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Forests facing the climate challenge and megafires

The lights are glowing red. Everywhere, the forests burn with an intensity and a recurrence exceptional yesterday, today usual.
“Global warming is the main cause, points out Bernard Prévosto, researcher in forest ecology at INRAE* in Aix-en-Provence. The repetition of droughts and heat waves dehydrates the forests, facilitating the spread and violence of fires. »
“This threat, he continues, no longer concerns the South of France alone, but also the Gironde and Brittany. On a global scale, boreal forests are also the prey of flames with the particularity that, since the layer of organic matter is greater there, the fire can smolder for a long time and start again at any time. This is also the case, to a lesser degree, in the Landes. »
But global warming only gives the general context, even if it is very favorable. The start of a claim depends on the combination of other factors.

Angles of attack

“We speak of a “triangle of fire”, explains Bernard Prévosto. It takes both fuel, wood, oxidizer, oxygen, and a spark, a production of heat. Drought and wind are thus aggravating factors. »
This triangle offers two angles of attack. On the spark side, first: “We can never insist enough on prevention because the spark is 90% of human origin and often accidental”, blows Bernard Prévosto.

In Gironde, the desire to “look ahead” and find the “little paradise” after the fires

On the fuel side, then: “We must, he enumerates, multiply the firewalls and extend to regions other than those around the Mediterranean the clearing around houses as well as along the roads. Facilitating the rapid intervention of firefighters involves increasing water points and access roads. »
Moreover, if nature fires all wood, all species are not equal. How to replant without “failing”?
“Natural regeneration – from pioneer plants to end-of-succession species – is not, as on the scale of large North American parks, possible in Europe, a fortiori in countries as anthropized as France, recalls the searcher. And this regeneration is hardly possible when the soils are heavily burned. The degradation and dispersion of organic matter makes them less fertile. Soil struggles to retain water. A slope facilitates erosion. »

Age privilege

Ecology is a balance as complex as it is fragile: “While mixtures of species are more suitable than monospecific crops to make plots less prone to burning, disease or pest attacks, these have a limit. linked to their vertical distribution, which provides fuel for the fire at all levels… Thus, old stands with only tall trees are less vulnerable, especially since their age gives them thick and resistant bark. »
“And essences, resumes Bernard Prévosto, have co-evolved with fire. The Aleppo pine, a conifer mainly planted in the Mediterranean, has serotinous cones which are relatively resistant to flames and then burst under the effect of heat so that their seeds spread. The holm oak leaves with buds. In fact, from fire resistance as with the thick-barked cork oak, to fire resilience as with the Aleppo pine, there is a continuum with the holm oak somewhere in between. »

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Soon tick weather maps in France

“Essences, he insists, have three strategies to last. Fewer leaves, moreover smaller, a more buried root system: acclimatization pushes them to consume less water by sweating less. With adaptation, the pressure of the environment selects the individuals who adapt best to it. With migration, assisted or not, species gain higher latitudes. The Aleppo pine goes further north. »
Nature remains open to dialogue, man… according to his interests: in this case, some species are more lucrative than others. Between soaring prices and conflagration of forests…

National Research Institute for Agriculture, Food and the Environment

Jerome Pilleyre

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