Among the factors that shape forest ecosystems, natural hazards play a major role. Fires, storms, insect invasions… these phenomena are characterized both by their singularity - they must be unexpected, uncontrollable and of an unusual magnitude - and by the direct consequences they have on human activities or people.
As species are perpetually adapting to their environment, a large-scale natural hazard will cause significant disruption of its conditions. But its occurrence also offers opportunities for certain species, and can be beneficial to biodiversity.
The so-called "pyrophilic" essences, that is to say those whose growth is stimulated by fire, constitute an interesting case: the seeds of the cones of the Aleppo pine ( propagating through a pine forest (Pinus halepensis).), a species installed on the Mediterranean rim, are for example released only after the passage of a fire.
Without the latter, their stands cannot prosper! In many cases, natural hazards can be of primary ecological interest, but they also pose a threat to forests.
Between 2002 and 2013, 67 million hectares of forest burned each year in the world, wiping out the equivalent of four times the surface of the French metropolitan groves.
Over this 12-year period, 85 million hectares have been affected by insects (attacking, for example, foliage or tree bark), 38 million by extreme weather conditions (storm, hurricane, drought, etc.) and 12,5 million by various diseases.
For comparison, the world's forest area is approximately 4 billion hectares.
- Marielle Brunette (@BrunetteMar) -
In Europe, the storm in mind
The storm was responsible for 53% of this damage, the fire of 16% and the biotic factors - that is to say of the world living on forests - of 16%, half of them being due to bark beetles, xylophagous insects (which feed on wood).
In France, more severe droughts
On a smaller scale, in France, numerous disturbances have affected forest stands. Storms Martin and Lothar of December 1999 are still remembered. They put down 140 million m³ of wood, mainly in eastern France.
With significant droughts since 2018, these are at least 300 hectares which were concerned by mortalities directly linked to drought or to bark beetle attacks in French public forests, the latter often being encouraged by the former, we will see later.
Ash chalarosis (Chalara fraxinea), a disease that emerged in the 1990s in Eastern Europe, decimates ash stands and advances a little more each year through French territory.
Finally, the pine tree nematode (Bursaphelenchus xylophilus), a species of worm found in Portugal, could have dramatic economic consequences if it were to settle in the Landes forests.
When hazards interact
The magnitude and frequency of these natural events are increasing as a result of climate change.
This intensification is already observed and tends to increase. For example, damage from wind, fire and insects has increased dramatically in Europe between 1958 and 2001 : on average + 2,6% / year for wind, + 4,2% / year for fires, and + 5,3% / year for bark beetles.
These examples also consider only one natural hazard at a time, whereas climate change favors interactions between them. We can define them through their temporal effects according to two types in particular: “simultaneous” or “compound” events, which take place at the same place and at the same time; “cascading” events, which occur in the same place but shifted in time. If applicable, a time period during which the effect of the first event lasts must be defined.
The case of the spruce bark beetle, discussed below, provides several examples of such events.
These effects can modify the vulnerability of the forest and / or the recurrence of the hazard, that is to say the average return time of the hazard in question or of another.
Multiple and complex phenomena
Scientists therefore expect to see the interactions between risks strengthen much faster than the risks themselves: more and more populations will therefore be affected in the future by multiple and complex hazards.
The issues surrounding their taking into account are numerous: from an economic and social point of view, the aim is to ensure the sustainability of the supply of environmental goods and services (wood, leisure, etc.); on the financial level, it is a question for the forest owners of return on investment but also of costs associated with the coverage of multiple hazards.
Finally, from an environmental and ecological standpoint, these multiple hazards endanger natural habitats, but also climate change mitigation services.
The spruce bark beetle, a textbook case
The spruce bark beetle, or bark beetle, is a wood-boring insect a few millimeters long. At an endemic population level, it plays a fundamental role in maintaining the forest ecosystem, since it only attacks spruces (Picea Abies) the most fragile.
As the latter tend to greatly increase evapotranspiration - that is to say the transfer of water to the atmosphere - their withdrawal allows better regulation of the water resource for the rest of the population, and also to generate water. place for the more dynamic younger generations.
This ecological balance can be disturbed in several ways and lead to the destruction of large areas of spruce forests (we thus designate forests mainly made up of spruce).
For example, a storm can produce a certain volume of chablis, that is to say trees uprooted and fallen to the ground as a result of natural events.
However, windfall, whose vigor and associated defensive capacities are reduced to nothing, constitute particularly easy targets for bark beetles. They therefore provide an opportunity for insect populations to grow very quickly. This interaction between storms and insect populations constitutes a first example of cascading hazards.
Climate change creates a combination of other simultaneous hazards. Like windfall, whose vigor is diminished, trees undergoing increasingly intense and regular summer droughts are very weakened and also become easy targets for bark beetles.
The time required for a larva to reach adulthood is also inversely proportional to temperature. The increase in the latter therefore helps insect populations to develop more quickly.
Historically, one or two generations of insects were seen each year. From now on it is up to three annual generations, which causes a much more brutal population growth.
Eastern France particularly affected
These hazards lead to profound changes in ecosystems and can cause disastrous levels of damage.
This is particularly what was observed during the summer of 2019 and 2020 in the Grand Est and Bourgogne-Franche Comté regions where several million cubic meters of spruce trees were affected, at the same time drastically reducing the selling price of wood.
Over 40 million cubic meters of spruces in 2018 and 2019 were also impacted in Germany.
The cascade of risks does not end there: the stands that have suffered attacks constitute a large quantity of dry wood (trunks, branches, needles) which could potentially increase the severity of fires in the affected region in the future. However, these areas were not particularly affected by the risk of fires in the past. Conversely, the burnt stands may encourage outbreaks of bark beetles.
Ecologists, economists and interactions
Scientists have been interested in natural hazards for a long time, but generally analyze them one by one or independently. Interactions between hazards have only recently been considered, particularly under the impetus of climate change, which tends to favor them.
However, they raise important questions, particularly in terms of modeling each interaction. Unlike isolated hazards, the impacts of multiple hazards are not linear due to synergy effects.
Depending on the disciplinary prism, the question will also be approached differently.
Ecologists often engage in the precise study of certain interactions, at the cost of results that are highly dependent on the ecosystem considered and therefore not very generalizable. For example, the interaction between storms and spruce bark beetles is particularly well known in mountain ranges in central Europe.
Economists, on the contrary, prefer to focus on much more general cases, therefore most of the time making the assumption that risks are independent of each other. For example, it is assumed that there are two unique and independent risks: fires and storms. The theoretical study of random and independent natural hazards is thus well dealt with in the recent economic literature.
More interdisciplinarity would be beneficial to each of these two disciplines. Economists would have a lot to gain by drawing inspiration from ecological models: they better describe the interactions within an ecosystem. As for environmentalists, economic models could provide them with interesting indications on the future of French forests, generally managed by man.
Market risk and financial risk
As mentioned with the case of the spruce market in eastern France, the bark beetle crisis has put a large quantity of wood on the market. In harvested volumes, this is the equivalent of a few normal years that have arrived annually on the local market.
This very large supply has strongly destabilized the markets and mechanically reduced the price of wood. A drop 70% of the price of spruces on foot sold by the ONF was observed between March 2018 (before the crisis) and March 2020.
Two other risks are therefore added to the risk of natural hazard.
The first concerns the market risk for forest owners, with falling prices. To our knowledge, it has not yet been studied in the context of multiple hazards by economists.
The second concerns the financial risk: the return on investment of these owners depends essentially on the profits, which no longer necessarily cover the costs of replanting. The management of this second risk is essential for the conservation of a sustainable forest cover and is therefore of major public interest.
Climate change is expected to increase hazards and their interactions in the future, thus reducing the amount of standing timber. As the forest constitutes an important stock of carbon, a feedback loop is therefore established between climate change and the volume of standing timber.
The more climate change intensifies, the more the hazards affect the forest too, destroying more surfaces, reducing the forest carbon sink and contributing to more pronounced climate change, etc.
Taking into account and managing these multiple hazards, as well as the various risks that result from them, is therefore essential for the sustainability of forest stands and the many associated ecosystem services.
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