Trees across French forests have been dying prematurely over the past decade, signalling widespread climate-driven forest decline, according to a new study.
The paper by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), published in Nature Communications, said there was a 1.5- to 4-fold increase in mortality rates among the nine most common tree species, including European beech, pedunculate oak, and Norway spruce.
The researchers reached these conclusions after analysing data of over 500,000 individual trees across 52 different species for the period 2015-2023 from the French National Forest Inventory (NFI).
Ecologically and economically important species such as Fagus sylvatica, Quercus robur, and Picea abies show steep increases. Many species showed a pronounced surge after 2018, highlighting the vulnerability of European forests to extreme drought events, while spatial patterns pointed to the compounding role of long-term regional warming and drying, the authors said.
Across all 52 species analysed by the scientists, nearly half (48 per cent) showed significant increase in mortality.
“These trends span size classes and coincide with reduced growth, indicating they do not reflect demographic turnover but widespread climate-driven declines in forest health. Similar reports across Europe reinforce that these processes are continent-wide, consistent with evidence that ongoing warming and drying are eroding forest health,” the study said.
The hotspots for this dieback were concentrated in northeastern France, particularly in the Jura, Vosges, and Grand Est regions. The study notes that these losses are not being offset by new growth or recruitment, indicating a widespread decline in forest health rather than a typical demographic shift.
Among the nine most common species, the temperature feature in almost 93 per cent of all models associated higher mortality risk with warmer anomalies during these seven years, the study stated.
While in 63 per cent of the models, the climatic water balance (CWB) feature associated higher mortality risk with drier anomalies, and in 34 per cent of models with wetter anomalies. When combining the temperature and CWB feature responses within each model, 62 per cent of models associated higher mortality risk with warmer-drier conditions and 31 per cent with warmer-wetter conditions.
“Such a strong association with warmer-drier conditions was expected, as these conditions are known to increase tree mortality, whereas the warmer-wetter response was surprising and suggests that even seemingly favourable anomalies may elevate mortality risk,” the study said.
The scientists in a statement said that across Europe, there have been signs for about 20 years that more and more trees are dying prematurely. In some regions of the continent, the state of the forests is now even worse than it was in the 1980s, when air pollution caused serious damage to trees in selected regions.
Even positive growing conditions, such as warm, wet springs, can increase the risk of trees dying.
“On-average warmer winters and warmer spring minimum temperatures stood out as particularly frequent and influential drivers of increased mortality risk. Although these anomalies occur in different seasons, they point to related physiological and biotic stress pathways. Warmer winters shorten or disrupt dormancy, leading to insufficient chilling, desynchronised spring leaf-out, and reduced investment in frost protection, which increases vulnerability to sudden cold spells,” it said.
The researchers noted that lower dormancy for trees may also demand to sustain higher metabolic activity during winter months, eventually depleting carbon reserves and potentially starving the trees.
Species such as Fraxinus excelsior, and Castanea sativa showed particularly high increases in the modelled additional mortality risk under warmer winter or warmer spring conditions.
This warming during winter dormancy and early spring can significantly increase mortality risks, long before the onset of summer stress.
Warmer spring minimum temperatures can also cause early bud break, increasing exposure to late frost, stretching growing seasons and further increasing chances of experiencing drought during growing seasons.
Collectively winter and spring warming exacerbate biotic pressures while milder cold-season conditions enable pests to survive, emerge earlier and breed rapidly.
Further, on-average warmer summer temperatures were less frequently identified than winter or spring anomalies yet exerted a comparable influence on mortality risk when present. Elevated summer temperatures intensify atmospheric water demand, contributing to “hotter” droughts”, a well-established driver of tree mortality globally and across Europe, the study observed.
“The relatively low frequency of summer temperature features across models likely reflects that temperature-driven evaporative demand was already captured by the drought index, such that impacts are expressed through CWB features rather than temperature features. Nevertheless, when warmer summers coincided with drier summer conditions, modelled mortality risk increased even more, indicating that warmer summers may amplify drought stress, although they may also increase stress through increased respiratory carbon losses (increasing the risk of carbon starvation) or accelerated pest development (e.g., bark beetles produce more generation in warmer summers, compounding stress in already weakened trees and enabling infestation of healthy ones),” it added.
Species such as Fagus sylvatica, considered relatively drought-tolerant were noted to fail to recover from 2018 drought.
Contrary to drought stress, wetter spring conditions that are generally considered favourable because they promote growth at the start of the growing season did not hold true as studies indicated increased mortality in favourable and wet growing conditions.
“Consistent with these findings, our models frequently associated wetter spring anomalies with increased tree mortality risk, with effects within the range of warmer temperatures and drought-related stress,” the authors said.
Citing reasons, the authors said structural overshoot that occurs when favourable growth conditions drive excessive canopy growth, leaves trees more vulnerable to hydraulic failure during subsequent drought.
They said the plausible explanation supported with their findings at species level as taller trees were found disproportionately more sensitive during wetter springs.
This is consistent with empirical observations of higher evaporative demand and hydraulic risk in larger trees, they said.
“The additional evaporative demand imposed by an enlarged canopy reduces hydraulic safety margins during drought because a higher minimum conductance (i.e., water loss through inefficient stomatal closure or cuticular transpiration) pushes xylem water potentials closer to hydraulic failure thresholds,” they explained.