The impact of climate change on forests and oak trees in particular

In the long term, climate change will result in a progressive reorganisation of the geographical distribution of the trees. The rise in temperatures will therefore result in species migrating northwards or to higher altitudes.

This northward shift in species distribution areas is expected to be at a rate of about 500 km within a century, whereas the natural migration speed of forest species does not exceed 50 km per century. It is therefore very difficult to anticipate the forest’s natural response to climate change.

The role of forests in combating global warming

Forests help to mitigate the effects of climate change: they absorb greenhouse gases (GHGs), regulate watercourses and protect coastal communities from extreme weather events and rising sea levels. Forests also create migratory corridors making the habitats of animal and plant species more resilient. Forests and trees are therefore one of the important solutions that can slow down climate change and make its progress more tolerable.

Oak trees and climate change

In mainland France, the Grand Est and Bourgogne Franche-Comté regions are among those most affected by climate change. The drought is exacerbated by the continental climate, by soils that are more likely to dry out and by the presence of particularly active insect pests.

For example, beech forests growing on limestone plateaux are suffering the full force of drought, mixed oak forests are under pressure from the processionary caterpillar and coniferous trees are under attack from the bark beetle.

However, as far as oaks are concerned, the results of an INRAE study dating from 2021* show that oaks are much more tolerant of drought than previously thought.

The results of a 2022** study also show that oak trees evolve rapidly and are capable of adapting to climate variations within a few generations. According to these findings, forest managers would be well advised to encourage the natural regeneration of forests to facilitate the rapid evolution of the tree populations.

Pedunculate oak and sessile oak and climate change

Pedunculate oak requires deep, well-watered soils and is vulnerable to severe drought. It should be noted that around a quarter of the pedunculate oak forests in the west of France are showing signs of dieback and that populations composed mainly of this species have a dieback rate four times higher than that of sessile oak. Conversely, sessile oak, which does not do well in waterlogged soils, can tolerate soils with a slightly deficient water content, making it more resistant to drought.

This is why, when renewing and improving forest populations, sessile oaks, which are less sensitive to global warming, should be preferred to pedunculate oaks. This is only holds true for a climate warming of +1 to +2°C on average per year; beyond this, the differences in behaviour between the two main hardwood species in France (4.5 million ha) become less pronounced and there is a risk of dieback, including on the sessile oaks.

According to the Centre Régional de la Propriété́ Forestière, downy oak should eventually replace the current pedunculate oaḱ tree holdings in New Aquitaine.

Lastly, to help oak trees adapt better to climate change, the ONF (French forestry office) recommends a few solutions:

• o Match the species to the site: pedunculate oak should be planted on sites with a good water supply, such as alluvial or hydromorphic environments, while sessile oak should be planted everywhere else.
• o Maintain a mixture of species and do not eradicate thermophilic oaks such as pubescent and pyrenean.
• o Opt for natural regeneration if the seedlings are of the desired species, in sufficient numbers and of good structure.
• o If planting has to be used, choose a source that is geographically and ecologically close.

According to the ONF, successfully adapting forests to climate change also requires the adoption of a new silviculture concept based on the principle of the “mosaic forest”, the aim of which is to reinforce the diversification of tree species, following the example of the experiments carried out in the “îlots d’avenir” (blocks for the future), as well as adapting the planting methods in the forest area.

*Histoire évolutive des chênes : la résistance à la sécheresse adaptée à leur habitat (France 2021, INRAE) 

**L’évolution suit le climat : les chênes se sont rapidement adaptés aux variations climatiques de l’Anthropocène (France 2022, INRAE) 

Photography © Christophe Deschanel