A once majestic oak is now almost dead: weakened by repeated heatwaves, it has shut down its vital functions to conserve water and is slowly dying in a forest in France. And it is not the only tree suffering.
It is one of countless trees across Europe’s forests that are “adopting” drastic – and often fatal – strategies to survive worsening heat and drought linked to climate change.
As France experiences a record‑breaking heatwave for the season, scientists warn that many of Europe’s forests already show the effects of years of rising temperatures and intense drought.
Just as humans cool themselves through sweating, trees regulate their temperature through transpiration, drawing water from the soil and releasing it as vapour through microscopic pores in their leaves.
As temperatures rise, the loss of water from leaves and soil accelerates, placing trees under severe strain to retain moisture.
Manuel Nicolas, from France’s National Forest Service, has been studying this phenomenon in the country’s public forests for decades.
All trees are “suffering,” but their response to heat stress varies depending on the species and location, said Nicolas, who leads the national forest ecosystem monitoring programme.
Trees are thirsting due to high temperatures
Some trees reduce water loss by quickly closing their stomata – tiny pores in their leaves or needles that regulate gas exchange and the release of water vapour.
Closing the stomata saves water, but slows or even halts photosynthesis, the process that converts sunlight, carbon dioxide and water into sugars essential for growth and survival.
Over time, the tree can effectively starve.
Another strategy, preferred especially by trees with deep root systems, is to draw water from the soil and keep their stomata open for as long as possible, Nicolas explained.
“But the risk in this case is that it dies of thirst,” he told AFP in Fontainebleau, a vast forest about 60 kilometres south of Paris.
“A tree is a column of water” that can absorb up to 200 litres a day, he said, transporting moisture from the soil to the canopy – the dense layer of leaves and branches formed by the tallest trees in a forest. But this system can be overwhelmed by heat or drought and collapse completely.
“Air bubbles enter the vessels and prevent the sap from circulating to the ends of the branches. This is called air embolism,” Nicolas explained.
“And when embolism affects too many vessels, the entire tree dies of dehydration.”
Adapt or die
Scientists say that as heatwaves and droughts become more frequent, trees are being pushed increasingly towards these limits.
In France, which hosts some of the most diverse forests in Europe, government data shows that tree mortality has doubled over the past decade.
Projections suggest that around 30 per cent of tree species may be at risk of population decline by 2050.
Experts say forests are being forced to adapt rapidly to climate changes that would normally unfold over thousands of years.
The planet has already warmed by around 1.4°C since pre‑industrial times, mainly due to the burning of fossil fuels for energy.
Scientists argue that the current period is likely the warmest the Earth has experienced in the past 125,000 years.
The impact on trees is visible to the naked eye in Fontainebleau, where some giant trees have shed their branches and leaves as a last attempt to survive.
Among them is oak number 37 – still standing, but stripped of branches and worn, its fate already sealed.
Nicolas and his team have been marking and observing trees in this area for 34 years and know that its time is approaching.
However, other trees in the same area remain tall and healthy for reasons scientists are still trying to understand.
An intense heatwave in 2018 destroyed a cluster of pines in Fontainebleau, but other conifers in the forest thrived for reasons not yet fully understood.
Some trees, such as Mediterranean species, may be more resistant to heat, Nicolas said.
Understanding “which species will be suitable 100 years from now” will be crucial for protecting forests from the heat to come, he explained.
“Against heatwaves, there is not much we can do. But we can try to help the forest adapt,” Nicolas concluded.
