What are the interactions between forests and climate?
In the collective mind, we readily associate the importance of forests on the climate with the ability to absorb CO2, one of the main greenhouse gases responsible for global warming. Forests would therefore help to mitigate the effect of this warming.
If this effect is very real, the interactions between forests and climate are much more complex than that and involve many parameters, depending on the geographical situation, the nature of the soil, or the orography.
In 2012, the UK’s Department of National Meteorology produced a paper that takes stock of studies conducted to date on the interactions between forests and climate. To date, this is the most comprehensive document I have been able to find on this subject, which highlights the importance of forests on the climate. The document (in English) can be downloaded from this page (see footer). We also provide an overview of scientific research in this area.
What must be understood is that the vegetation cover of a continent has a significant influence on the climate that prevails there. It is not only the climate that makes the vegetation. The interaction is two-way.
How important is the forest for the climate?
The MET office document has listed several factors that link forest importance and climate. To do this, a distinction must be made between the global impact (for the planet) and the local impact (on a region).
Importance of forests for the local climate
The various studies conducted to characterize the interactions between forests and climate indicate that four main effects link the importance of forests and the local climate:
- Variations of albedo.
- Aerodynamic effects.
- Hydrocarbon emissions.
Variations in albedo
The albedo measures the ability of a surface to retain or reflect a received light energy. An albedo of 1 indicates that the surface completely reflects the received light, while an albedo close to zero indicates a surface that absorbs this energy, which will be transformed into heat or other energy. Snow, the upper surface of clouds are high albedoes, and send much of the energy received back to space. Forests have a rather low albedo. They absorb light energy, in particular to make trees grow by the physico-chemical processes that take place there.
The upper surfaces of forests are less smooth than soil with no vegetation or clean vegetation. This feature has the following effects:
- To brake the wind speed on the surface.
- To make the flow in the lower layers more turbulent, which favors other phenomena of exchange between the trees and the atmosphere.
The mechanics of tree growth involve exchanges of water between soils and the atmosphere. The trees return through the stomata leaves of water vapor pumped into the soil. This physical effect increases heat exchange between the different layers of the atmosphere and promotes the recharge of water to the layers of air that circulate over forests.
Trees release hydrocarbon compounds into the atmosphere that decompose by oxidation in the atmosphere into aerosols. These aerosols can have impacts on the onset of precipitation.
Impact of forests on the global climate
These are the absorption of CO2 by trees and the emission of greenhouse gases by trees. The overall effect, however, is positive and helps to mitigate global warming. As such, the importance of forests for the climate is significant.
In addition to interactions with climate, forests promote:
- Plant and animal biodiversity.
- Soil regeneration.
- Reducing soil erosion.
They also make it possible to produce wood for future generations.