[LUM#21] We won't go to the woods anymore
Storms have long been the greatest threat to French forests. But for the past twenty years or so, another threat has been hanging over their future, forcing humans to face a painful reality: not only will forests not help us combat climate change, but worse still, this change is already killing them.
"We are seeing a 50% increase in tree mortality and a decrease in their growth. Based on current scenarios, by the end of the century, the forest could give way to a wooded savanna in the south of France. In the north, competition for water will thin out the ranks," notes Isabelle Chuine, a researcher at the Center for Functional and Evolutionary Ecology.1 and coordinator for the Academy of Sciences of a report on the state of French forests.
Drought is one of the most lethal consequences of climate change for trees (Climate Change and the Biosphere). "When it is warmer, evaporation and transpiration from trees are greater, so the water stored in the soil is released more quickly into the atmosphere, leaving them with fewer water resources available." This trend is exacerbated by the decline in rainfall, which is affecting many regions of France to varying degrees.
Extensive damage
Also linked to climate change is the increasingly frequent occurrence of early heatwaves, as was the case in June 2019. "Even the Mediterranean forest, which is particularly well adapted to these phenomena, has suffered considerable damage." (The fertility of the holm oak is not adapting to a drier world), recalls the researcher. The reason for this is the specific functioning of these evergreen species, which do not lose their leaves in winter and produce new foliage later in the year. "This adaptation allows them to avoid late spring frosts. But if a heatwave occurs before the leaves have reached maturity, they will not be able to withstand these temperatures as they would have done in the middle of summer."
Drought and heat waves trigger an immediate response in trees, whose leaves are equipped with stomata. These tiny pores open and close to capture the CO2 needed for photosynthesis, but they also allow water to escape. "In the event of drought, the leaves close their stomata to conserve water. But in doing so, they no longer absorb CO2 and therefore no longer photosynthesize," summarizes Isabelle Chuine. If the drought continues, the leaves, and even the branches, or even the entire tree, will die, depending on the severity of the episode. The survivors will emerge weakened and become prime targets for pests and pathogens. Not to mention fires, which are also exacerbated by drought.
The laurels are cut
Another threat is beginning to emerge: warmer winters. "In our latitudes, trees produce new leaves and flowers in the spring and fruit in the fall before losing their leaves," the specialistpoints out . This fall, in the Massane beech forest in the eastern Pyrenees and in the Puechabon forest, scientists observed a massive burst of buds. In 2015, massive autumn blooms were also observed throughout France. "This is extremely risky behavior because these young leaves do not survive the winter, and the flowers do not produce fruit, which is a net loss for the tree. All of these situations will become increasingly frequent by the end of the century." (Understanding the multiple effects of climate change on the timing of leaf emergence) How can we hope to combat climate change with forests that are dying?
French forests currently store nearly 3 gigatons of carbon, but their carbon sink capacity has been halved in just ten years, "and if we look at each region individually, many are no longer carbon sinks." The political response so far has been to increase logging, in other words, to cut down more trees. " France's national low-carbon strategy in France was based on the idea that, since the forests were going to die, it was better to harvest them and store the carbon from the trees in long-lasting wood products, then replant more resistant species.The problem is that only 3% of the wood harvested is currently transformed into long-lasting wood products ."
Life-size
Another approach is to let nature take its course. Diversifying tree species and genetic material in forests to make them more resistant is one option. "No one really knows if we will be able to preserve the forests we know in certain regions. Having failed to anticipate this situation despite warnings from scientists, we are now conducting a full-scale experiment,"concludes Isabelle Chuine.
Your papers, please!
What would the identity card of French metropolitan forests look like? "They cover about 30% of the territory and three-quarters of them are privately owned," explains Isabelle Chuine. Two-thirds of the trees are deciduous, while one-third are coniferous." The most common species are oak "and even very old oak trees," beech, spruce in the northeast, and maritime pine, which is very common in the Landes region and is now the largest forest in France.
French forests are characterized by their composition of regular high forests. "This is a centuries-old practice that has been included in the national inventory of intangible cultural heritage. It is the image of a forest with tall trees of a single species, large trunks..."For the rest, there are mainly coppices, which are lower tree formations that are often cut down, or coppices with undergrowth.
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- Cefe (CNRS, UM, IRD, EPHE) ↩︎