We won't be going to the woods

We won't be going to the woods

For a long time, storms were the greatest threat to the French forest. But over the last twenty years or so, another threat has been weighing on its future, bringing Man face to face with a painful realization: not only will the forest not enable us to combat climate change, but worse still, climate change is already killing it.

A network of footbridges provides access to the tree canopy.

" We have observed a 50% increase in tree mortality and a reduction in tree growth. Based on current scenarios, by the end of the century forests could give way to wooded savannah in the south of France. In the north, competition for water will thin out the ranks," notes Isabelle Chuine, researcher at the Centre d'écologie fonctionnelle et évolutive.¹ and coordinator of a report on the state of French forests for the French Academy of Sciences.

Drought is one of the first lethal consequences of climate change for trees (Climate change and the biosphere). " When the weather is warmer, trees evaporate and transpire more, so the water stored in the soil returns more quickly to the atmosphere, leaving them with fewer available water resources ". This trend is exacerbated by the drop 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 Mediterranean forests, which are particularly well adapted to these phenomena, have suffered considerable damage. (Holm oak fecundity does not acclimatize to a drier world), recalls the researcher. The reason: the specific functioning of these evergreen species, which don't lose their leaves in winter and produce new foliage later. " This adaptation enables them to avoid late spring frosts. But if a heatwave occurs when the leaves have not yet reached maturity, they won't be able to withstand these temperatures as they would at the height of summer.

Drought and heatwaves provoke an immediate response in trees, whose leaves are equipped with stomata. These are small holes that open and close to capture the CO2 essential for photosynthesis, but which also allow water to escape. "In the event of drought, leaves will close their stomata to save water. But in doing so, they no longer absorb CO2 and therefore no longer photosynthesize ", summarizes Isabelle Chuine. If the drought continues, leaves, twigs and even the whole tree will die, depending on the severity of the episode. Survivors are weakened and become prime targets for pests and pathogens. Not to mention fires, which are also favored by drought.

The laurels are cut

Another threat is beginning to emerge: warmer winters. " In our latitudes, a tree produces new leaves and flowers in spring, and fruit in autumn, before losing its leaves," recalls the specialist. This autumn, in the Massane beech forest in the Eastern Pyrenees and in the Puechabon forest, scientists observed a massive bursting of buds in autumn. In 2015, massive autumn blossoming was also observed throughout France. "This is extremely risky behavior, as these young leaves do not survive the winter, and the flowers do not produce fruit, which is a dry loss for the tree. All these situations will become increasingly frequent between now and the end of the century. " ( Deciphering the multiple effects of climate change on the time lag of leaf unfolding) How can we fight climate change with forests that are dying?

French forests currently store almost 3 gigatons of carbon, but the carbon sink they constitute has been halved in just a decade, " and if you look region by region, many are no longer carbon sinks". So far, the political response has been to increase withdrawals, in other words, to cut back. " France's national low-carbon strategy in France was built on the idea that, since forests were going to die, it was better to harvest them and store the trees' carbon in long-lasting wood products, then replant with more resistant species. Problem: only 3% of the wood harvested today is transformed into long-lasting wood products.

Life-size

Another approach is to let nature take its course. Diversifying the species and genetic material of forests to make them more resistant is one approach. " No one really knows whether we'll be able to maintain the forests we know in certain regions. Having failed to anticipate this situation, despite scientists' warnings, we are now in the realm of full-scale experimentation ," concludes Isabelle Chuine.

1. Cefe (CNRS, UM, IRD, EPHE) ︎

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