It is repeated everywhere that the forest is the lungs of the planet and that burning wood would be "neutral" for the climate. Photosynthesis, nighttime respiration, humus, fossil coal... this is what really happens, and why the famous balanced carbon account of trees is a nice simplification.
This is the image we all have in mind: the tree grows, it absorbs carbon dioxide, it produces wood, then it dies and returns exactly what it took. A clean cycle, closed in on itself, a zero carbon balance. It's nice, it's reassuring... and it's a bit too simple.
This vision has very concrete consequences. It fuels the idea that burning a log in your stove would have no impact on the climate. After all, what's the difference between a trunk digested by fungi and the same trunk turned to ashes in the living room? The gas ends up in the air in both cases, right?
Except that the forest does not operate like a two-column accounting system. It resembles a gigantic CO2 vacuum that continuously captures and stores carbon compounds, some of which never return to the atmosphere. Let's see how, step by step.
Everything starts with the leaves and needles. On their underside are tiny slit-like openings, like very small mouths. It is through these that, during the day, the tree absorbs carbon dioxide from the air and expels oxygen.
Inside, chlorophyll captures light energy and acts as the engine for the reaction: CO2 and water are transformed into sugar, cellulose, and various carbohydrates. Part of it serves as immediate fuel, while the other is stored to produce wood. The oxygen we breathe is simply the product of this reduction of CO2. In other words, the trunk of a beech tree is solidified atmospheric carbon.
For a tree, a perfectly blue sky is nothing romantic: it’s the opening of the buffet. Maximum light intensity means photosynthesis is in full swing, which means reserves are filling up.
Nota Bene: if the forest appears green, it is precisely because chlorophyll is not perfect. It absorbs almost the entire visible spectrum except for green, which it cannot utilize and must reflect. What we find so beautiful is actually a luminous waste, unusable for the tree.
Here’s a point that is almost always forgotten: the air in the forest is not rich in oxygen all the time. During the day, yes, and generously. In summer, trees release about 10,000 kilograms of oxygen per square kilometer every day. Since a human consumes about one kilogram per day, such an area meets the needs of 10,000 people. A summer walk under the trees is indeed a breath of oxygen.
But at night, the process reverses. Photosynthesis stops, there is no more reduced CO2, and the tree continues to live: sugar is burned in the mitochondria, the energy factories of cells, just like in us. As a result, a quantity of carbon dioxide is released. The stomata then function in reverse, releasing CO2 and absorbing oxygen.
In addition to this, there is all the small life in the soil: foliage, dead wood, decomposing plants harbor microorganisms, fungi, and bacteria that eat and digest without ever stopping, day and night, even in winter (the soil hardly freezes beyond five centimeters, even in polar cold).
Rest assured, we do not suffocate in the forest at night. A constant airflow mixes the gases in the lower atmosphere, and marine algae release oxygen all year round, regardless of the season. The nighttime deficit is filled before we even notice it.
Let's revisit the classic reasoning in detail, because it is not absurd: a tree stores carbon throughout its life, then it dies. Fungi and bacteria then attack the wood, digest it, and release it, transformed, into the atmosphere. A quantity of greenhouse gases strictly equivalent to that which was absorbed, says the theory.
On paper, the operation is therefore neutral. And this is where the famous conclusion comes from: since everything goes back into the air anyway, one might as well heat their home with it.
The problem is that this scheme stops at the soil's surface. It considers decomposition as a simple return to sender, while in reality it is a long journey downward, with stops, detours, and above all losses along the way... losses for the atmosphere, gains for the ecosystem.
When a trunk rots in place, part of its carbon returns to the air. But the largest portion remains locked in the forest. The decayed wood is broken down into smaller and smaller pieces, absorbed by countless species that, centimeter by centimeter, bury it deeper. The ultimate residue is taken care of by the rain, which allows organic matter to penetrate the soil.
Burning the same log short-circuits this entire process: in just a few minutes, all the carbon is released back into the atmosphere. Nothing is buried, nothing is set aside. Therefore, the comparison does not hold.
And yet, there is a disturbing nuance. Today, logging requires constant thinning of the stands. The sun then reaches the ground, warms it, and stimulates the growth of lower-layer species. To develop, these species consume the last reserves of humus from the deeper layers and release them as gas.
The volume thus released is roughly equivalent to that of combustion. In clear terms: for every log you burn at home, outside, the same amount of CO2 escapes from the exposed forest soils. In our latitudes, soil carbon reserves are depleting as quickly as they are being formed. It is not combustion that is neutral; it is the storage that has been canceled upstream.
How much does a tree actually store? Throughout its life, it can accumulate up to 20 tons of CO2 in its trunk, branches, and root system. Twenty tons, in a single living being that merely stands and breathes.
A hasty conclusion is often drawn: since young trees grow quickly, we should "renew" forests to capture more. In practice, this means cutting down the old ones and replanting, with the slowdown in growth expected to occur, depending on the species, between 60 and 120 years.
However, a large international study involving about 700,000 trees across all continents says exactly the opposite: the older the trees, the faster they grow. A tree with a trunk that reaches one meter in diameter produces three times more biomass than an individual half its size.
Old does not mean weak and unproductive, but vigorous and efficient. With age, the risk of fungal rot inside the trunk increases, which lowers the market value of the wood, but this does not slow down growth. If we want forests to play their role against global warming, the rule is simple: let them age. On the scale of a tree, 120 years is barely the end of schooling.
The forest not only breathes, it accumulates. The carbon that escapes decomposition descends slowly, carried by soil organisms and rainwater. And the deeper you go, the lower the temperature becomes.
This is where everything happens: as the temperature drops, life slows down, almost coming to a complete stop. Microorganisms cease to work, so they no longer release anything into the air. Carbon finds its final resting place in the form of humus and begins a very, very slow process of transformation and enrichment.
In other words, as long as we do not disturb this soil and expose it to the sun, each generation of trees adds a layer to a deposit that never pays back. It is not a closed cycle; it is a cumulative subtraction: carbon material removed from the atmosphere for durations that far exceed human timescales.
This also explains why dead biomass is just as important as living biomass. The more a forest hosts standing wood and wood on the ground, the thicker the layer of humus becomes, and the more the whole system retains water and carbon.
This burial work is not magical; it is carried out by an invisible army. Half of a forest's biomass is beneath your feet. A handful of forest soil contains more living organisms than there are humans on Earth, and a single teaspoon already holds a kilometer of fungal filaments.
Among the workers in this chain are oribatids, tiny mites less than a millimeter long, brown-beige, resembling small, stout spiders. They live in fallen leaves and voraciously feed on them. Without them and their colleagues, litter and bark flakes would pile up to several meters thick. Other species have specialized in fungi, while others focus on decaying wood.
You can observe the result on your next walk. Scrape the forest floor until you reveal a layer of lighter color. All the dark soil above this boundary is heavily enriched with carbon: this is the very beginning of the carbonization process, right under your fingers.
Nota Bene: humus is that black organic matter resulting from the decomposition of leaves, wood, and soil organisms. It is not mineral soil; it is carbon in transit, capable of retaining water like a sponge and nourishing trees for decades.
If this soil were no longer disturbed, what would happen to these dark layers? A first degree of coal, natural gas, or oil. This is exactly what happened about 300 million years ago.
At that time, the trees were somewhat different: they resembled more to ferns or giant horsetails. But with their 30 meters in height and trunks that could reach two meters in diameter, they had the stature of our current species. Most grew in swamps. At the end of their life, their trunks fell into the marshy waters where they decomposed almost not at all.
Over the millennia, thick layers of peat formed, gradually transformed into coal under the pressure of debris that accumulated on top. Thus, it is literally fossil forests that today fuel several major power plants around the world.
Today, this formation of coal is almost nonexistent, precisely because of repeated clearings. And it is not only a matter of modern forestry: the Romans and Celts, who already practiced regular logging, were among the first to interrupt these transformation processes. The mechanism, however, can occur again without disturbance in large protected areas, such as the integral reserves of national parks.
Trees are not the only ones filtering carbon dioxide from the air: all plants do, including marine algae. When they die, their carbon sinks into the depths and gets stored in the sediment as organic compounds. This is supplemented by animal residues, such as the limestone from corals, which constitutes one of the largest reservoirs of CO2 on the planet.
Over hundreds of millions of years, enormous amounts of carbon have been removed from the atmosphere. One figure illustrates the scale: during the Carboniferous period, when large coal deposits were formed, the concentration of CO2 in the air was nine times higher than it is today. Forests, among other factors, helped bring it down to three times that value.
Forests also regulate the climate in another, more immediate way. In summer, they release up to 2,500 cubic meters of water per square kilometer back into the atmosphere through transpiration. This vapor reforms clouds that move inland and fall as rain, over and over again.
Without this pumping system, the interior would be arid: beyond 600 kilometers from the sea, the climate becomes so dry that the first desert areas appear. A forest is therefore not just a carbon stock; it is also a rain-making machine.
The question of how long forests will continue to store carbon is no longer really up for debate. For quite some time, we have reversed the process: oil, gas, and coal are extracted, burned as fuels, and released into the air. We are cheerfully depleting reserves that took hundreds of millions of years to form.
Curiously, trees benefit from this in the short term. The increase in CO2 concentration in the air has a fertilizing effect, and the latest forest inventories confirm it: trees are growing faster. Productivity estimation tables have had to be revised, as the volume of biomass produced today is about one-third higher than it was just a few decades ago.
But this is a poisoned gift. For a tree, slowness is a guarantee of longevity, and this accelerated growth, further boosted by massive nitrogen inputs from agriculture, is not healthy. The rule of "less CO2, more longevity" remains more valid than ever.
So, to remember: yes, trees do absorb CO2, and a lot, up to 20 tons each. No, their balance is not neutral; it is favorable... provided we allow carbon to descend into the soil. This requires forests that are not constantly opened to light, dead wood left in place, undisturbed soils, and old trees spared. Specifically: prefer wood from management that mixes ages and species, support the reservation of part of the stands, and above all, stop viewing an old forest as a forest "to be renewed." Old trees are our best allies; we just need to let them age.
Author Loïc on 25 July 2025
Sports and Leisure : Leisure
Author Audrey on 23 September 2024
Sports and Leisure : Leisure