anthropogenic variables, such as water leakage rates and tourism demand, to assess the sensitivity of drought risk to management assumptions. The findings from this study would demonstrate the ML-based framework is efficient to predict urban drought risk, supporting region-specific drought mitigation and climate adaptation strategies.
Key words: machine learning, urban drought, anthropogenic factors, drought risk mapping, climate change
Acknowledgement
Following are results of a study on the "Convergence and Open Sharing System "Project, supported by the Ministry of Education and National Research Foundation of Korea
How to cite: Bae, Y., Jung, Y., Choe, J., Lee, Y., and Lee, S.: A Machine Learning Framework for Urban Drought Risk Assessment under Future Climate Scenarios: A Case Study of Gangneung City, South Korea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16457, https://doi.org/10.5194/egusphere-egu26-16457, 2026.
Warming shifts isotherms upwards and previously cold-limited stands can reveal signs of moisture-limited growth. However, the pace of this transition and its characteristics are unclear. To bridge this gap, we present outputs of an intense monitoring effort of tree growth and phenology from a Picea abies stand located originally at the treeline in the Krkonoše Mts. at the Czech-Polish border. This site has been gradually lagging behind the advancing treeline isotherm. During a 12-year period between 2014 and 2025, we collected xylogenesis data, measured stem expansion and shrinkage using dendrometers, and monitored microclimate and leaf phenology. For each year, we derived critical growth dates (start, end, peak growth date), and mean and maximum growth rates for both xylogenesis and dendrometer data. In addition, we evaluated the time series of the tree water deficit. Our results show that despite high inter-annual variability, there was a trend towards a longer duration of xylogenesis, mainly associated with the extension of the cell wall thickening phase. Secondly, we found a reduction in the mean daily rate of cell formation. These trends observed at the cellular level were consistent with observations from dendrometers. The period of growth extended towards the end of the summer and the mean growth rates slightly decreased over time. Interestingly, tree water deficit increased over time with more frequent summer periods with negative climatic water balance and strongly negative soil water potentials. This was reflected in stem growth mainly in the driest years (2018, 2019, 2024) when growth cessation was the earliest within the entire period of monitoring. Our intense growth monitoring witnesses a transition from a strictly cold-limited treeline stand towards tree growth with occasional signs of moisture limitation. Although tree growth was unambiguously affected by drought only during the warmest and driest years, mean growth rates have been slightly declining due to increasing latent tree water deficits.
How to cite: Treml, V., Kuželová, H., Lange, J., Mašek, J., Yele, P., and Tumajer, J.: Intense tree growth monitoring reveals increasing moisture limitation in a former treeline stand, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18037, https://doi.org/10.5194/egusphere-egu26-18037, 2026.
Adaptation measures are needed to protect forests from the impacts of climate change. To do so, forest management throughout Europe has largely shifted toward climate-smart forestry, i.e. focusing on species portfolios which are suitable under a wide range of possible future climates. These strategies primarily take into account different climate scenarios based on the Shared Socioeconomic Pathways outlined and consistently updated by the IPCC. However, evidence is mounting that the risk of the Atlantic Meridional Overturning Circulation (AMOC) collapsing in the second half of the 21st century is increasing. Such a collapse would likely entail drier and cooler conditions across Europe. This possibility is not accounted for in current climate-smart forestry approaches an complicates the task of forest management regarding suitable species choices to ensure the integrity of European forests throughout the next century.
To determine optimal species portfolios for climate-smart forestry, dynamic vegetation models (DVMs) have been used due to their ability to model ecological processes under different future scenarios. However, as of yet, DVMs have not been applied to investigate the consequences of a possible AMOC collapse. Here, we use LPJ-GUESS to model the impact of an AMOC collapse on European forests from both a species composition and a carbon perspective taking into account current forest management and species selection practices.
Our results suggest that an AMOC collapse in the second half of the 21st century will lead to diverging responses across Europe. Northern Europe, including the British Isles and Scandinavia is at risk of "shrubification" and subsequent decrease of forest carbon. On the other hand, coastal areas, particularly in the Mediterranean region are likely to experience an increase in forest area due to the cooler climate. Across Europe, our simulations suggest that a shift in species selection will need to occur to ensure the continued productivity and integrity of forest ecosystems. Our results underscore the need to consider the possibility of an AMOC collapse in forest management plans to ensure that the forests established today will remain viable tomorrow.
How to cite: Meyer, B. F., Wittenbrink, M., Rammig, A., and Buras, A.: A world run AMOC: Simulating the forest carbon and water cycle past the tipping point of the Atlantic Meridional Overturning Circulation with a dynamic vegetation model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19684, https://doi.org/10.5194/egusphere-egu26-19684, 2026.
In recent decades, Central Europe has experienced an increase in the clustering of hot and dry compound extremes, with significant implications for forest health and carbon uptake. The multi-year drought from 2018 to 2022 is a prime example of this, but the severity of persistent drought and its impact on growth likely varies among species and landscapes. Here, we investigate how recent and historical droughts influence radial growth across a hydroclimatic gradient in Germany.
Tree-ring cores were sampled from multiple sites spanning western and eastern Germany to capture contrasts in water availability and elevation. The dataset includes broadleaf and conifer species common in managed and semi-natural forests (Fagus sylvatica, Quercus robur, Q. petraea, Pinus sylvestris, Pseudotsuga menziesii). To quantify drought impact and persistence, we relate growth to multi-timescale drought indices (SPEI) and compare the 2018–2022 sequence against earlier drought episodes. Species-specific growth responses are estimated using a non-linear hierarchical modelling framework (generalized additive mixed models, GAMM) that can simultaneously account for size and age effects, stand context, between-tree variability, and the repeated-measures structure of annual rings.
We present results on (i) how exceptional recent drought persistence is in a historical context, and (ii) which species–site combinations are most sensitive to sustained water limitation, by linking multi-timescale drought metrics to species-specific growth responses across contrasting environments. Our findings reveal that the 2018–2022 period stands out as the most severe multi-year drought event at longer accumulation scales across regions. Meanwhile, growth responses demonstrate pronounced species dependence and site modulation along the gradient. Our work provides valuable insights into recent forest growth anomalies and helps to inform expectations under increasing climate variability.
How to cite: Pohl, F., Neuwirth, B., Abdoli, M., Müsgen-von den Driesch, M., Steger, D., Blume, T., Heinrich, I., Bogena, H., Leuchner, M., and Hildebrandt, A.: Historic analogues and species-specific tree growth responses to the 2018-2022 drought sequence in Germany, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20083, https://doi.org/10.5194/egusphere-egu26-20083, 2026.
Drought and temperature trends are shifting under recent climate change, resulting in decreased water availability. These changes are leading to widespread tree growth declines, increased mortality, and regeneration impairment, having direct implications for forest biodiversity and functioning. Despite growing evidence of drought-induced impacts on forests, characterising drought effects at large spatio-temporal scales remains challenging due to the limited availability of long-term continuous data over space and time and the multidimensional nature of drought. As a result, assessment of drought effects on forest responses varies widely across studies. National Forest Inventories (NFIs) systematically record forest structure and composition, enabling demography and biomass estimation. Their extensive spatial coverage and systematic data collection make NFIs invaluable tools for long-term monitoring of climate-driven spatio-temporal changes in forests. To understand how drought impacts on forests are assessed using NFIs, we systematically analysed the scientific literature on the use of NFIs to evaluate drought effects on forests. We conducted a scoping review using the Scopus database to identify studies published in English that explicitly link drought to forest responses based on NFIs. We found that most of the studies are conducted in North America and Europe, reflecting NFI data availability, and focused on species- or stand –level responses- growth or mortality- and accounted for only one drought dimension (e.g. intensity). Our review aimed to consolidate existing knowledge on drought impacts on forests using NFIs, identify dominant methodological approaches, and highlight critical gaps that must be addressed to improve understanding and prediction of forest responses to drought under ongoing climate change.
How to cite: Ruiz-Benito, P., Rodes-Blanco, M., Tijerín-Triviño, J., Rebollo, P., Serra-Maluquer, X., Astigarraga, J., Bravo-Hernández, M., Cruz-Alonso, V., Fernández-Blas, C., Grajera-Antolín, C., Herrero, A., and Zavala, M. A.: Large-scale forest responses to droughts using national forest inventories: a systematic review, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20335, https://doi.org/10.5194/egusphere-egu26-20335, 2026.
The increasing frequency and persistence of drought and compound hot and dry extreme events have raised growing concerns about the future of forest ecosystems, given the strong link between climatic stress, tree mortality, and declining biomass carbon stocks. European forests are particularly vulnerable, as the continent is among the regions most affected by compound hot and dry extremes in terms of both spatial extent and duration. Assessing how forests respond to repeated drought events is therefore important in understanding ecosystem vulnerability under ongoing climate change and to pinpoint adaptation strategies.
In the presented study, we investigate the dynamics of the resilience of stable European forests where repeated drought events occur. Using a 2003-2022 time series of Normalized Difference Vegetation Index (NDVI) anomalies at an 8-day temporal resolution from MODIS satellites, we quantify ecosystem resilience via the lag-1 temporal autocorrelation (AC1). Drought events are retrieved from the Dheed global database of dry and hot extreme events based on ERA5 (Weynants et al., 2025).
Trends in AC1 in between the events are then assessed to identify dynamics in forest resilience across Europe and to explore their correlation with drought frequency and a set of drought metrics. The link between forest resilience and drought events is also explored at the biogeographical scale. This approach assesses the potential cumulative impact of repeated extreme events on forest resilience beyond a single-event recovery analysis. By understanding if and how repeated droughts shape European forests' response to extreme events we aim to eventually identify preconditions, such as ecosystem heterogeneity, that positively influence their resilience.
How to cite: Elia, A., Pickering, M., Forzieri, G., Cescatti, A., and Fernandez Prieto, D.: Resilience dynamics of European forests under consecutive drought events, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21486, https://doi.org/10.5194/egusphere-egu26-21486, 2026.
Drought is a major abiotic stressor that constrains key physiological processes in forest trees and influences their resilience. Climate models predict increasing frequency and intensity of drought events, which will shorten the intervals available for recovery between successive stress episodes. This study investigated the effects of water deficit on photosystem II (PSII) functioning in European beech (Fagus sylvatica L.) saplings, and recovery of PSII-related functional parameters following rewatering.
A total of 41 F. sylvatica saplings were grown under a transparent roof with controlled irrigation in a forest gap in central Poland. The experimental design included a well-watered control and a water deficit treatment consisting of a 30-day irrigation withdrawal followed by 40 days of rewatering to track recovery. Eight measurement series were conducted during the summer of 2024 at regular 7–10-day intervals. Chlorophyll a fluorescence was measured after 20 min of dark adaptation using a HandyPEA fluorimeter.
The progressive decrease in soil water content led to a significant decline in PSII efficiency (FV/FM) in F. sylvatica saplings. The applied stress prolonged the time required to reach maximum fluorescence (TFM) and decreased the maximum fluorescence level (FM) indicating slower and incomplete reduction of QA molecules and a reduced pool of available reaction centres (Area). These changes increased excitation pressure per reaction centre, reflected by higher ABS/RC, thereby elevating the risk of photodamage. However, we also observed increased dissipation of excess energy as heat (DI₀/RC), providing evidence for the activation of PSII protective mechanisms. Following rewatering, F. sylvatica saplings exhibited partial recovery of PSII performance, suggesting that drought-induced impairments of photochemical efficiency were at least partly reversible under the applied experimental conditions. Taken together, our results suggest that while F. sylvatica can engage photoprotective responses under drought, incomplete post-drought recovery may increase vulnerability under scenarios of recurrent drought with short recovery intervals, with implications for the management of beech-dominated forests.
How to cite: Dubińska, A. and Niemczyk, M.: What chlorophyll fluorescence reveals about PSII functioning during water deficiency in European beech (Fagus sylvatica L.) saplings?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22265, https://doi.org/10.5194/egusphere-egu26-22265, 2026.
A dry climate and intense grazing by livestock are dominant drivers of vegetation structure and ecosystem processes in Mediterranean woodlands. Ecological studies predict contrasting effects of grazing on tree drought stress, reflecting a balance between browsing damage and reductions in competition for water with the herbaceous layer.
We investigated the combined effects of grazing and drought on the water status of Quercus coccifera, the dominant evergreen oak of the Eastern Mediterranean region. We set up an ecophysiological experiment in Southern Israel, at the dry edge of distribution of oak woodland ecosystems. We compared mature oak trees exposed to continuous cattle grazing with four nearby individuals protected from grazing by fencing. Trees have been monitored using high-frequency sensors measuring sap flow, stem water content, and radial stem growth, complemented by continuous meteorological observations and soil water content. We analysed the effects of grazing, season, and their interaction effects using linear mixed-effects models. Furthermore, we applied structural equation modelling to disentangle direct and indirect relationships between climatic drivers and ecophysiological variables.
All ecophysiological variables exhibited strong seasonal patterns, with a significant buffering effect of grazing on tree water use in the dry season. Grazed trees maintained higher stem water content and higher transpiration rates relative to ungrazed trees during periods of high atmospheric and soil drought. The climatic control on tree water deficit and sap flow differed between seasons, with vapor pressure deficit dominating during the wet season and radiation controlling water fluxes during the dry season. Stem water content functioned as an internal water reservoir, buffering tree water deficit in winter and sustaining transpiration during summer drought.
Our results suggest that grazing can buffer drought stress during periods of low water availability, likely by reducing competition for soil water with the herbaceous layer. By improving the tree water status, grazing may increase tree resilience under drought stress and mitigate climate change effects.
How to cite: Ludovicy, S., Grünzweig, J., and Sheffer, E.: Livestock Grazing Alters Seasonal Water-Use Strategies of Mediterranean Oaks , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13257, https://doi.org/10.5194/egusphere-egu26-13257, 2026.
Boreal forests are increasingly exposed to extreme heat and altered precipitation patterns, leading to periods of water stress that threaten their capacity to provide important ecosystem services. Management interventions can improve the resilience of forests to water stress, but our ability to implement such adaptation methods is contingent upon accurate identification of areas most susceptible to the adverse effects of this water stress. Recent advances in dynamic vegetation modelling have improved our ability to predict water stress responses in forests, including the integration of plant hydraulic processes into the ecosystem model LPJ-GUESS. Here, we evaluate the ability of this new adaptation, LPJ-GUESS-HYD, to detect water stress in three forests across Sweden. We identified periods of moderate, severe and extreme drought based on the Standardized Precipitation-Evapotranspiration Index (SPEI), and compared LPJ-GUESS-HYD carbon flux simulations with ICOS eddy-covariance flux data and satellite-based vegetation indices in drought and non-drought periods from 2015 to 2022. We found that LPJ-GUESS-HYD could accurately capture many water-stress-induced shifts in carbon fluxes and vegetation indices. However, its ability to detect these water stress responses varied largely between sites and years, and depended on the duration and intensity of the water stress. Our results provide insight into the factors determining the efficacy of LPJ-GUESS-HYD for predicting water stress responses, and highlight areas where improvement is needed.
How to cite: Brinkhoff, R., Gomes de Almeida, F., Pugh, T., Akselsson, C., and Kljun, N.: Assessing the ability of LPJ-GUESS-HYD to predict water stress responses in boreal forests, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1004, https://doi.org/10.5194/egusphere-egu26-1004, 2026.
Global warming is expanding the length of the growing season. However, within this longer window, the number of effective growing days is decreasing due to drought, so that warming often leads to growth losses on dry sites. Tree species mixtures can enhance productivity and biodiversity and may mitigate climate-change impacts. To better understand the effects of climate and species mixing, high-resolution seasonal growth observations are valuable. Nevertheless, mixing effects have rarely been examined at the seasonal scale.
In pure and mixed oak–pine stands at Maissau (475 m a.s.l.; mean annual temperature 10.1 °C; precipitation 475 mm year⁻¹), hourly radial growth of 48 trees has been monitored since 2017 using band dendrometers installed on dominant, intermediate, and suppressed trees. A hierarchical generalized additive model (GAM) with components for seasonal growth, shrinkage and swelling following rainfall, and diurnal water uptake was fitted to the data.
The model explained 95% of cumulative diameter increment. Increment differed significantly among years, species, dominance classes, and mixtures. There was pronounced interannual variability, with lower growth rates and a shorter growing season in drier years. Growth of Quercus robur and Q. petraea started earlier and lasted longer than that of Pinus sylvestris. Dominant trees grew for approximately one month longer than suppressed trees. Mixture effects were small. Both species exhibited pronounced diurnal cycles of water uptake, which were stronger around the summer solstice than in spring and more pronounced in P. sylvestris than in Quercus spp.; differences due to mixture were again minor.
Pinus sylvestris was more affected by drought than Quercus spp., which can be traced to their differing physiology. As a conifer, P. sylvestris pursues a conservative water-use strategy, closing stomata earlier than Quercus spp.; under drought this leads to strongly reduced photosynthesis and lower growth. Mixture had only a small beneficial effect for the two species studied, likely because niche complementarity is limited.
How to cite: Vospernik, S.: Seasonal growth dynamics in pure and mixed oak–pine stands under drought: insights from hierarchical generalized additive models, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8147, https://doi.org/10.5194/egusphere-egu26-8147, 2026.
Under climate change, not only are droughts becoming more severe, but seasonal precipitation patterns are changing, as well as the timing of snowmelt. Especially in mountainous or northern forests, where trees may depend strongly on snowmelt, it is therefore crucial to understand how the timing of droughts affects tree health and performance. Trembling aspen (Populus tremuloides Michx.) is the most widespread tree species in North America, and, like many other species, it has experienced large-scale, drought-related diebacks over recent decades. Depending on where they grow, aspens rely to various degrees on snowmelt for growing season transpiration, however it remains unclear whether droughts during different seasons affect the trees differently. Using a field experiment, we tested the effect of spring and summer drought on aspen leaf gas exchange, growth and hydraulics.
Over two years (2024 & 2025), we conducted a fully factorial experiment in a mature aspen forest in northern Utah, USA. In the year before the experiment, plots were trenched to ca 70cm depth. Each year, we induced a spring drought by removing most of the snowpack (ca 1.7m3 m-2; > 95 % of total snowpack) in early spring, while for the summer drought, starting in June, we covered ca 60 % of the forest area with rainout shelters. We measured stem growth and various physiological parameters including water potentials (Ψ) and leaf gas exchange up to weekly throughout the growing season. Additionally, we assessed hydraulic conductivity and vulnerability, in early and late summer each year.
Snow removal led to slightly more negative predawn Ψ in early summer, but these effects were small and short-lived. Otherwise, to our surprise, neither drought treatment affected Ψ, leaf gas exchange, growth or hydraulic conductivity, despite extremely dry topsoil. Interestingly however, during the much drier growing season 2025, trees showed reduced predawn Ψ, stomatal conductance and assimilation compared to 2024 irrespective of experimental treatment. These findings suggest that our trees were more affected by regional-scale droughts than by plot-level precipitation manipulation. One possible explanation for this is that these trees may have access to the water table. This is surprising as aspens are typically thought to mostly rely on water from shallow soil layers.
How to cite: Zahnd, C., Fickle, J., Congram, L., Campioli, M., and Anderegg, W.: The surprising resistance of trembling aspen (Populus tremuloides) to experimental drought., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10096, https://doi.org/10.5194/egusphere-egu26-10096, 2026.
Posters virtual: Thu, 7 May, 14:00–18:00 | vPoster spot 2
EGU26-16743 | ECS | Posters virtual | VPS6
Restoring Mediterranean holm oak forests: ecosystem functioning and climate mitigation in the LIFE RECLOAK projectThu, 07 May, 14:36–14:39 (CEST) vPoster spot 2
Mediterranean forest ecosystems are currently facing severe challenges due to the combined pressures of biotic and abiotic stressors. In particular, Holm oak (Quercus ilex L.) forests are experiencing a widespread decline driven by the effects of climate change-induced drought and the aggression of the soil pathogen Phytophthora cinnamomi. This decline threatens not only forest biodiversity but also the stability of essential ecosystem services. In response to this problem, the LIFE RECLOAK project aims to restore and improve the conservation status of these threatened habitats. The project adopts an approach involving the genetic selection of drought-tolerant and pathogen-resistant genotypes, their micropropagation, and subsequent planting in pilot sites across Italy, Spain, and Malta. Within this framework, Work Package 7 (WP7) is dedicated to the "Evaluation of ecosystem functioning and climate mitigation effects," validating the success of the reforestation efforts. Therefore, the primary objective of WP7 is to quantify the restoration of ecosystem processes and the enhancement of climate change mitigation potential provided by the selected resistant genotypes compared to traditional forest stock.
The activities of WP7 integrate field monitoring and modelling tasks. Firstly, the project will assess vegetation growth and biodiversity. In the pilot sites, key morphological parameters of Q. ilex (such as Basal Diameter (BD), Plant Height (PH), and Leaf Area Index (LAI)) will be measured annually to track biomass accumulation. Concurrently, biodiversity indexes (BI) will be calculated to monitor the recovery of understory vegetation. To evaluate the restoration of below-ground processes, soil quality will be assessed through measurements of soil respiration, Soil Organic Carbon (SOC), erosion rates, and Water Holding Capacity (WHC). Recognizing the slow growth rate of holm oaks, these monitoring activities are planned to continue for ten years after the project's conclusion, ensuring a long-term perspective on ecosystem recovery. Data collected from vegetation and soil monitoring will be used for the assessment of carbon sequestration, calculating the CO2 absorbed by both the woody biomass and the soil compartment. WP7 will also develop a monitoring tool using the Biome-BGC Musso ecohydrological model. By integrating site-specific pedoclimatic data with the physiological traits of the resistant genotypes, this model will be calibrated to simulate carbon and water pathways (e.g., Water Use Efficiency) over the plantation's lifespan. This approach quantifies the added value of the resistant genotypes, demonstrating their superior resilience under changing environmental and management conditions.
Finally, under the coordination of the Democritus University of Thrace, WP7 will integrate these findings to analyze broader Ecosystem Services (ES), including provisioning, regulating, and supporting functions. Ultimately, by validating biological indicators and establishing a robust carbon monitoring protocol, WP7 will demonstrate the effectiveness of using improved genotypes, offering a replicable model for restoring Mediterranean areas affected by forest dieback.
How to cite: gori, A., beltrami, S., alderotti, F., ferrini, F., dibari, C., ferrise, R., paffetti, D., and brunetti, C.: Restoring Mediterranean holm oak forests: ecosystem functioning and climate mitigation in the LIFE RECLOAK project, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16743, https://doi.org/10.5194/egusphere-egu26-16743, 2026.
