Trees and climate change

The consequences of climate change on forests

Climate change is changing how trees and forests work. Some effects may be beneficial but most are harmful, and will remain so in the absence of adaptation.

It is climate extremes that will play a decisive role in the future, because living organisms have difficulty adapting to sudden changes.

The challenge is therefore to know if the forests will be able to adapt or if they will wither in the next 50 to 100 years. In addition, deforestation and forest degradation account for 11% of global greenhouse gas emissions.

A potentially positive aspect of climate change is the faster growth of some tree species. This is due to an increase in the concentration of carbon dioxide in the atmosphere since the twentieth century, which stimulates the photosynthetic activity of trees. The increase in temperatures makes it possible to lengthen the period during which trees grow, but multiplies their water requirements accordingly.

By 2100, it is estimated that forests could be less productive in the southern half and on the west coast of France, due to the seasonal cycle of rainfall.

Using the most recent scenarios from the Intergovernmental Panel on Climate Change (IPCC), climatologists predict negative impacts for hardwood and coniferous forests throughout France. It is estimated that the holm oak will expand greatly due to temperature increases and could even go back to the Loire by 2100, whereas today it is a typical Mediterranean species. On the other hand, beech could strongly decrease because of its sensitivity to lack of water.

The main constraints for tree development are soil and air droughts. An international study ([1]) concluded that most trees are in danger of dieback because of hydraulic failure, whether in arid or humid areas. It is mainly the trees at the limit of the species' range that could be affected in the near future.

Source : PNRHL-G.Martin Horcajo

In Occitania, following the drought of 2003, significant dieback in summer observed in softwood plantations. They caused high mortality in the Tarn, and the phenomenon continued in the following years, mainly on spruce stands.

Forests are still in shock from the droughts of 2003 and 2006, which made them even more vulnerable and led to loss of growth, deterioration of their health, increased susceptibility to bio-aggressors, and especially abnormal mortality.

INRA scientists have shown that forests are more sensitive to the recurrence of droughts than to their intensity. This fragility depends on the species of tree, its place of origin, its growth but also its age.

The expansion of pine processionary caterpillars to the north and at altitude is representative of the spread of forest bio-aggressors due to global warming. The most optimistic scenario indicates a colonization reaching Paris by 2025.

Forest trees have greater genetic diversity than humans, which provides excellent insurance to adapt to environmental changes during their long life. Research in this direction will indicate which species and which sources to introduce rather than others, but also to give advice on good practices to put in place. The introduction of hardwoods in the middle of conifers may, for example, have a protective effect against predatory tree insects.

The role of the forest vis-à-vis climate change

The forest is a real carbon sink since it allows the storage of atmospheric CO2. Each year, it absorbs between 10 and 15% of carbon emissions worldwide.

For example, deforestation and forest degradation account for 11% of global greenhouse gas emissions.

Trees can mitigate the effects of climate change by playing a role at several levels: 1 m3 of oak wood stores one tonne of CO2. The forest plays a key role in mitigating and combating climate change, so-called "3S":

  • Sequestration: trees capture CO2 from the atmosphere through photosynthesis and sequester carbon in the wood and soil;
  • Storage: Forest products sequester carbon throughout their life, prolonging carbon storage, especially when using long-lived products such as wood in construction;
  • Substitution: Wood products can substitute for other materials that consume more energy in construction (material substitution) or non-renewable fuels that emit greenhouse gases (energy substitution).

The forest-wood sector has an important role to play in the climate field, and it is important to inform and use the products in cascade: from cutting to recycling through the use for construction for example.

he long-term solution would therefore be partly to increase the area of forested land, but above all to sustainably manage forests by minimizing the risk of destruction following fires or storms, for example. This will prevent forests from becoming sources of greenhouse gases by preserving stockpiles.