Why the French forest needs a "basic treatment"

• For the past two decades, we have witnessed an increasing mortality of forest stands and an overall decline in their productivity, according to our partner The Conversation.

• Climate change is undoubtedly an essential cause, but also the indicator of forest ecosystems weakened by silvicultural practices focused on wood production.

• This analysis was conducted by Guillaume Decocq, professor of plant and fungal sciences and Serge Muller, researcher at the Institute of Systematics, Evolution and Biodiversity.

The conclusions of the National Forest and Wood Conference, launched by the government in October 2021 with the aim of “thinking the French forest of tomorrow”, should be released in the coming days.

One of the major axes of this reflection concerned the strengthening of the resilience of forests and the preservation of biodiversity.

Because the French forest is today in crisis: for two decades, we have indeed witnessed an increasing mortality of forest stands and an overall decline in their productivity.

If the wooded area in mainland France has continued to grow since the middle of the 19th century, it is due to the afforestation – spontaneous or artificial – of agricultural land, since the area occupied by ancient forests continues to decrease.

The French forest in figures © National Forest and Wood Meetings 2021

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 CO₂ emissions, aggravating the greenhouse effect and the associated climate change.

In such a context, adapting the French forest is more than a necessity, it is an emergency.

The tree should no longer hide the forest

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 for 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.

The necessary adaptation of 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.

Northern Vosges Mixed Forest – Sept. 2021 © Evrard de Turckheim, CC BY-NC-ND (via The Conversation)

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.

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".

Clearcutting a plot of Douglas fir in a forest in Oise © Guillaume Decocq, CC BY-NC-ND (via The Conversation)

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,

, wood production capacity.

​A condemnation without trial of indigenous species

Several planning documents, such as the 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.

Mixed forest in the south of Amiens (Somme) where spruces are spared from bark beetle attacks – Oct. 2021 © Guillaume Decocq, CC BY-NC-ND (via The Conversation)

The effectiveness of these different mechanisms in the face of rapid climate change is still poorly understood, hence the importance of being able to observe the response of native species in a "natural" context, i.e. outside the 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.

A false good idea: the use 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).

Map showing the progression of ash dieback © Department of Agriculture (via The Conversation)

Because the introduction of exotic species in the forest is not new.

Many have resulted either in failures of acclimatization, or in serious ecological crises: accidental introductions of exotic pests (the current epidemic of ash dieback is one example among dozens of others), biological invasions (the cherry tree late, once praised for its merits in forestry, has today become the enemy of the forester), erosion of native biodiversity (the ghostly undergrowth of many coniferous plantations in the plains are a glaring example of this);

or again, aggravation of the consequences of certain hazards (the mega fires experienced by the Iberian Peninsula are closely linked to eucalyptus plantations, which are very flammable, and yet promoted in the French Mediterranean region).

In the forest of Compiègne (Hauts-de-France), invasion by black cherry – June 2005 © Guillaume Decocq, CC BY-NC-ND (via The Conversation)

A detailed analysis of these risks is presented in a white paper on the introduction of exotic tree species into the forest, recently published by the Botanical Society of France.

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.

Better take into account the results of scientific research

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.

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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.

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This analysis was written by Guillaume Decocq, professor of plant and fungal sciences at the University of Picardie Jules Verne (UPJV) and Serge Muller, professor emeritus and researcher at the Institute of Systematics, Evolution, Biodiversity of the National Museum of History natural (MNHN).

The original article was published on The Conversation website.

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Declaration of interests

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• Forestry

• Tree

• Global warming

• Forest

• Biodiversity

• Video

• The Conversation

• Planet

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