In the south of Gironde, beech trees have been growing in the Ciron valley for 43,000 years. Today, the Bordeaux-Toulouse High Speed Line (LGV) project planned nearby threatens this forest, the oldest in France. Former researcher at the National Research Institute for Agriculture, Food and the Environment (INRAE), Alexis Ducousso studied the exceptional ecosystem of the Ciron valley. Today, he warns of the multiple threats to biodiversity from the LGV project.
On October 12, between 800 and 1000 people gathered in the south of Gironde against the Bordeaux-Toulouse LGV project to denounce the artificialization of land. It would concern 5000 hectares according to them. The works also threaten the Ciron forest, the exceptional age of which scientific discoveries have established.
In the Ciron forest, a beech forest « unique » 43,000 years old
Indeed, the threatened beech forest of the Ciron valley is the oldest in mainland France: it is at least 43,000 years old. “This is an exceptional case where a species of tree is still present in its glacial refuge”explains Alexis Ducousso, former researcher at INRA (now INRAE). A glacial refugium is an ecosystem that allowed species to withstand the Ice Age. It is in itself “walking” that he made the discovery that would become his subject of study. “When I arrived in this beech forest for the first time, I had a shock,” he says. “I had to drop all my forest ecology courses at once because it was so atypical. »
“When I arrived in this beech forest the first time, I had a shock”
The beech forest makes the Ciron valley a place « unique » equipped with a “extraordinary diversity”. “Diversity accumulates over time”explains the scientist, who estimates that the forest could even be more than 43,000 years old. Carbon-14 dating cannot date beyond 50,000 years but “you have to go back to the Eemian, around 110,000 years ago, to have trees in the region”he argues.
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Deforestation, groundwater, invasive species: the multiple impacts of the LGV on the forest ecosystem
The Bordeaux-Toulouse LGV project which passes near the valley poses numerous threats to the forest. It aims to serve Toulouse by TGV and reduce the journey with Bordeaux by 73 minutes. First of all, the constructions will lead to a “deforestation” which will affect the local microclimate » dependent on the forest, predicts Alexis Ducousso.
The route of the Bordeaux-Toulouse LGV (in red) passes near the Ciron forest (circled in green). ©SNCF-GPSO
The risks associated with “hydroecological functioning” of the Ciron forest are more complicated to evaluate, specifies the scientist. The sub-basement of the track will disrupt the water tables which supply the valley with water. “fresh and well oxygenated”. The alteration of these aquifers would involve a risk of upheaval of the “cool microclimate” of the valley, he warns. In addition, the route of the track will disrupt “tributaries of the left bank of the Ciron” . The “railway triangle” separating the route towards Toulouse or Bayonne will be located on a valuable marshy area to the south of the forest.
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Another aspect of the installation of the LGV, “biological invasions” favored by human constructions. Invasive plant species in the region, “like black locust, boxelder maple, black walnut”risk taking advantage of the upheaval in the microclimate to invade the forest to the detriment of the beech trees which have been growing there for more than 40,000 years.
However, the risks are not limited to the direct impacts of construction. THE “related works linked to the LGV” can cause the most significant disruption. Alexis Ducousso has already observed such a phenomenon on the construction of the Paris-Le Mans Paris-Tours route. “ The proximity, the possibilities that farmers had to drain more easily” due to the « remembrements » for the construction of the track caused “ the biggest damage »he explains. Risks that are very difficult to assess, the consequences of which may only be seen in a few years.
The Ciron beech forest, closely linked to the region’s ecosystems
Alexis Ducousso is not optimistic for the future of the forest if work on the LGV continues. “The impact of the work will be very rapid”he warns. “In a few years, it will be completed. » All the “fauna” and the “fagéticole funds” of the beech forest risk being permanently lost, with its specificities like its “five species of endemic mushrooms”. A possibility that alarms him: “if the beech trees disappear, that means that all of the ecosystems in the area have been seriously damaged”. “The Ciron valley has extremely high biodiversity due to its history but we will also lose biodiversity outside the beech forest”he warns. Compensation measures would be unimaginable given the ecological value of the site.
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“The Ciron valley has extremely high biodiversity due to its history but we will also lose biodiversity outside the beech forest”
A crisis “often underestimated”
For Alexis Ducousso, the case of the Ciron forest is symptomatic of a “biodiversity crisis” still very little known to the general public and “often underestimated”. However, she is “at least as serious as the climate change crisis”alerts the scientist. Biodiversity is not just about “little bird stories”. Elle “provides numerous ecosystem services, climate regulation, regulation of water regime, regulation of parasites and crop pests”he emphasizes. Benefits that benefit society beyond the forest ecosystem. Finally, “the higher biodiversity you have, the higher your level of happiness”says the researcher.
Audrey Bonn
To go further:
Good action for the climate is not always good for biodiversity
Decline of biodiversity: alert on ecosystem tipping points
Francis Hallé: “the primary forest offers maximum biodiversity, maximum CO2 capture, maximum soil fertility”
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