Species diversity, the key to finding resilient forests

• Thousands of hectares of forests are devoured each year by forest fires which are more intense and more frequent with climate change.

• Emmanuel Macron announced on Wednesday “a major national project mobilizing forest professionals to be able to replant, redo the forest and prevent the risks of today and tomorrow.

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• To rebuild these spaces, two experts insist on the importance of diversifying species in order to protect them from future fires.

  But to find a forest with a fully functioning ecosystem, it takes between 70 and 100 years.

20,600 hectares of forest gone up in smoke in Gironde in one week, 15,007 hectares in 2021 and 17,400 hectares throughout France in 2020... Each year, forest and vegetation fires destroy the French forest area.

And with climate change, they are becoming more intense and more frequent.

But what happens to these devastated natural spaces years later?

How quickly do trees grow back to regain their original structure?

What role does man play in this reconstruction?

asked itself these questions and tries to answer them with the help of two specialists in the matter, who insist on the importance of monitoring the start of fires, diversifying species and fighting against climate change. which weighs more and more in the damage caused to our environment.

What happens to forests destroyed by fires?

The images of charred forests, stripped of their sometimes century-old trees, accumulate over the years.

Once the fire has been brought under control and extinguished, there remains a heavy workload to restore them.

And they will only regain their brilliance decades later.

A year after the fires that affected the Var in 2021, we surely find "a few seeds, a few shoots" but we are still far from a forest structure and the places will keep "traces of the flames for another two to three years", considers Eglantine Goux-Cottin, President of the Consulting Engineer in Environment and Forestry (ICEF), contacted by

.

"It all depends on the vegetation kit, trees, species that were present before the fire, as well as the type of soil," adds Brigitte Musch, head of the Genetic Conservatory of Forest Trees at the National Forestry Office ( NFB).

The more leafy the trees, like the oaks of Gironde, the more likely they are to restart from their stumps.

For Aleppo pines, which are found in Mediterranean forests, seeds released during the fire will be able to germinate the following spring.

However, this also depends on the intensity of the fires.

And what we saw this week “will make things more complicated given the violence of the flames which in places exceeded 100 meters high”, warns Brigitte Musch.

When can trees be replanted?

Again, this will depend on the structure, the species of the forest, and the intensity of the fires.

“If the root system has not burned, the trees will be able to start again and we will see some next year, but given the surface area affected in Gironde, we will have to replant in the affected areas”, anticipates Brigitte Musch.

But before thinking about replanting, several fronts of struggle are organized.

First of all, you have to "fight the wind which risks sweeping away the humus", essential for regrowth, then "take out the charred wood, cut, raise the plans... All of this will take one or two years", estimates-t-t -she.

How long does it take for a forest to regain its appearance before destruction?

The other technique is to let nature take its course.

The duration of natural regeneration will depend on the environment, the context, the species of the forest, whether it is temperate or Mediterranean.

But generally, “after ten years, the trees begin to structure themselves, then it takes between 20 to 30 years to find a structure similar to that before the fire.

Finally, from 70 to 100 years old, we will recover a forest with interactions and a functioning ecosystem,” explains Eglantine Goux-Cottin.

"The best solution is to support this natural regeneration, monitor and follow up before replanting at all costs," she insists.

Indeed, when man intervenes, it takes about the same time because the growth of the trees remains the same.

What trees to plant?

For Brigitte Busch, these desert lands are an opportunity to try new things, to experiment with silviculture methods, to test other species... Because if certain trees are more adapted to these fires, such as the Aleppo pines or the cork oaks, "today, they can no longer support the increased intensity and frequency of these fires", explains Eglantine Goux-Cottin.

They are increasingly “water stressed due to increasingly extreme drought periods.

Forests are under pressure.

The mistake to avoid at all costs is then to plant the same species in a single space.

Diversity is indeed an effective asset for the rebirth of an ecosystem.

“The most resilient forests are those with the most biodiversity,” agrees Eglantine Goux-Cottin.

It is then necessary to favor mixtures of species with deciduous trees, conifers, as well as Mediterranean species.

And what topography to adopt?

The space between the trees is "a question that agitates the community quite a bit", according to Brigitte Musch.

For the question of fires, it is better that the tops do not touch each other in order to avoid too rapid a spread.

At the same time, it is also necessary to fight against ferns and other tall grasses which promote the start of fires, and thus prevent too much light from penetrating the ground.

Especially since "the closer the trees are to each other, the more a selection of the most resistant to climate change will be put in place", abounds the manager at the ONF.

Moreover, fires are not the only threat.

“The way of planting will influence the forest structure, if it is very dense, we will have very tall trees, very thin trunks, continues Eglantine Goux-Cottin.

In this case, there is a risk of stress with trees that are more fragile in the face of storms, you have to find a good balance”.

In general, to prevent these forest fires from multiplying, it is necessary to monitor the forests, maintain them, prevent the outbreak of flames and above all, globally, to fight against climate change because of which this threat is more intense and more frequent each year. year.

And today, France is losing forest areas because they burn much faster than they grow back.

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