Trees and climate change

Consequences of climate change on forests

Climate change is changing the way trees and forests work. Some effects may be beneficial but most are harmful, and will remain so if no adaptation happens.

A potentially positive effect of climate change is a 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 also makes it possible to lengthen the period during which trees grow, but multiplies their water requirements accordingly.

On the contrary, by 2100, we estimate that forests could be less productive in the Southern half and on the West coast of France, due to changes in 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 productivity throughout France. We also estimate that the holm oak range will expand greatly due to temperature increases and could even reach the Loire by 2100, despite being a typical Mediterranean species. On the other hand, the beech could strongly decrease its range because of its sensitivity to the lack of water. 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 main constraints for tree development are soil and air droughts. An international study 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 their species range that could be affected in the near future.

In Occitanie, following the 2003 drought, we observed significant dieback during the Summer in conifer plantations. It caused a high mortality of trees in the Tarn, and the phenomenon continued through the following years, mainly on spruce stands.

Forests are still in shock from the 2003 and 2006 droughts, which made them even more vulnerable and led to a decrease in the growth rate, a deterioration of their health, an increase in their sensitivity to pests, and more importantly an abnormal mortality rate.

The expansion of pine processionary caterpillars to the north and in altitude is representative of the spread of forest pests due to global warming. The most optimistic scenario indicates a colonization reaching Paris as soon as 2025.

Source: G.Martin Horcajo - PNRHL

However, trees are able to adapt to new environmental conditions. Forest trees have a higher genetic diversity than humans, which provides them with an excellent insurance to adapt to environmental changes along the generations. More research on this topic should indicate which species and which provenance to introduce, and should also advice us about the good pratices to adopt. For examples, introducing deciduous tree species in conifer stands may have a protective effect against insect pests.

Therefore, climate extremes will play a decisive role in the future, because living organisms hardly have time to adapt to sudden changes. The challenge is then to know whether forests will be able to adapt or whether they will if they will wither in the next 50 to 100 years.

Role(s) of forests regarding climate change

Forests are true carbon sinks given that they allow the storage of atmospheric CO2. Each year, they absorb between 10 and 15% of worldwide carbon emissions. Thus, deforestation and forest degradation account for 11% of the global greenhouse gas emissions.

Trees can mitigate the effects of climate change by playing a role at several levels: 1m3 of oak wood stores one ton of CO2. Forests play a key role in mitigating and fighting against climate change through the so-called "3S":

  • Sequestration: trees capture CO2 from the atmosphere through photosynthesis and confine carbon in the wood and the soil;
  • Storage: forest products keeps the carbon they captured 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).

Therefore, the forest and wood industry has an key role to play concerning climate change, and it is highly important to inform about this and to use wood products with a cascade approach: from cutting to recycling through the use for construction for example.

The long-term solution to climate change would then include increasing the area of forested land, but more importantly sustainably managing the forests by minimizing the risk of destruction following fires or storms, for example. By preserving current sotcks, this will prevent forests from becoming sources of greenhouse gases instead of sinks.

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