Forest fires: “Less burnt surfaces. But for how much longer? »

A few days ago, Weather report France published its forecast for the meteorological summer – which covers exactly the months of June, July and August – to come. A summer that promises to be drier than normal. Especially in the southern half of the country. All after 3^e driest spring ever recorded in mainland France. And in a situation of drought already worrying in some regions.

With an even greater probability, this summer of 2022 should be hot. All that is missing then is the wind so that the three weather conditions that most influence the spread of Forest fires are reunited. This is what could happen in the south-east of France this weekend.

“At the start, you need a trigger. It is usually of human origin. Negligence, more often than not, like a cigarette butt thrown on the side of the road”points out Nicolas Martin, specialist in Mediterranean forests at the National Research Institute foragriculture, food and the environment (INRAe). Then, it takes wind to push the flames. But between the two, there must be a combustible dried up. Following a lack of precipitationsure. “When soils or subsoils are dry, the roots do not have enough water to nourish the leaves of the trees. »

Because, let’s remember, a forest consumes a lot of water. “For a mature tree, it is several tens to several hundred liters per day during the vegetation period, when the conditions are favorable and there is water in the soil”, explains Nicolas Martin. The reason why “the idea that trees could help fight against soil drought is rather wrong”.

The role of air dryness

The trees are therefore sensitive to the lack of precipitation. But also the heat. It plays on a magnitude that specialists call the relative humidity of theatmosphere.

” Reality physical, is that the trees transpire. Water travels from the ground to the atmosphere through the tree, which can be seen as a network of vertical pipes. A tension is created at the level of the leaves of the tree, which makes it possible to take water from the ground and the basements. However, water is bound to the ground. Thus, the less water there is in the soil, the greater this tension must be. Similarly, the less humidity there is in the atmosphere — that’s what happens when it’s hot and dry — the greater the tension there too. During a drought, or worse, when a drought is associated with vague heat, this tension can thus become extreme. The water gradually passes into the gaseous phase, a phenomenon called “the cavitation “. The canals of the shaft, intended to be waterlogged, then fill withair. It’s’embolism. The leaves are more and more difficult to feed and dry up »explains Nicolas Martin.

And finally when the leaves dry up, they provide the ideal, abundant fuel that will trigger a fire at the slightest spark.

Prevention to limit forest fires

the climate change has been going on for several decades now. And studies show that fire danger has increased since the 1990s. “However, in France, the number and surface area of ​​fires has generally decreased. In particular thanks to the strong investment in the forces of struggle and prevention », explains Nicolas Martin. The missions of Defense of the French forest against fires (DFCI) played a major role. ” According to the data baseof fire, the effect was dramatic after the 1990s. A notable exception is that of the year 2003, a year when it made extremely hot and dry . There was an exhaust with several large fires that could not be stopped. Enormous areas were lost, far greater than the areas burned in other years. »

The means injected into prevention have also been significant. These include, for example, fuel cuts, clearing of undergrowth and roadsides and approaches to houses. “DFCI policies organize patrols in the massifs to detect fire outbreaks early. » Because, very logically, the earlier we intervene, the greater the chances of quickly extinguishing the fire. The DFCIs, particularly in the south of France, also regularly measure the water content of shrubs in the forests. A way to precisely set the level of risk. And, if necessary, to deploy combat forces or preposition Canadairs.

“All these measures have shown their effectiveness. But the models also show that, in a context of climate change which increases the risk of occurrence of forest fires, and with the current level of prevention and fight, the teams could find themselves overwhelmed and the burned surfaces could go up again”points out the specialist in Mediterranean forests from INRAe.

The risk of forest dieback

And this risk would not be the only one hovering over our forests. Severe droughts as announced in the years to come if we do not manage to control climate change can also be at the origin of a dieback of the massifs. “We have already seen it in recent years”, remembers for us the researcher. The mechanism is still the one mentioned above. When the tree lacks water, the tree’s metabolic organs — the leaves — die. Organs of survival — the buds— also dry out. “But you need very intense droughts like the one Europe experienced in 2003 or 2018. The lack of water was the cause of high mortality on the side of firsspruces or beeches. »

The trouble is that droughts, even if not severe enough to cause a forest to dry out, can weaken them. “The decline will be gradual. Especially since the trees then become more susceptible to pathogens . Bark beetles, for example. In 2019-2020, we had to deplore hundreds of hectares of dead spruce trees after two or three droughts in a row and a bark beetle attack”tells us Nicolas Martin.

Among the solutions, of course, the drastic reduction of our emissionsof greenhouse gas . To limit climate change responsible for an increase in the number and severity of droughts. “We don’t have full control over it.notes the researcher. But we have to be a leader to show that it is possible, to set an example and ensure that other countries follow us! »

Adapt to drought

Variability genetic nature is often mentioned as a solution. Intraspecific variability. Trying to identify the likely resource-rich places most tolerant of water scarcity. Selecting seeds from these regions could help enrich other regions that will be more exposed in the future. Nicolas Martin quotes for us the example of the beech which presents “a very wide spatial distribution and high genetic diversity. They are found from southern Europe to the north. With beech trees that probably have different tolerance levels.” The intrinsic potential sounds interesting, but researchers still don’t know how much. And the operation could harm the productivity of the forest. “There is often a compromise to the resistance to drought, which results in lower productivity. »

The change ofspecies can also be considered. Replace native species with species exoticmore resistant. “It may seem attractive, but it remains very controversial, because it is very difficult to anticipate the ecological consequences of a change of species for the other functions of the forest (biodiversity , food web, etc.). » For insects or birds, habitats change. And sometimes, in the most exposed regions, this means a real upheaval. With the added risk of importing new pathogens with the trees, for example.

Another adaptation considered is that of stand thinning. “By removing individuals, we lightensnaturally the water balance of the massif », explains Nicolas Martin. What may reduce exposure to water stress . “According to some models, it works. But in practice, we sometimes observe the opposite effects. Dieback appears to increase in some settings with increased thinning. Probably because cutting trees breaks the continuity of forest cover and alters the micro-climate. After a cut, the light and heat can increase in the vicinity of formerly sheltered crowns. » It would therefore seem that the practice does improve the dryness of the soil. But let it harm the dryness of the atmosphere. “It also poses regeneration problems by exposing the young shoots to radiation and temperatures that they struggle to withstand. »

Stand diversity could also be used. In the Landes, for example, the production forests of maritime pinesare monospecific . “In France, this is the case for 85% of plantations. A historical heritage and a question of profitability”, the researcher tells us. But the principle is today criticized. Because the monocultures are more fragile than mixed stands. “One pathogen can be enough to destroy everything. » And conversely, therefore, researchers are wondering if diversity could not help stands to better resist drought. “Regarding pathogens or biodiversity, the benefit is clear. Regarding the lack of water, the answer is not unequivocal. Depending on the species and associations, the effects can indeed be positive or neutral. Even negative. » Work is underway.

“No solution is perfect. But a whole range is offered to us today. These options can already be implemented. Research works to solidify and formalize them. Given the magnitude of climate change and other societal challenges around the forest and the wood (autonomyenergy and materials, etc.), we would be tempted to act quickly by universally applying economically profitable solutions. This would be neglecting when it can have disastrous consequences on other services”concludes for us Nicolas Martin.