Future-Proof Tree Species

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Future-Proof Tree Species, a key element of our forest adaptation strategy

In Belgium and across Europe, climate change is impacting the health and functioning of forests. In this article, we discuss the reasons why PlantC considers the introduction of so-called future-proof tree species as one of the essential links in an adaptation strategy that must enable our forests to maintain the services they provide us daily.

Climate Change: Multiple Consequences for Our Forests

Water Deficits

Successive droughts, such as those in 2017, 2018, 2020, and 2022, do not allow trees enough time to replenish their reserves. While a single drought can affect a tree’s metabolism, repeated droughts can be fatal. And the entire forest suffers. Moreover, climatologists confirm an increase and intensification of drought episodes. Water deficits affect tree metabolism in two ways:

  • The tree’s photosynthetic potential is degraded, affecting the entire glucose production cycle, from which the tree draws its energy. The tree is forced to draw on its reserves, and the succession of droughts does not allow it to replenish them.
  • The cavitation phenomenon occurs when the tree’s hydraulic continuity is compromised: air bubbles appear in the sap-conducting vessels, rendering them permanently inoperable.

Consequences are already observed in stands with a growth slowdown and a reduction in foliage mass. Perhaps you have also observed the drying out of a tree’s crown, usually accompanied by the regrowth of lower branches? This is known as crown dieback.

Extreme Temperatures

They can have a direct impact on trees, through the burning of needles and leaves. Many species are susceptible to burning when the foliar tissue reaches a temperature of 43°C due to sun exposure in a hot atmosphere. Combined with droughts, extreme temperatures increase the probability of fires occurring. Forest fires from which our country has been relatively spared… for now.

Milder Winters… Making Young Trees More Susceptible to Frost

You have probably noticed in recent years a prolonged autumn, with trees remaining in leaf longer. Mild temperatures do not help to ‘harden’ the plant, which then becomes susceptible to frost during sudden temperature variations.

More Frequent Extreme Rain and Wind Events

If droughts are becoming more frequent, so are episodes of intense precipitation. These contribute to the leaching of forest soils, which already tend to become impoverished and acidified. An increase in storms and the forest damage they can cause is also to be feared.

Proliferation of Pathogens and Weakened Trees

As a consequence of the preceding points, pests and fungi are seeing their range extend to our latitudes, or are multiplying due to rising average temperatures. Many species, weakened by climatic conditions, are experiencing a decline in their overall health.

Some Native Species Struggling

The evolution of the natural range of the beech (Fagus sylvatica) is gradually pushing it northward, so that it is now to be excluded from many formerly suitable forest sites in Belgium.

Our ash trees (Fraxinus excelsior) are massively affected by a fungus, ash dieback, while Phytophthora attacks the common alder (Alnus glutinosa). More recently, the sycamore maple (Acer pseudoplatanus) has been affected by Cryptosoma corticale, a fungus responsible for sooty bark disease of maple.

Ash Dieback in Young Trees

A non-native species, but omnipresent in Belgian silviculture, spruce trees (Picea abies) have seen their stands ravaged by bark beetles.

Future-Proof Tree Species: Stand Affected by Bark Beetles
Example of a stand affected by bark beetles

However, it should be noted that certain species, particularly native ones, do not seem negatively affected by the climatic conditions of recent years. This is the case for the small-leaved lime (Tilia cordata), one of the rare native species that has seen its average annual growth increase.

Forest Adaptation Strategies

A forest adaptation strategy must lead it to better absorb climatic and health disturbances, in order to maintain the services provided by the forest: biodiversity, regulation of the carbon and water cycle, raw material, well-being…

While this article does not aim to be fully exhaustive in this regard, it should nevertheless be noted that these strategies primarily rely on two concepts:

  • The management methods. This is probably the main adaptation lever. Since the mid-century, silviculture has primarily been conducted as even-aged high forest (trees of the same age class) and often monocultural. This is evidenced by the high proportion of spruce in our forests, which have also been severely affected by bark beetles. To make our forest stands more resilient, it is necessary to diversify management methods, notably by introducing mixed silviculture under continuous cover, and by resorting to natural regeneration on sites where it remains appropriate.
  • The diversification of species and the increase in genetic diversity. In the face of environmental challenges, not all species react in the same way. By diversifying our forests, we increase our chances of seeing stands grow healthily. It is on this point that we will focus in the remainder of this article.
Future-Proof Tree Species: Strategy

Finally, it should be noted that these two strategies combine in the concept of a mosaic forest.

Assisted Migration

Glaciations severely impoverished European flora, which, during its migration southward, encountered insurmountable obstacles such as the Mediterranean or mountain ranges. Furthermore, post-war silvicultural choices led us to strongly favor certain species at the expense of others. In a context where many native species are struggling, increasing genetic potential involves two approaches:

  • Promote the assisted migration of southern species and provenances.
  • Introduce species originating from other continents.

Assisted migration consists either of accelerating the movement of species potentially better adapted to future climates, to environments outside their original distribution areas, or of importing plants of native species, but of southern genetics (example: sessile oak from Southern France).

In all cases, these species must be able to tolerate hard frosts, whether early or late, which remain and will remain frequent at our latitudes. This fact significantly restricts the potential for using typically Mediterranean species.

Species Introduction: Scientific Research and Risk/Benefit Analysis

The introduction of new species and provenances is, however, debated. Two major questions emerge:

  • What will be the impact on biodiversity and on ecosystem functioning?
  • What is their place in our forests?
Illustration of a reforestation consisting of 5 species, including red oak

Furthermore, it must be kept in mind that the risk of invasion or negative impact on the ecosystem will never be ruled out. The introduction of a new species must be subject to a risk analysis and a risk/benefit balance assessment.

It is in this context that the research project Trees for Future was launched. Its ambition is to test different provenances and tree species in forests within a network of experimental plots distributed across the country. The future-proof species currently studied are: Atlas Cedar; Turkey oaks, sessile oaks (of southern origin), Hungarian oaks, downy oaks; Small-leaved lime (differentiated origin); Liquidambar; Scots pine; Maritime pine; …

Cedrus atlantica
Liquidambar styraciflua L.
Liriodendron tulipifera Linnaeus
Pinus nigra subsp. Laricio Maire
Quercus cerris L.

The objective is to evaluate these species based on different criteria:

  • Adaptation to current and future climate.
  • Resistance to pests (insects) and pathogens (diseases, fungi).
  • Productivity and wood quality for timber production.
  • Effect on biodiversity (carrying capacity for fauna and flora and invasive risk).

Introduction Strategies

Different introduction strategies exist:

  • Introduce native species of southern provenances. That is, introduce a sessile oak from Southern France; the risks are negligible.
  • Introduce European species of southern origin (e.g., downy oak, maritime pine…). These are species biologically close to our native species, their biological potential is similar, and the risk of invasion is limited. However, beware of possible hybridizations, especially in areas where an endemic subspecies is present.

Introduce species originating from other continents. In this case, the risk of invasion cannot be ruled out from the outset. These species should be tested in experimental plots. It is not recommended to proceed with this type of enrichment in rare forest ecosystems (threatened or of high heritage value). The Trees for Future project aims to minimize these impacts by monitoring the behavior of these new species and deciding on their deployment in our forests or to destroy them if the risk of invasion is too high.

Forests & Biodiversity

Forest biodiversity depends on several factors: species present, the environment, forest management. According to the Potential Biodiversity Index (IBP), forest biodiversity relies more on the forest structure (and thus on the applied forest management) rather than the composition of the species present.

A diversified forest generates diversified food resources and refuges.

A stand composed of non-native species can potentially have a good reception for biodiversity. Evidently, it is reasonable to believe that biodiversity reception will remain higher in a stand composed of native species, which have co-evolved for thousands of years with local fauna and flora. But what about this fact in a climate where the natural distribution areas of species are constantly evolving?

PlantC’s Position on Future-Proof Tree Species

The effects of global changes are visible both in forests and in agriculture. One of PlantC’s raisons d’être is to contribute to the resilience of our agricultural and forest territories, by considering Humanity’s place in its ecosystem.

PlantC therefore looks to the future, considering that, in the face of climatic and health uncertainties, it is urgent to experiment, without being reckless. We believe it is our duty to fully consider all adaptation strategies for our forest, while also considering the risks and questions that persist. We therefore remain constantly vigilant regarding the results from both Belgian and European research programs.

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