French forests have grown significantly over the past decade, covering 17.6 million hectares in 2024—nearly a third of the territory of Metropolitan France and Corsica. This is an increase of 900,000 hectares compared to ten years ago and nearly double the area from the early 20th century. Despite this expansion, the forests are absorbing less carbon than they did fifteen years ago, which challenges the assumption that more forest area automatically means greater carbon storage. The national forest inventory conducted by the French Institute of Geographical and Forest Sciences (IGN) shows that forests are increasingly affected by climate change, with rising tree mortality and recurring crises that are slowing their ability to act as a carbon sink. Over the past forty years, the volume of standing timber has grown by almost 50%, which might suggest that forests are still playing a strong role in carbon sequestration. However, the actual carbon sink function has weakened significantly. Tree mortality has increased by 125% over ten years, rising from 7.4 to 16.7 million cubic meters of dead wood per year between the periods 2005–2013 and 2015–2023. Bark beetles are a major cause of this mortality, particularly affecting common fir trees, which account for 2.4 million cubic meters of dead wood annually—more than fires. As a result, the forest carbon sink has declined from 63 to 39 million tons of CO2 per year over the same period. The weakening of the carbon sink is due to three interrelated factors: increased tree mortality, slowed tree growth, and continued strong harvesting by the timber industry. These factors are working together, rather than balancing each other, and are causing a decline in the overall health of the forests. Today, French forests sequester 39 million tons of CO2 annually, down from 63 million in the period 2005–2013. The amount of dead wood has doubled in ten years, reaching 159 million cubic meters. Approximately 8% of the 2.3 billion trees assessed show signs of decline, amounting to nearly 193 million trees. In some areas, such as the Northeast, the rate of tree death and harvesting has exceeded the rate of new growth, making these forests temporarily lose their ability to absorb carbon. The main threats to trees are not natural disasters like fires or storms, but rather pests and diseases. While fires are dramatic and visible, especially in the South and Southwest, they are not the primary cause of the overall decline in forest health. The most affected regions are the Northeast, Vosges, Jura, and Ardennes, where bark beetles and other pests are causing significant damage. The common fir is the most affected species, with 2.4 million cubic meters of dead wood per year due to a bark beetle infestation. The ash tree is losing 1.6 million cubic meters annually to a fungal disease called chalara. The chestnut is also suffering from two diseases, ink disease and canker, affecting 1.7 million cubic meters of trees. Even traditionally resilient species like the beech and oak are showing increased signs of decline. This shift is partly due to rising temperatures, which have disrupted the natural balance that once controlled insect populations. For two years, foresters have observed unseasonably high temperatures that create ideal conditions for bark beetles to thrive. This has raised concerns about France’s climate strategy, which has relied heavily on its forests as the second-largest carbon sink after the oceans. The slowing of the forest’s carbon absorption capacity now requires a reassessment of how much flexibility France has to offset emissions from other sectors, such as transport, industry, and construction. As a result, the country’s ability to maneuver within its climate goals is shrinking. To address these challenges, a French Forest Observatory was launched in July by seven partners to monitor forest degradation and guide public policy. On the ground, the National Forest Office is experimenting with assisted migration, planting tree species from southern France in forests in the North and East. The RENEssences research network is testing new species in 525 "future islands" covering 922 hectares. These efforts are taking place as foresters prepare for the forests of 2050. At the Guémené-Penfao nursery in Loire-Atlantique, an assisted migration project initiated in 2011 is testing southern French seeds to prepare for the future of the beech and sessile oak in the Verdun forest.