VSs underlined by the conclusions of the National Forest and Wood Meetings, launched by the government in October 2021 with the aim of “thinking the French forest of tomorrow”, the French forest is today in crisis. For two decades, we have witnessed an increasing mortality of forest stands and an overall decline in their productivity. If the wooded area in metropolitan France has continued to grow since the middle of the 19e century, it is due to the afforestation – spontaneous or artificial – of agricultural land, because the area occupied by ancient forests continues to decrease.
This “dieback” is generally attributed to climatic changes. The recurrent summer droughts weaken the trees and the mild winters favor the pullulations of bio-aggressors, in particular bark beetles and beetles. Climate change is undoubtedly a key cause of this, but it also reveals forest ecosystems weakened by decades of silvicultural practices focused on wood production. Not only do French forests fix less carbon per unit area, but the exploitation of dying stands induces additional CO emissions2 aggravating the greenhouse effect and associated climate change. In such a context, adapting the French forest is more than a necessity, it is an emergency.
Treating the Causes of Wasting
Forests are not fields of trees, but ecosystems with multiple interactions between the different components. The walker quickly notices that the descents of the crowns and the mass mortalities mainly concern monospecific plantations, made up of trees of the same age, often corresponding to species introduced outside their native territory. This is the case with many spruce plantations in the plains, while the natural high-altitude spruce stands are holding up quite well. The former form simplified stands that are sensitive to climatic hazards (storms, droughts, fires) and attacks by pests (insects, fungi, etc.), while the latter, much more heterogeneous and diversified, are more resilient.
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Even if there is a sensitivity specific to each species and each site situation, the direct and indirect impacts of climate change are modulated by the functional integrity of the forest ecosystem, which is itself largely influenced by forestry. Adapting the forest means acting on the health of the ecosystem and not simply replacing dying trees with others. It is a fundamental treatment of the causes of the wasting that must be undertaken, and not a simple treatment of the symptoms. The forest can no longer be reduced to its trees and its production function: only an ecosystemic vision can be salvific.
New silvicultural practices
The main lever for adapting the French forest is based on the promotion of silvicultural practices that take greater account of the functioning of forest ecosystems as a whole; this to sustainably improve their state of health, their resilience, and increase their ability to sequester and store CO2.
First, each species must be reserved for stations presenting optimal conditions for it, currently and taking into account the modeled evolution of the climate over time steps consistent with the silvicultural cycle. It is also necessary to favor mixed stands (several species) and structurally heterogeneous (several heights and shapes of crowns), so as to strengthen resistance to weather hazards and pest attacks.
Privileging natural regeneration makes it possible to increase genetic diversity subject to natural selection and local adaptive capacities, unlike plantations. This implies better management of the silvo-hunting balance, in particular by favoring the accompanying vegetation which protects sensitive plants and provides an alternative food resource.
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There are already silviculture methods implementing these principles, such as irregular or selection high forest. This type of silviculture is not new, it has been adopted since 2017 by the National Forestry Office for all public forests in the Ile-de-France region in order to avoid “clear cuts”.
Faced with recurring droughts, it is necessary to adapt the density of stands to the water balance of the station and to preserve the water supply to the soil, including by limiting its compaction. More generally, increasing the resilience of forests requires promoting biodiversity within all compartments of the ecosystem. This is still too often perceived as a constraint for the forester, as an obstacle to management, even though it is his long-term insurance for maintaining soil fertility, resistance to bioaggressors and, ultimately, wood production capacity.
Unsuspected adaptation capacities of trees
Several planning documents, such as regional forest-wood plans (PRFB), consider somewhat hastily that native species are no longer adapted to the “new” climate. This fixist vision of the living world forgets that European forest species have already experienced many climatic changes (notably a Little Ice Age and a medieval optimum). Worse, it ignores a number of recent scientific works which highlight the unsuspected adaptive capacities of trees.
At least three sets of mechanisms allow the spontaneous adaptation of trees in a changing environment: genetic mechanisms, via natural selection which acts over the long term, which requires a certain genetic diversity; epigenetic mechanisms, which predispose individuals to environmental conditions that their parents experienced, via induced marks capable of modulating gene expression and inducing mutations; holobiontic mechanisms, via the symbioses resulting from the co-evolution between the tree and its microbiota, the latter contributing to many vital functions.
It is important to increase the areas of protected forest areas.
The effectiveness of these different mechanisms in the face of rapid climate change is still poorly known, hence the interest in being able to observe the response of native species in a “natural” context, i.e. outside a forest subject to forestry. In this regard, it is important to increase the areas of protected forest areas and their representativeness of the different climatic contexts and types of soil, as underlined in the contribution of the National Council for the Protection of Nature to the Assises de la forêt and wood. These areas of high naturalness are not only reservoirs of preserved biodiversity, but also life-size laboratories for understanding the biology of species and spontaneous forest dynamics, essential for acquiring references for designing the silvicultural itineraries of tomorrow.
The false good idea of exotic essences
The alleged “unsuitability” of indigenous species justifies the use of exotic species, often coming from other continents, whose interest and harmlessness are more than doubtful… The idea of favoring species naturally resistant to water stress would be attractive , if it did not show a certain amnesia (in addition to ignoring millions of years of evolutionary history).
Because the introduction of exotic species in the forest is not new. Many have resulted either in acclimatization failures or in serious ecological crises: accidental introductions of exotic pests, biological invasions, erosion of native biodiversity; or even worsening of the consequences of certain hazards.
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The risks associated with exotic species, which are difficult to predict, but real and costly for society, justify that new plantations be more closely regulated. These should be the subject of a prior impact study with risk analysis. More generally, it is urgent to assess the medium and long-term risk/benefit ratio of these plantations, and, pending such an assessment, to submit to a moratorium the political and financial measures encouraging their introduction in forest.
Update engineers’ tools
This essential effort to adapt forest management to climate change should not be limited to engineering actions, but should be based on an interdisciplinary scientific approach, based on all the recent contributions of conservation sciences and techniques. Scientific research in forest ecology in particular is highly mobilized on the question of the impacts of climate change on the forest and the adaptive capacities of species. The numerous research results would make it possible to support forest management and planning strategies on a solid scientific basis. However, these results have so far been little or not taken into account by decision-makers.
However, sustainable forest management cannot be based on engineering alone, just as it cannot be reduced to trees alone. Acting in a changing environment and in an uncertain universe implies integrating our scientific knowledge in all fields, taking into account the evolution of societal expectations and updating the tools of engineers.
*Guillaume Decocq is professor of plant and fungal sciences, director of UMR Edysan at the University of Picardie-Jules-Verne. Serge Müller is professor emeritus, researcher at the Institute of Systematics, Evolution, Biodiversity (UMR 7205), National Museum of Natural History (MNHN).