The importance of trees – Foundation for the Living

The importance of trees – forests and climate

A forest is not only an accumulation of trees placed next to each other, but must be understood as a complex system consisting of the sum of a very large number of interactions, while being able to interact with outside as a single organism.

It is a very rich ecosystem capable of assuming a wide range of ecological functions (ecosystem services) that play a vital role for humans. In addition to wood production, they are a reservoir of biodiversity and habitats for a large number of species. The high biomass production of these ecosystems allows the creation of a very rich soil with a thick layer of humus. Also, these soils are consolidated by the presence of roots, which has the effect of reducing their vulnerability to slope and natural hazards such as avalanches or landslides.

Forest ecosystems also greatly influence the quality of the air. They produce a significant amount of oxygen in the air we breathe, but are also able to capture dust or other non-degradable pollutants.

The importance of forest ecosystems is not limited to the local scale. On the contrary, forests play a role in major terrestrial biogeochemical cycles, particularly those of water and carbon. In the growth phase, a forest fulfils the function of a natural reservoir of carbon, called a sink. It uses the carbon present in the atmosphere (in the form of carbon dioxide) and fixes it in the wood and in the soil. This carbon is found there and blocked in another reservoir that the atmosphere, and therefore does not participate in the increase of the greenhouse effect.

For the water cycle, their role is even more significant. The concept of evapotranspiration is important: it defines the amount of water transferred to the atmosphere depending on the type of surface. This concept includes evaporation at ground level, the interception or not of precipitation by this surface and the transpiration of organisms found there. Through these processes, a large amount of water vapor is transferred to the atmosphere. This water vapor will then condense (due to the lower temperature) and fall back on the forest as precipitation.

Gorshkov and Makarieva, two Russian physicists, go even further. They show that this forest evapotranspiration forms a column of air where they are. This column generates a difference in partial pressure of water vapor that will draw water from ocean surfaces on the continent, because the evapotranspiration force of an ocean is lower than that of a primary forest. They call this phenomenon the biotic pump.

Here we reach an absolutely fundamental point: a forest is therefore able to attract the ocean air masses charged with moisture and thus generate additional input. On the other hand, this phenomenon only works if the evapotranspiration of this forest or other surface is greater than that of an oceanic surface. It works for a primary rainforest such as the Amazon rainforest, but it does not work for grazing, monoculture or deforested soil, or for growing forest. Significant evapotranspiration requires a fairly warm and humid climate and a large leaf area.

This difference in evapotranspiration according to the type of forest shows us the absolute importance of protecting our primary tropical forests such as the Amazon rainforest. If deforestation continues to the point where evapotranspiration will be lower than that of the oceans, the pump phenomenon may be reversed and the air masses loaded with moisture over the forest may be attracted to the oceans. In this case, we would enter a negative spiral of degradation in which the dewatering of the continental soil could occur at a completely crazy speed.

Besides that, the Amazonian forest is so large that the ecological functions it fulfils are not only regional or continental, but global and global. It is rightly given the function of thermostat, or rather air conditioner of the planet. It generates such a large amount of clouds that it is reflected in the global climate and in the atmospheric circulation of the earth. As clouds are able to reflect or absorb sunlight, the Amazonian forest contributes indirectly to the reduction of the greenhouse effect, in addition to contributing directly through its carbon storage.

We hope that all these arguments will convince you, as we do, that reforestation is not only very urgent, but is above all a truly ecological and inexpensive solution in the fight against global warming. A recent Swiss study (Thomas Crowther, ETH Zurich) argues that 1200 billion trees would be needed to absorb two-thirds of the carbon dioxide we have emitted since the industrial era, which corresponds to the United States’ surface area. ‘America. This seems important, but this solution has the advantage that the trees that are planted absorb the CO2 we have already emitted into the atmosphere, while producing oxygen. For example, the development of renewable energies, a solution that is often put forward, certainly makes it possible to reduce our carbon emissions, but does not reduce its atmospheric concentration. Reforestation is therefore a much more effective solution, at least involving a much shorter transition period, at a much lower cost and at the same time helping to reduce the loss of biodiversity that we are experiencing. Reforestation and the fight against deforestation are ultimately the best weapons we have in the face of the challenges posed by climate change.

Beyond a simple recycling of precipitation at the local level, forests are responsible for the transfer of atmospheric moisture from the oceans to the interior of continents, thanks to repeated cycles of evapotranspiration and condensation. The discovery by Russian researchers Victor Gorshkov and Anastasia Makarieva of this hitherto unknown role will lead us to re-evaluate the importance of natural forests such as the Amazon and the need to maintain them to ensure the functioning of terrestrial hydrological regimes. global scale.

Indeed, water requires considerable energy to evaporate over the forest (600 calories per gram), energy that it returns in the form of heat in the upper atmosphere at the time of condensation and the formation of rains. Thus, the extreme impact of solar radiation at the equator is absorbed by ecosystems rich in water and biomass in these areas of the globe. At the same time, the speed of the condensation process compared to the slowness of evapotranspiration creates a pressure difference with suction effect. The Amazonian rainforest then functions as a gigantic hydrological core, attracting the air masses of the Atlantic and enriching them with water, to perform half a dozen cycles of evapotranspiration-precipitation, progressing from east to west – finally rise in the Andes massif and deviate to the north (United States, Gulf Stream) and south (Argentina) giving rise to hot rains at latitudes far from the equator. The Amazon River flowing from West to East is superimposed on a huge atmospheric river flowing in the opposite direction, from the Atlantic to the Andes and well beyond. Tropical forests can thus be seen as a component of the biosphere, ensuring both the functioning and the stability of the great geoclimatic cycle.

In this context, the current situation is probably much more critical than hitherto imagined. Indeed, some experts believe that the Amazonian forest should not fall below the threshold of 70% of its initial surface if this “climatic heart” must be able to continue to beat. The danger lies in the clear cuts carried out on large surfaces, starting the closed natural massifs very rich in water, which one knows that they cannot burn spontaneously. This results in cutting edges exposed to intense sunlight, causing the bare trees to dry out. Forest fires can then be triggered, self-sustaining faster and becoming uncontrollable. This phenomenon has already wreaked havoc in Indonesia and Malaysia as a result of large-scale deforestation in favor of oil palm plantations. A medium and long-term perspective, if this trend continues in the Amazon Basin: an irreversible disruption of the geoclimatic system, followed by a dramatic desertification of the most vital ecosystem of our planet.

Retto Knutti, climatologist, in Die Wochenzeitung of last September 5th

What consequences would the disappearance of the Amazon rainforest have ?

If the Amazonian forest disappears completely, it implies a diffusion of trapped CO2 equivalent to half of the historical emissions of humanity. In other words, we would release into the atmosphere the equivalent of 20 years of our current emissions. This alone would cause an increase in temperature of 0.5 C °; not overnight, but surely in a few decades …

The importance of trees – forests and climate

Retto Knutti, climatologist, in Die Wochenzeitung of last September 5th

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