INFOGRAPHICS. Warming, diseases, insects… How the decline of French forests threatens our climate objectives

For one hundred and fifty years, the forest has been regaining ground on French territory. However, this dynamic hides a massive decline in trees. To the point of jeopardizing France’s carbon neutrality ambition.

COP28, which takes place until Tuesday, December 12 in Dubai (United Arab Emirates), is an opportunity for the countries present to take stock of the commitments made in 2015 during COP21 in Paris, in order to “limit the rise in temperatures to 1.5°C” and to reach “net zero emissions”. Lhe France displayed in 2019, in its law relating to energy and climate, its intention to achieve “carbon neutrality by 2050”. LThe State has therefore taken the gamble of massively reducing its greenhouse gas emissions, while offsetting the remaining ones. Among the levers mobilized to achieve this compensation is the French forest.

The national low carbon strategy, France’s roadmap to combat climate change, predicts that nearly half of the 80 million tonnes of greenhouse gases that will still be emitted in 2050 will be absorbed by the forest. France is relying all the more on its forest carbon sink since, for almost a century and a half, the forest has been expanding. “It is linked to the abandonment of agriculture which began in the middle of the 19th century and which continues today, especially in less productive or less easy to exploit areas, in small or medium-sized mountains for example”explains Laurent Augusto, research director at the National Institute of Agricultural Research (Inrae).

Nearly a third of the territory covered by forest

Agricultural land that has fallen into neglect in the countryside is being reclaimed by vegetation. In the Jura or the Seine valley, the forest has regained its rights. “Most of the time, these are phenomena of spontaneous reforestation”, explains the researcher. In 2022, according to the National Institute of Geographic and Forestry Information (IGN), the area of ​​wood represented 17.3 million hectares in mainland France and Corsica, or 31% of the territory.

These forests act like CO2 pumps. “The engine of this pump is photosynthesis, explains Laurent Augusto. To produce matter, the tree captures CO2 from the atmosphere.” Conversely, it also releases CO2 when it breathes or when its roots, its dead leaves or the tree as a whole die and are degraded. At this time, the carbon, previously stored in particular in the wood of the tree, finds itself partly stored in the soil and partly emitted into the atmosphere. “The forest is therefore a carbon sink when what comes in is greater than what is returned to the atmosphere,” summarizes the specialist. This is what normally happens. When it is not faced with catastrophic events, such as storms or heatwaves, the forest plays its role as a pump.

It is on the hope that this mechanism operates at full capacity that the calculation of the national low carbon strategy is based. With an expanding forest, it must be able to absorb around 40 million tonnes of CO2 equivalent in 2050. But since around 2013, this objective has been significantly compromised. The health of forests is deteriorating and, year after year, their growth slows down, which undermines their capacity to play this role as a carbon sink.

More than 600,000 hectares of forest dying

IGN estimates the area of ​​declining forest in France to be around 670,000 hectares in 2022, or 3.9% of all French massifs. A forest stand is said to be dying when 20% of the trees which have access to light have either died recently, or have part of their upper branches which are dead. This area of ​​forest that is dying is equivalent to all those affected by fires over the past thirty-five years. A figure that is all the more worrying as it is a minimum estimate. “When a tree is dying, we sometimes have to act very quickly. To avoid the proliferation of certain insects or diseases, we cut it down. It then does not appear in the count of dying trees but in that of felled trees”notes Nathalie Derrière, head of the inventory results department at IGN. If the phenomenon is global, certain regions are particularly affected, notably in the North-East, where conifers have been decimated.

Global warming, which has favored hot conditions in recent years, is primarily responsible for this decline in forests. Since 2018, France has experienced exceptionally hot summers and marked by severe droughts, with the exception of 2021. “To adapt to this drought, trees stop their growth and stop photosynthesis”, explains Isabelle Chuine, terrestrial ecologist and research director at the University of Montpellier. The tree then stops growing and playing its role as a carbon pump.

“Drought can lead to embolism or even death of branches or the entire tree.”

Isabelle Chuine, terrestrial ecologist

at franceinfo

This dieback of trees is also caused by pathogens and pests that attack forests. “When the tree is weakened, it becomes prey for fungi, diseases and insects”, explains the researcher. Whether insects already present in France for a long time or new pests arriving thanks to global trade, all have benefited from this weakening.

Accelerating tree mortality

“For spruce trees, it’s a double penaltydescribes Lilian Duband, climate change project manager at the National Forestry Office of Burgundy-Franche-Comté. This species was already weakened by drought and the bark beetles took advantage of this to proliferate. Especially since these insects, which dig galleries under the bark of trees, are favored by increasing temperatures.”

To these factors can sometimes be added human management errors, past or recent. “In the 1950s and 1960s, at a time when we were not aware of global warming, there were many spruce plantations in low and medium mountains. But it is a species that is adapted to high altitude, not repeated droughts, analyzes Laurent Augusto. Hence the fact that these trees suffered greatly during the recent heatwaves.” Although spruces are the trees most affected, all species have seen their mortality increase. Due to this accumulation of factors, tree mortality increased by almost 80% between the periods 2005-2013 and 2013-2021, according to IGN.

This deterioration in the health of trees and the slowdown in forest growth have undermined the forest carbon pump.. And that’s without taking into account harvesting: wood cutting increased by almost 9% per year between 2005-2013 and 2013-2021, according to IGN, even if many of them concerned diseased trees.

Almost half as much CO2 absorbed

Result: in a few years, the capacity of the forest to absorb CO2 has been halved. About five years ago, forests captured 50 million tonnes of CO2 equivalent. Today, this well is only 27 million tonnesalerts Mélanie Juillard, engineer at Citepa, the association which establishes each year an assessment of greenhouse gas flows in France. This does not leave much room for maneuver and the forest could become a net source of carbon if this dynamic continued. In certain regions such as Franche-Comté, the shift has already taken place: the forest there now emits more greenhouse gases than it absorbs.

This trend goes against French ecological planning. The national low carbon strategy planned to capture up to 80 million by focusing on an increase in the carbon sink, of which forests are the main contributors. But the discourse is changing and we are now mainly talking about securing the existing well.”notes Mélanie Juillard.

“The essential means we have to mitigate climate change and achieve carbon neutrality is failing us.”

Isabelle Chuine

at franceinfo

Still, some hope to find ways to help the forest adapt. “We know, for example, that putting several species of trees in the same forest makes it less vulnerable to diseases. and to the attackers”, recalls Laurent Augusto. “But it remains a race between, on the one hand, our ability to help the forest adapt and, on the other hand, the worsening of hot climatic conditions and the stress that trees undergo.tempers the expert.

For experts contacted by franceinfo, the preferred solution to safeguard French forests and their capacity to absorb greenhouse gases, and thus achieve climate neutrality, is to fight against emissions, from the outset. “The more the carbon sink decreases, the more reduction efforts will have to be increased in other sectors.warns Mélanie Juillard.

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