The 2018 European Drought: A Turning Point for Continent's Forests
In the summer of 2018, Europe experienced one of its most severe droughts in modern history, characterized by prolonged heatwaves and minimal rainfall that scorched landscapes from Scandinavia to the Mediterranean. Forests, long seen as resilient green lungs absorbing carbon dioxide and regulating climate, began showing alarming signs of stress. Satellite imagery captured vast areas where lush canopies turned prematurely brown, a phenomenon known as leaf browning or scorching. This event wasn't just a temporary setback; it marked the beginning of ongoing challenges for forest ecosystems amid climate change.
Researchers across European universities quickly mobilized to understand the scale and mechanisms. Institutions like the University of Basel and ETH Zurich in Switzerland, along with the University of Freiburg in Germany, led early assessments using high-resolution satellite data from Sentinel missions. Their findings revealed that about 10% of central European forests lost leaves too early, with deciduous species like European beech (Fagus sylvatica) hit hardest.
Satellite and Ground Observations: Mapping the Damage
European academics employed advanced remote sensing techniques to quantify the damage. A collaborative effort between the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL)—closely partnered with University of Neuchâtel—and University of Grenoble Alpes developed algorithms analyzing Sentinel satellite images every 2-3 days at 10m resolution. This distinguished deciduous from coniferous trees and modeled water balance, pinpointing hotspots in warm, dry regions with shallow soils.
Beech forests suffered most, dropping leaves weeks ahead of schedule, while oaks proved more tolerant due to deeper roots. Ground surveys confirmed tree sizes and terrain as key vulnerability factors. These studies, published in journals like Ecological Indicators, provided the first Europe-wide map of premature defoliation, informing forest managers and policymakers.
Mechanisms of Leaf Browning: Hydraulic Failure and Heat Stress
University researchers delved into physiological processes. At the University of Freiburg, teams led by S. Haberstroh investigated Scots pine (Pinus sylvestris) forests, finding the 2018 hot drought pushed ecosystems to a tipping point. Hydraulic failure—where water columns in xylem cavitate under tension—combined with carbon starvation led to stomatal closure, halting transpiration cooling. Leaves overheated, scorching from green to brown.
ETH Zurich and University of Basel studies on beech showed slower pre-drought growth predisposed trees to crown dieback. Premature senescence (leaf aging) signaled irreversible damage, with severe crown transparency and leaf browning indicators correlating to higher mortality. This step-by-step failure: soil dries → roots can't supply water → stomata close → leaves heat up beyond 45-50°C → proteins denature → cells die → browning spreads.
New PNAS Study: Limits of Thermal Acclimation Under Drought
A groundbreaking 2026 study in PNAS, led by Alyssa T. Kullberg at WSL with collaborators including University of Lausanne affiliates, simulated conditions in a Swiss open-air lab. Young beech and downy oak (Quercus pubescens) trees faced +5°C warming and chronic drought over five years. Warming alone allowed acclimation, raising thermal safety margins (TSMs)—difference between leaf temp and lethal threshold—via adjusted transpiration. But drought narrowed hydraulic safety margins (HSMs), forcing stomatal closure and breaching TSMs, triggering photosystem II failure and scorching.
Beech showed severe scorching; oaks fared better. This hydraulic-thermal cascade explains 2018 browning: satellite images showed Europe-wide patches mirroring lab results.
Photo by Lucas George Wendt on Unsplash
Emissions and Carbon Cycle Disruptions
Drought-stressed forests alter emissions. University of Freiburg research on Scots pine noted sharp declines in volatile organic compound (VOC) emissions—gases like isoprene and monoterpenes trees emit for cooling and defense—post-mid-July 2018, alongside reduced photosynthesis. ICOS network studies across Europe showed net carbon uptake dropped 56-62 TgC in summer, turning sinks into sources temporarily. Legacy effects persisted into 2019-2020, with doubled disturbances like bark beetles.
Universities like Wageningen (Netherlands) and University of Helsinki quantified methane and CO2 shifts in mires, linking drought to GHG feedbacks.
Species-Specific Vulnerabilities Across Europe
- Beech (Fagus sylvatica): Highly susceptible; premature senescence led to dieback in Switzerland, Germany.
- Scots Pine (Pinus sylvestris): Tipping point reached; defoliation, mortality surge (University of Freiburg).
- Oak (Quercus spp.): More resilient, deeper roots; less browning (WSL/ETH Zurich).
- Spruce (Picea abies): Secondary bark beetle deaths post-drought.
Central Europe worst hit; Mediterranean forests faced compounded fires.
Long-Term Legacy Effects and Forest Disturbances
University of Natural Resources and Life Sciences Vienna (BOKU) tracked disturbances post-2018: unprecedented levels persisted 2 years, with mortality rising. Technical University of Munich noted hydraulic failure indicators in temperate species. Five-year review (2018-2022) by Knutzen et al. consolidated Europe-wide data, showing multi-year heat/drought damages.
European Universities Spearheading Adaptation Research
Leading institutions include:
- ETH Zurich & University of Basel: Phenology, frost-drought interactions (Yann Vitasse).
- University of Freiburg: Ecosystem tipping points, BVOC dynamics.
- University of Neuchâtel & WSL: Lab simulations, remote sensing.
- Technical University of Munich: Hydraulic assessments.
- University of Granada (Spain): Mediterranean impacts.
Strategies for Building Resilient European Forests
University-led initiatives propose:
- Diverse species mixes, favoring drought-tolerant oaks, firs.
- Soil improvement, thinning for water access.
- Assisted migration of genotypes.
- Monitoring networks like ICP Forests (pan-European uni collaboration).
For more on forest research careers, explore opportunities at AcademicJobs.com research jobs.
Future Projections: Hotter Droughts and Research Needs
By 2100, disturbances could double; universities urge predictive models, genetic studies. EU-funded projects like FORESTWARD involve multiple unis for forward-looking adaptation. Young researchers are pivotal, with growing demand for expertise in remote sensing, physiology.
European forests' fate hinges on integrated uni-led science and policy.
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