Our forests are dying: this is the alarming observation of the ONF, the National Forestry Office. In six years, more than 300,000 hectares of public forests have suffered early mortality. This represents 30 times the area of Paris.
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French forests are suffering from climate change. The Corgebin national forest, on the edge of the town of Chaumont (Haute-Marne), is a good example. On 1,101 hectares, there are beeches, oaks, hornbeams and spruces… Or at least what remains of them, as explained by Jean-Claude Tissaux, responsible for “reconstitution and adaptation to climate change” at the ‘NFB: “Until 2017 or 2018, it was a relatively dense forest. In the middle of summer it was dark, with a lot of leaves on the trees. Now, it has cleared up considerably.”
Around him: branches, trees on the ground, others still standing but marked with an orange dot. These will be slaughtered before they die. In Haute-Marne, 40% of the forest is experiencing early dieback. “During the storm of 1999, we saw the woods on the ground and it was a shock because overnight the landscape changedcontinues Jean-Claude Tissaux. We are now in a kind of silent storm : there are beginning to be gaps, natural thinnings which are caused by natural mortality, the fall of trees.”
“It’s the equivalent in volume of what the 1999 storm did. I don’t know how far we’re going to go…”
Jean-Claude Tissaux, NFBat franceinfo
Among the explanations: Haute-Marne has just suffered seven years of exceptional drought and heat (with the exception of 2021). Climatic conditions which signify the death sentence for many trees: “When water begins to run out in the soil, these trees will close their stomata, to avoid internal water loss. The stomata are small holes on the leaves, which allow photosynthesisexplains the ONF specialist. The tree will recover CO2, transform it into sugar which will be used for its metabolism ; When this closure of stomata lasts too long over consecutive years, the tree can no longer make sugar and ends up dying of starvation.”
Weakened trees are no longer able to defend themselves. Bark beetles, pest insects, decimate the spruce trees of this national forest. Other species are no longer able to develop their branches. The clearings created allow heat to pass through, which amplifies drought and the risk of fires. A vicious circle that we find everywhere in France, but mainly in the east of the country.
“In a pessimistic scenario, the beech disappears completely”
On his computer in the ONF premises in Chaumont, Jean-Claude Tissaux presents maps based on IPCC data, to determine “the climatic compatibility of a species by 2070, depending on climate change scenarios. There is a map for beech, a map for oak, for the same forest. We realize that if we are really in a pessimistic scenario, the beech completely disappears from the forest.”
This scenario, with global warming of four degrees, would have numerous consequences: economic (less wood to sell), ecological (impact on biodiversity, on the maintenance of dunes or mountainous terrain) and climatic: the disappearance of the forest amplifies drought phenomena. “The forest causes a lot of evapotranspiration, this reduces the air above. When you have a mass of warm air arriving, there is precipitation on the forest. And when you remove the vegetation, you break the cycle some water : the precipitation does not return and the forest cannot establish itself.”
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Based on this alarming observation, the ONF is trying different approaches: first carrying out selective cutting to limit competition between trees and encouraging their growth, then planting different species so as not to put all its eggs in the same basket, and finally test new species. In front of a fenced plot, Jean-Claude Tissaux explains that “the species that works relatively well here is the Arizona cypress. With the relatively hot and dry summers like in 2022, and a dry year of 2023 over a long period of time, period even in autumn, this species does quite well. We are entering the third year.”
“The goal is to see, at 10 years, at 15 years, at 20 years, at 30 years or 40 years, whether the species is suitable for reforestation or not.”
Jean-Claude Tissaux, NFBat franceinfo
It is a long-term solution, therefore, and uncertain given the number of parameters which today explain the decline of our forests.