Introducing new species and provenances in response to climate change: daring or reckless?

The Royal Belgian Forest Society's position on future forests

Introduction

There are very few forest areas that are not currently affected by climate change. A large majority of foresters agree that we need to change our practices. The introduction of new species/provenances adapted to a warmer, drier climate is being experimented with as part of the Trees for Future or similar projects in neighbouring countries. This is just ONE of the proposed strategies for adapting forests to climate change, along with others such as greater use of natural regeneration, soil protection, more dynamic forestry, mixed forestry with continuous cover, increasing biodiversity, etc. All these methods have their merits, and SRFB advocates combining them rather than opposing them.

However, the introduction of new species/provenances into forests gives rise to a great deal of debate. These mainly concern two aspects:

• the potential impact that these new species/provenances have/have on biodiversity and ecosystem functioning.
• their role in adapting forests to climate change and health crises. Some advocate their introduction because they are better adapted to current and future conditions, while others maintain that our native species will adapt naturally.

On both the first and second aspects, SRFB is taking a nuanced approach, because no adaptation option can be ruled out and none can be advocated as the only solution.

Essences from here and elsewhere: a long history

In the collective unconscious, the forest often inspires an idea of eternity and immutability. However, this is not the case: the forest has always changed, it's just a question of timing.

On a broad time scale, these changes are linked to natural phenomena such as continental drift and ice ages, which have shaped forests over geological time. For example, the dominant position of beech in our regions dates back only 3,000 to 4,000 years, a very short time compared with the 400 million years that separate us from the appearance of the first trees!

On a «shorter» scale, a very recent animal species has had a particularly strong impact on its environment, including forests: mankind, through its many civilisations! From overexploitation to reforestation, man has profoundly modified not only the surface area but also the composition of forests.

European history, in particular, is full of examples, both happy and unhappy, of the introduction of species, particularly plant species, into forests and elsewhere. What would our landscapes and our food supply be without maize, beans, tomatoes, wheat, plane trees, poplars, potatoes and so on? Not all of these plants are native to the European continent, yet they make a major contribution to its landscapes and economy.

On the other hand, the disastrous consequences of the introduction of Japanese knotweed, Giant hogweed and Himalayan balsam no longer need to be proven.

In our forests, too, many species are the result of introductions, and while an old chestnut tree poses no problems for anyone, a late cherry tree can pose major management problems.

We feel it is important to define certain terms here¹.

• Indigenous or autochthonous: describes a taxon, population or animal or plant community recognised as originating in the territory where it is currently found. (Note: the notion of administrative territory (region, country) is obviously irrelevant here).
• Exotic or allochthonous: describes a taxon, population or animal or plant community introduced voluntarily or involuntarily outside its natural range.
• Naturalized: describes an allochthonous species that has settled in a region other than its area of origin, and is capable of reproducing there on a long-term basis and spreading spontaneously, thus integrating the existing ecosystem (e.g. Robinia speudoacacia).
• Invasive species: a species that locally takes up a position that can be considered excessive within an ecosystem, in relation to a dynamic power that is locally superior to others (e.g. the sycamore maple, by forming dense and pure facies, can be considered invasive within a beech wood with woodruff).
• Invasive species: an allochthonous species which, due to the absence of its own set of regulators, has particularly effective reproduction strategies and forms locally invasive populations. It can have a significant impact on the ecosystems it colonises, replacing native species which it helps to make rare or extinct, and blocking certain ecosystems through its dynamic cover (e.g. Asian knotweed, Reynoutria japonica and Reynoutria sachalinensis).

These definitions evoke a relatively fixed and rigid situation. However, as indicated above, the distribution area of a species varies over time as a function of climate variations (for example, the natural distribution areas of holm and downy oaks are currently declining in southern Europe while they are expanding in the north). The time scale envisaged should not, therefore, lead us to consider the current state of the forests as definitive, all the more so in the current context.

Benefits and risks of introducing new species/provenances

A list of recommendations has been issued by a panel of experts² to adapt forests to climate change and make them more resilient. It includes the following points in particular:

1. Strictly adapting species to the site and silviculture to the species;
2. ensure good soil structure using appropriate farming techniques;
3. maintain game populations in balance with the forest's carrying capacity ;
4. diversify tree species and maximise genetic diversity ;
5. encourage the assisted migration of southern species and origins.

The introduction of new species/provenances is clearly in line with the last two recommendations. The ultimate aim of all these measures is to maintain healthy, multifunctional forests in a context of climate change, in terms of production, regulation and cultural services.

This type of introduction must take into account the risk of negative impacts on the host ecosystem. These impacts may be biological (competition with and suppression of native species), structural (e.g. destabilisation of banks by Japanese knotweed), economic (e.g. cost of combating late cherry in regenerations) or landscape-related.

A new species that spreads effectively across the landscape could lead to a loss of biodiversity linked to its ability to eliminate others and its inability to host the local flora and fauna. The loss of biodiversity could be accompanied by disruptions to ecosystem functioning and lead to the loss of associated ecosystem services.

It is therefore both legitimate and essential to consider the potential for a new species to cause harm before deploying it on a large scale. The introduction of a new species, as with any new species, must therefore be subject to a risk analysis and an assessment of the risk/benefit balance.

Towards a differentiated approach

In order to consider the introduction of a new species as potentially problematic, it must be difficult to control its dispersal in the environment into which it is introduced AND its arrival in the ecosystem must significantly disrupt its functioning.

The levels of risk, both in terms of ecosystem disruption and excessive dispersal, depend in particular on the origin of the species. An introduced species can become invasive in its new environment if the latter lacks the pathogens and predators that regulate it in its area of origin (Enemy release hypothesis³). As a result, a species from southern Europe that has co-evolved with the herbivores, insects, bacteria and fungi present in Europe is less likely to become a problem when introduced here than a species from East Asia or North America, as the latter would arrive in an environment devoid of its suite of regulatory organisms. Conversely, and for the same reasons, the biological potential (capacity to host native biodiversity) is higher for European species than for species from other continents (for example, downy oak or Hungarian oak are more capable of hosting the animal, fungal and plant species associated with our native oaks than American oaks).

In view of the above, it is clear that the approach can be differentiated according to the nature of the species in question, the ecosystem into which it is introduced and the way in which it is introduced (on what surface, as a mixture, to enrich natural regeneration, or as a monospecific species, etc.).

The Belgian forest, and more generally the forest of north-western Europe, has different facies and not all forests have the same challenges. For example, the adaptation of old sub-natural forests, whether protected or not, should not be considered in the same way as that of more recent forests planted in the 20th century with deciduous or coniferous trees on farmland or moorland. The former are of great heritage interest. They concentrate most of the biodiversity that is typical of forests, and their adaptation should preserve their functioning and structure as much as possible. The latter are more «cultivated» forests with less heritage and biodiversity interest. Their adaptation will aim to maintain sustainable timber production, while increasing the resilience of stands to climatic and health hazards, using appropriate silvicultural techniques that also benefit the other functions of sustainable forest management.

Sub-natural old-growth forests

In sub-natural ancient forests, the major challenge is to maintain the integrity of these forests, the animal and plant species they shelter, their structure, their genetic diversity, their functioning and their productivity. In a context of stable climate, the natural regeneration of existing tree species, provided they are in good condition, is a key factor.⁴, Natural regeneration is often the best solution for meeting all these challenges. In a changing climate, however, natural regeneration may not be enough to ensure the long-term resilience of stands.

For example, for the renewal of sub-natural old-growth forests, we could adopt a gradual approach based on a prior diagnosis.

1. If the species in place are in station, that the prognosis for their future is good (see for example, the maps of climatic compatibility of Climessence), and that the adult trees are of good quality and show no symptoms of decline, then natural regeneration alone remains a perfectly valid option. However, care should be taken to ensure that regeneration is carried out in such a way as to maintain the most drought-resistant species (increasing the proportion of sessile oak in beech forests, for example) and rare species (wild cherry, wild cherry, etc.).
2. If the species in place are currently doing well but the climatic prognosis for them is uncertain, the introduction of individuals of the same species but from a more southerly origin (e.g. Sessile oak from the south of France, common beech from Italy, etc.) into the natural regeneration could be envisaged to strengthen the stand's genes and increase its capacity to select (naturally and via hammering) robust and adapted individuals. This is known as assisted provenance migration.
3. In the most difficult sites, where existing species are likely to be in great difficulty (sessile oak stands on filtering soil, lowland beech stands, chalarose ash stands, etc.) and where dieback has already been observed, natural regeneration should be used with caution, favouring less represented but better adapted species (e.g. sessile oak in ash stands): sessile oak in the ash grove) and supplementing this with species that are potentially better adapted to the new/future conditions. These could be uncommon native species (small-leaved lime, plane maple, nannyberry, etc.), native species of southern provenance or new species. However, given the great heritage and environmental value of sub-natural old-growth forests, there are strong reservations about introducing southern provenances and new species, which are reflected in a number of legislative restrictions.

Recent forests (with or without conifers)

The SRFB proposes to favour new experiments in recent forests where the ecosystem has been impoverished by old agricultural practices. This leaves more room for daring and experimentation, and it seems to us that the restrictions on the use of new exotic species should be less here than in sub-natural old-growth forests.

Thus, in recent coniferous or deciduous forests, stand renewal, particularly when the transformation (change of species) follows a health problem, should be able to rely on the full range of introductions, subject to a few specific precautions:

• use southern provenances of native or exotic species already listed in the Ecological Species File (e.g. Douglas fir of Californian origin).⁵) ;
• use southern species that are biologically close to native species (e.g. Hungarian oak in relation to pedunculate oak) or exotic species listed in the Ecological Species Register (e.g. Mediterranean fir in relation to silver fir and/or Vancouver fir);
• use new species, in particular softwoods of American, North African or Asian origin, but only on a select basis and only in small areas. These plantings must be carried out on an experimental basis and in a very controlled manner (see also below).
• comply with all relevant legislation, particularly in terms of installation surface area, as stipulated in Article 40 of the Forestry Code⁶.

In terms of planting schemes, we can of course proceed, as in sub-natural old-growth forests, by enriching the natural regeneration at specific points. However, full planting of these new species, possibly mixed with others with complementary functional traits, is a good option (islands of future in the sense of the mosaic forest).

For trials of new species originating from other continents, a thorough evaluation (prior to trials and during trials) is required before any large-scale deployment. In the context of Trees for Future, on the basis of the available literature and the experience accumulated in historical arboretums, several potentially promising species at the outset were discarded. We can cite, for example, Manchurian ash, which is potentially invasive and a healthy carrier of chalarosis, or Lawson cypress, whose potentially invasive nature has recently been highlighted⁷.

Selected species which have passed these initial checks but which nevertheless prove to be invasive, within or from the test plot, must naturally be abandoned and the plot destroyed.

These trials of new species are likely to increase in size over the next few years. In France, for example, where there is a deliberate policy of setting up "islands of future" in public forests, the objective over the next 20 years is to achieve forest cover equal to 0.5% of the total forest area. In France, the surface area for these islands is of the order of two hectares, limited to 0.5 hectares in Wallonia.

Conclusion

Assisted migration of southern provenances and species and the introduction of species from other continents are just some of the strategies used to adapt forests to climate change. They complement methods such as increased use of natural regeneration, dynamic silviculture, respect for the soil and mixed forestry with continuous cover...

While we cannot reasonably rule out any risk of invasion or negative impact on ecosystem functioning linked to these introductions, the approach proposed by the SRFB in this article minimises these risks as far as possible. It has the merit of proposing a way forward for the adaptation of forests to climate change, with a view to maintaining ecosystems in good working order. In our view, it is more risky to continue management «as usual» than to experiment with enriching the forest with new species and new methods.

The task of today's foresters is no longer primarily to maintain the integrity of typical forest associations (the alder-beech forest, for example), but above all to maintain the structure and functioning of our forests. As a result, they will be able to continue to exist and perform their essential social, environmental and economic functions.

About the introduction of new species or provenances, SRFB considers three categories of new species/provenances, each with its own level of associated risk and deployment strategy.

• Provenances méridionales d’essences indigènes : risques négligeables (exemple : chêne sessile du Sud de la France). Elles devraient pouvoir être utilisées en enrichissement de plantations et de régénérations naturelles de provenances locales de même espèce dans toutes les situations, y compris en forêts anciennes sub-naturelles. Elles augmenteraient ainsi la diversité génétique globale du futur peuplement.
• Essences européennes d’origine méridionale : risque faible (exemple : chêne pubescent, pin maritime…). Ces essences sont biologiquement proches de nos essences indigènes (certaines peuvent même s’hybrider avec elles), leur potentiel biologique est similaire et le risque d’invasion très limité. L’introduction de ces essences en enrichissement est peu risquée et devrait être favorisée, y compris pour la diversification des forêts anciennes sub-naturelles si les essences indigènes en place sont en limite de station.
• Essences originaires d’autres continents : risque potentiel plus élevé (entre autres son caractère invasif) et potentiel biologique plus faible (pas d’historique de coexistence avec la faune et la flore indigènes). La SRFB recommande de ne pas introduire ces essences en forêt ancienne sub-naturelle et de les réserver à des reboisements de forêts récentes, idéalement en mélange. Pour les essences nouvelles non encore testées en milieu forestier en Belgique ou en régions voisines, les plantations sont à réaliser dans le cadre d’essais bien cadrés (par exemple Trees for Future). Les parcelles seront bien identifiées afin de surveiller leur comportement sur le long terme, avant un éventuel déploiement plus important et, le cas échéant, de pouvoir les détruire facilement.

En conclusion, ce sont bien le savoir-faire forestier et les connaissances scientifiques qui font toute la différence entre l’audace et l’inconscience lors de l’introduction de nouvelles essences ou provenances en réponse aux changements climatiques. Nous vivons une période de fortes incertitudes et il nous semble important d’ouvrir le champ des possibles, de savoir se remettre en question et prendre des risques mesurés. Dans cette optique, nous plaidons pour une combinaison des stratégies plutôt que leur opposition, pour la nuance plutôt que le dogmatisme. Les nouvelles essences/provenances ont leur place dans une approche globale de l’adaptation de nos forêts aux changements climatiques, pour peu qu’elles soient utilisées en toute conscience des risques et bénéfices potentiels.