On a global scale, forests are the main continental carbon sink. However, on a more local scale, while some forests are carbon sinks, others are sources. The net carbon flux depends on various natural and anthropogenic factors. In particular, human activities can both promote carbon storage (protection and expansion of forest areas, fertilizing effect CO2 released by human activities) and cause its release (deforestation, degradation).
1. Forests in the global carbon balance
The increase in the concentration of greenhouse gases in the earth’s atmosphere contributes to global warming. Carbon dioxide (CO2) is one of the main greenhouse gases and is currently responsible for about 60% to 80% of the anthropogenic greenhouse effect. The concentration of CO2 in the atmosphere increased strongly during the pre-industrial era due to emissions linked to deforestation, and then following the industrial revolution with emissions linked to fossil fuels (mainly coal, petroleum products and natural gas). This concentration has increased from about 278 ppm in 1750 to 410 ppm in 2019. Currently, it is increasing by about 2.4 ppm per year (See Focus Carbon and forest biomass).
The forest is, according to the FAO definition , an area characterized by a tree cover rate higher than 10% and a tree height higher than 5 meters.
Forests represent an immense stock of carbon via (i) biomass, (ii) soil carbon (in the form of organic matter and carbonate minerals), (iii) dead wood and (iv) litter (the thin layer of dead organic matter above the soil). This stock evolves according to natural factors (deaths of old or diseased trees, growth of others) and anthropogenic factors (deforestation, plantations) (Figure 1). When the stock of carbon stored in the forest increases, the forest is a “carbon sink”, whereas if its stock decreases because carbon is released into the atmosphere, it is a “carbon source”. Depending on the region, the forest can be a source or a sink of carbon. For example, tropical deforestation leads to a significant emission of CO2 into the atmosphere, while the growth of trees contributes on the contrary to capture atmospheric CO2.
The Global Carbon Project (GCP) , establishes each year a balance sheet of carbon fluxes on Earth. This project is particularly interested in carbon emissions of anthropogenic origin and the resulting fluxes between the different carbon reservoirs of the Earth (land surfaces, ocean and atmosphere). In the following, we list these different terms with their estimates over the last decade (2010-2019) and the associated uncertainty (± 1σ) (Figure 2) :
- CO2 emissions from fossil fuel use and cement production (EFOS ≈ 9.4 ± 0.5 GtC/year). If we estimate the population of the planet at 8 billion people, this represents an emission of about 1.2 tons of carbon per person per year, with, however, in reality a very large disparity between countries.
- Emissions related to land use changes, mainly deforestation (ELUC ≈ 1.6 ± 0.7 GtC/year). These emissions are uncertain and estimated with inventory models (book-keeping ) from lost or gained forest areas.
- Terrestrial (SLAND ≈ 3.4 ± 0.9 GtC/yr) and oceanic (SOCEAN ≈ 2.5 ± 0.6 GtC/yr) CO2 sinks, estimated from an ensemble of 3D numerical ocean carbon cycle models, and from an ensemble of diagnostic models estimating air-sea fluxes from numerous in situ measurements of ocean surface CO2 partial pressure (SOCAT).
At the atmospheric level, the difference between the incoming (EFOS + ELUC= 9.4 + 1.6 = 11.1 GtC/yr) and outgoing (SOCEAN + SLAND= 2.5 + 3.4 = 5.9 GtC/yr) carbon fluxes leads to an accumulation of 5.1 GtC/yr. Thus, of the 9.4 GtC/year released by human activities, 5.1 GtC/year is stored in the atmosphere, i.e. 55% of the emissions.
At the continental level, the difference between incoming (SLAND = 3.4 GtC/yr) and outgoing (ELUC = 1.6 GtC/yr) fluxes means that continental surfaces store 3.4 – 1.6 = 1.8 GtC/yr, equivalent to 20% of the anthropogenic carbon emissions. Finally, the oceans absorb 25% of the carbon released by human activities (see also Figure 4).
Forests are estimated to capture between 70 and 100% of the 1.8 GtC/year absorbed from continental surfaces. Non-forested areas (grasslands, crops, tundra, etc.) would therefore capture between 0 and 30% of these 1.8 GtC/year. The uncertainty about the relative contributions of forests and non-forest areas to CO2 sequestration reflects the fact that different methods, with their associated uncertainties, are used to estimate carbon stocks and their variations, and that they do not converge on the same results. In any case, forests are the main carbon sink in the continental environment. In addition, they are also a source of carbon through deforestation. Understanding the role of forests in the carbon cycle is therefore essential for dealing with global warming.
The impact of deforestation (ELUC) was relatively stable, but it is rather increasing in recent years (Figure 3). The terrestrial sink (SLAND) is increasing, possibly related to replanting efforts and the fertilizing effects of CO2 , but tended to stagnate in recent years. The total residual terrestrial carbon sink (SLAND– ELUC) is roughly stable over the last two decades (Figure 3).
Before looking at changes in carbon stocks, whether positive (forest = sink) or negative (forest = source), let us first consider the distribution of these stocks on a global scale.
2. Distribution of forest carbon stocks
Forest carbon stocks correspond to the carbon stock present in the vegetation (above and below ground), the soil, dead wood and litter. Forests cover about 3,900 million hectares of the planet and are distributed in three main geographical domains: tropical (about 50%), boreal (1,100 Mha) and temperate (750 Mha)  (see Table S2 of this reference ).
According to a study published in 2011 by Yude Pan and colleagues , the stocks of the world’s forests are of the order of 860 GtC, of which 380 PgC (44%) is in the soil (to a depth of about one meter), 360 GtC (42%) is in living biomass (above and below ground), 75 GtC (9%) is in dead wood, and 45 GtC (5%) is in litter. Geographically, the tropical forest accounts for more than half of the stock (470 GtC, 55%) and the rest of the stock is distributed in the boreal (270 GtC, 32%) and temperate (120 GtC, 14%) forests (Figure 4).
Stock density is similar in tropical and boreal forests (242 vs. 239 MgC.ha–1 or tonne C. ha–1), but is 60% lower in temperate forests (about 150 MgC. ha–1 or tonne C. ha–1). A fundamental difference in the structure of carbon stocks in tropical and boreal forests should be noted. In tropical forests, the stocks are mainly located in the biomass (56%) against 32% for the soil, whereas in boreal forests, they are mainly in the soil (60%) against 20% for the biomass. Thus, if we consider only the living biomass (above and below ground), out of a total of 360 GtC, the tropics account for about 70% (260 GtC) of the carbon stocks of the world’s forests, of which about 40% (140 GtC) is in the American tropics alone and 25% (93 GtC) if we consider only the above ground living biomass of the Amazon (Figure 5) .
As mentioned above, the carbon stock of forests changes according to climate, human activities, etc., and these changes can be positive (carbon accumulates in the forest, called a sink) or negative (called a source). Let’s consider the main factors behind these changes, whether they are positive or negative.
3. Increase of the stock: the forest as a carbon sink
Photosynthesis converts mineral carbon from the atmosphere into organic carbon, which then accumulates in the trees (leaves, branches, trunks, roots) and in the soil as the forest grows. If the forest has been in equilibrium for many, many years (centuries), carbon losses due to the mortality of older or dying trees are more or less compensated by the growth of younger trees (see specific section 3.1 about old-growth and intact forests) and old-growth forests can therefore be sinks or sources, but of low intensity per unit area.
Even if the total biomass (above and below ground) of a forest remains relatively stable, the forest can accumulate carbon in the litter layer that can feed the soil carbon pool over the long term. Thus, the litter layer and soil are important terms in the carbon balance of boreal forests because the rate of decomposition is low.
Following Pugh et al , we distinguish forests into two broad classes (i) regrowth forests and (ii) old-growth and intact forests (defined by age greater than 140 years). These authors estimate a carbon sink of 1.30 GtC/year for regrowth forests and 0.85 GtC/year for old growth forests. In total, they estimate a global forest sink of about 2.15 GtC/year, which is comparable in magnitude to the residual terrestrial sink (1.8 GtC/year) estimated by the Global Carbon Project.
3.1. Old and intact forests in pseudo-equilibrium
These forests correspond to forests that have not undergone (in theory) any human intervention for many decades: they are assumed to be relatively “intact”. These are essentially tropical forests, either protected or still relatively inaccessible to human activities.
In a very old forest, in pseudo-equilibrium with its environment, the carbon stock remains relatively stable or, under certain specific conditions, slowly increases with an accumulation of carbon in the soil. Indeed, the accumulation of carbon during the growth of trees is compensated by the emission of carbon linked to the decomposition of dead wood and litter (via autotrophic respiration in the litter and the soil), which results from the fall of leaves, branches and a natural mortality of the oldest trees (renewal of the biomass). As the decomposition of dead wood debris is not instantaneous, carbon emissions to the atmosphere from tree mortality are not immediate: there is a buffering effect associated with this decomposition over several years , .
But does a balance really exist in intact, ancient forests (Figure 6)? In reality, it is likely that in most cases, even these forests undergo disturbances. These include the fertilizing effect of CO2 (and to a lesser extent the effect of atmospheric deposition of other elements such as nitrogen), increasing temperatures, and changing rainfall patterns. The fertilizing effect of CO2 on the growth of tree biomass is well demonstrated by models and in situ experiments, and according to some authors, it would explain a large part of the carbon sink of forests on a global scale. This sink would be of the order of 1.2 GtC/year over 1990-2007 in the tropics according to in situ inventory data . However, given the difficulties in estimating the carbon sink of forests over large areas, nothing is yet well established in the scientific community at a continental scale.
For example, a highly cited article by Luyssaert et al  challenges the idea that old-growth forests are carbon neutral. They estimate from a few sites that old-growth and intact forests in temperate and boreal regions constitute a significant carbon sink (about 4.6 GtC/year). This value would be strongly overestimated according to Gundersen et al , who propose a value of 1.6 GtC/year for the old-growth forests (> 200 years) of these regions. This second estimate remains high, as it is of the same order of magnitude as the global terrestrial sink (see section 1), and does not take into account losses by fire and insects. Furthermore, Pugh et al  estimate that there are very few old-growth forests in temperate and boreal regions, which confirms that the results of Luysseart et al. would be strongly biased by the inclusion of “managed” forests in their study .
Also of note are two recent reference studies in the tropics that include most of the “intact” forests. Brienen et al , based on a very large in situ database of “intact” sites, find that the Amazon forest is indeed a carbon sink, but that this sink is declining, linked to a decrease in productivity and an increase in mortality. They conclude that their results contradict those of models (generally taking into account the fertilizing effect of CO2, but not that of mortality) that predict a continuous and intense carbon sink of tropical forests. The study by Hubau et al  confirms that we have passed the peak of carbon sink in tropical forests. The sink in the biomass of intact Amazonian forests continues to decline rapidly, while that of Africa (about 0.66 MgC/ha/yr or tonne C/ha/yr) is only beginning to decline (mortality being lower than in Amazonia because African tropical forests are more adapted to drought and high temperature events).
In conclusion, it appears that old-growth and intact forests, found mainly in tropical forests, are a carbon sink. However, it should be noted that this “sink” effect is possibly due to the fertilizing role of CO2 and that, if it exists on a large scale, it is probably small and declining due to increasing tree mortality associated with drought and increasing temperatures.
3.2. Regrowth forest
3.2.1. General case
The sink here is the result of a disruption in the demography of the forests that are regrowing following natural or anthropogenic disturbances that have greatly affected their carbon stock: cutting as part of a managed forest, deforestation, degradation, fires, etc. (Figure 7) These forests correspond mainly to:
- regions where the forest is managed (Western Europe, United States, Canada) including very large regions where a generally rather young forest is developing in connection with massive plantations (e.g. China);
- the abandonment of cultivation or less intensive forest management (Western Russia);
- “natural” secondary forests that regrow after fires, degradation or deforestation (mainly in the tropics and boreal regions).
3.2.2. Special case of managed forests
Among the regrowth forests, we present here the special case of managed forests. A well-managed forest that is regularly cut by plots will have a relatively stable biomass stock over time, unless management tends to increase stand density. But such a forest can also be a carbon sink if the cuttings are used to make durable products such as housing components (framing, siding, furniture). This carbon storage is not eternal and, eventually, these products will add to the atmospheric carbon stock. However, they can have a delaying effect over decades. This buffering effect is increased if the use and life span of these wood products increases. Durability is particularly important for species that are highly resistant to pest attacks (larch, chestnut, black locust, etc.) and require little or no chemical treatment. These species should therefore be favored whenever conditions are suitable. A forest is well managed if the harvests respect the sustainability of the forest and avoid overexploitation. This also implies preserving the biodiversity that makes the forest more resilient to disturbances (pest attacks, storms, fires, etc.). Thus, while monospecific crops may appear more profitable in the short term, they are more exposed to major risks (e.g., the storms – including Klaus in 2009, Figure 8 – that ravaged the Landes forest).
Overexploitation of the forest (e.g. to fuel a biomass power plant) through frequent felling, harvesting of stumps and dead wood, etc., can increase the harvest in the short term. However, in the long term, this weakens the forest (decreasing soil fertility), can cause soil compaction and erosion, etc. and is ultimately very negative in terms of carbon balance.
4. Decrease of the stock: the forest as a source of carbon
Many factors can cause a decrease in forest carbon stocks.
Deforestation corresponds to the transformation of a forest surface into another type of surface over the long term: area of crops, pastures, shrubby wasteland, urban elements, etc. This does not include forest cutting conducted as part of long-term forest management, which has only transitory effects. More generally, deforestation is classified as land-use and cover-change (LUCC). As we have seen, this term is one of the main ones in the global carbon balance: deforestation leads to CO2 emissions into the atmosphere of about 1.6 ± 0.7 GtC/year. This term, noted ELUC, corresponds to the algebraic sum of carbon fluxes going out to the atmosphere (tree felling and soil respiration after deforestation) and carbon fluxes coming into the biosphere (regrowth of forests after changes in land use, e.g. on abandoned cropland).
Nowadays, deforestation can be monitored fairly well by remote sensing using optical or radar methods. The latter allow images to be obtained even through clouds, which are very common in the tropics and boreal zones (Figure 9) . For example, the Global Forest Watch project tracks deforestation using the Landsat series of optical satellites . Since 2015, the optical (Sentinel 1) and radar (Sentinel 2) satellites of the European Space Agency (ESA) offer a finer spatial resolution (10 m) and repeatability (two images per week) than before . As a result of international pressure (e.g. the FAO REDD+ program to reduce emissions from deforestation and forest degradation), and the implementation of remote sensing monitoring, deforestation has tended to decrease, with the notable exception of Brazil in recent years, in relation to the “environmental” policy of this country.
4.2. Forest degradation
In addition to deforestation, forest degradation also play an important role in carbon losses. Unlike deforestation, during degradation, the forest loses biomass, but it remains a forest surface. Degradations can be of anthropic origin (selective cutting of trees or undergrowth) or natural. In the latter case, massive degradation can be linked to fires (e.g. in Siberia), to pest attacks (in particular in the United States, Figure 10 , and in the boreal forests of Canada), to episodic droughts (El Niño climatic phenomena in the tropics ; Figure 11) or repetitive droughts that lead to mortality and dieback of trees on sometimes very large surfaces. All these phenomena are exacerbated by climate change, which makes extreme events (droughts, storms, floods, etc.) increasingly frequent and intense. The result is a vicious circle: the ongoing climate change leads to increased degradation of forests, which intensifies their emission to CO2 emissions into the atmosphere of about 1.6 ± 0.7 GtC/year. This term, noted ELUC, corresponds to the algebraic sum of carbon fluxes going out to the atmosphere (tree felling and soil respiration after deforestation) and carbon fluxes coming into into the atmosphere and thus accentuates climate change. However, unlike deforestation, degraded forests can store lost carbon again if there is a decrease in pressure, either natural (e.g., decrease in pest populations) or anthropogenic (e.g., stop of logging activities). Unlike deforestation, human-induced degradation, which reduces the carbon sink effect of forests in a discrete way sometimes, is very difficult to monitor by remote sensing.
5. Where are the major forest carbon sinks and sources located?
5.1. Study methods
First, it should be noted that there are still large uncertainties in estimating and quantifying forest carbon sources and sinks at the global scale.
Indeed, while estimating the carbon stock of a forest can be done relatively accurately at the local scale on a few sites, it is a very laborious and difficult task that involves taking detailed measurements of tree characteristics (height, average trunk diameter at the standard height of 1.3 m, density, etc.) on a plot representative of the forest stand . Spatialization of these estimates on a national or continental scale is still very imprecise. Spatialization can be based on:
- on remote sensing data , via optical or radar observations, but these measurements saturate for high forest biomass values. There is therefore a debate in the community on the validity of these estimates .
- on lidar data (which measure the height of trees which is well related to their biomass) such as those provided by the GEDI satellite ; but several years of GEDI observations are needed to cover the whole planet. Thus, if global maps of forest aboveground biomass are available , they do not allow yet a dynamic monitoring of stock changes.
Estimates of these changes are based on inventory work  that relies on monitoring data of changes in land use types (including that of the FAO) that are weighted by estimates of the carbon stocks of these different land use types. However, the quality of the inventory data varies greatly from one country to another, and the used biomass estimates are still rather coarse.
Very recently, passive microwave observation data (L-VOD vegetation index at long microwave wavelength, about 20 cm) have made it possible to map the major continental carbon sinks and sources in terms of aboveground biomass . Products based on optical observations are also used to estimate changes in forest canopy cover  (see Figure 12 and Focus Forest Carbon & Biomass).
5.2. Main results
Here we outline some very recent results, some of which are still under analysis. As mentioned earlier, regrowth forests are important carbon sinks. For example, the reforestation program conducted in China for several years, particularly in the south of the country, is responsible for a carbon sink of about 0.15 GtC/year. The western part of Russia also sequesters a lot of carbon, probably due to the abandonment of some cultivated areas and a decrease in forest exploitation. There are also sinks distributed throughout the tropics in connection with (i) secondary forests, i.e. those growing on formerly exploited areas, (ii) a decrease in degradation linked to certain political contexts (e.g. civil war in the Central African Republic), (iii) more humid conditions that are conducive to the growth of forests (south-eastern Africa and central-eastern South America).
It should be noted that these regions generally correspond to the so-called greening regions highlighted by optical satellites (Figure 13) . However, the greening phenomenon does not necessarily reflect a carbon sink. For example, following a fire or a forest cut, low vegetation can recover very quickly and optical satellites measure greening via vegetation indices (NDVI, leaf area index), whereas L-VOD microwave images, which penetrate the vegetation and probe the entire biomass, do indicate a drop in carbon stocks.
Studies based on the L-VOD index confirm that the main sources of carbon are in tropical rainforests related to deforestation and degradations (Congo Basin and Amazon, including the infamous arc of deforestation in its south-eastern part) and in forests affected by dieback and fires associated with drought (Siberia, Australia, extreme southern Africa). Thus, as suggested by Edward Mitchard , a recent study has shown that the tropics as a whole have become carbon neutral: losses due to deforestation/degradation being compensated by sink regions .
More recently, a study showed that the Brazilian part of the Amazon was no longer a sink, but a source of carbon over the last decade . The news, which is very symbolic, went around the world and was reported in many newspapers (Figure 14). The article also shows that the impact of hidden degradations (“hidden”, as they are generally not detected by optical satellites) exceeds that of deforestation.
6. Mitigation, adaptation and reforestation
In the face of ongoing climate change, it is vital to act in terms of mitigation and adaptation. Mitigation consists of using the capacity of forests to absorb CO2 to reduce its atmospheric content. This can be done, for example, by increasing the use of wood in the sustainable elements of the habitat (wall, floor, frame, insulation, etc.).
Adaptation of forests aims to make them less vulnerable to climate change in order to prevent them from becoming carbon sources. One of the key elements is adaptation to high temperatures and drought. Indeed, climate change is so rapid that forests do not have time to adapt, especially through changes in species distribution where those better adapted to a hot and dry climate would replace species that are not adapted to the new conditions. The latter suffer mortality and dieback due to insect or fungal attacks and abiotic damage (spring frost and summer drought). In France, the most affected species are beech, chestnut, some oak and pine species as well as spruce which will disappear from regions where they are no longer adapted. Conversely, some species are thriving, such as the holm oak (Figure 15) in Aquitaine, which is gradually replacing the pedunculate oak, which is less tolerant of water stress, in some places .
One of the solutions to accelerate the adaptation of forests is to replant with species and varieties better adapted to current and future climatic conditions (see, for example, the RENEssences project of the French National Forestry Office).
There are also many requests for replanting as part of advertising campaigns or from companies that want to green their image. In some cases, these programs are serious and useful if it is a question of avoiding soil erosion phenomena, restoring disappeared ecosystems, planting species better adapted to a hot and dry climate, etc. However, one must be extremely careful because reforestation can pose many problems.
For example, massive replanting can put pressure on water resources, as trees consume a lot of water, which depletes soil reserves and can make reforestation unsustainable . It is therefore preferable to replant in areas where water availability is sufficient and where forests are disappearing due to anthropogenic disturbances (deforestation, degradation, fires). Particularly noteworthy are large areas of the Amazon basin and Russia.
Moreover, replanting has a very negative carbon footprint because of the energy spent to grow the plants, the use of mechanical equipment, road construction, etc. Moreover, the planted trees can suffer massive mortality episodes (due to lack of irrigation) and the plantations are often done with a single species, which is not very conducive to the biodiversity of the forest. The latter is however a guarantee of resilience of forests to biotic and climatic disturbances. Thus, when conditions are favourable, for example in the case of a large thinning in a tropical forest, it is largely preferable to leave room for natural regeneration. Natural regeneration is generally very favorable to biodiversity and does not require human intervention, synonymous with greenhouse gas emissions. In summary, rather than replanting, the priority should therefore often be given to the protection of huge forest areas (especially in the tropics and in Russia), where soil moisture conditions are favorable to tree growth (and natural regeneration) and where carbon stocks are decreasing due to lack of protection. If large international companies really want to invest in a sustainable and selfless way, this is probably the best choice.
7. Take-home messages
- Forests represent a huge carbon stock through (i) biomass, (ii) soil carbon, (iii) dead wood and (iv) litter.
- Depending on the region, the forest can be a source or a sink of carbon. Tropical deforestation leads to a significant emission emission of CO2 into the atmosphere, while the growth of trees contributes on the contrary to capture atmospheric CO2.
- Forests are estimated to capture between 70% and 100% of the 1.8 GtC/year absorbed by continental surfaces. Non-forested surfaces (grasslands, crops, tundra, etc.) would capture between 0 and 30% of these 1.8 GtC/year.
- The density of stocks is similar in tropical and boreal forests; but in tropical forests, stocks are mainly located in biomass (56%) against 32% for the soil; vs. 20% and 60% for boreal forests.
- Out of a total of 360 GtC of living biomass (above and below ground), the tropics account for about 70% (260 GtC) of the carbon stocks of the world’s forests, of which about 40% (140 GtC) is in the American tropics alone and 25% (93 GtC) if we consider only the above ground living biomass of the Amazon.
- Old and intact forests (mainly in tropical forests), constitute a carbon sink, but this “sink” effect is probably weak and declining due to increasing tree mortality linked to drought and increasing temperatures.
- Many factors can cause a decrease in forest carbon stocks, such as deforestation or forest degradation. Thus, over the last decade, the Brazilian part of the Amazon is no longer a sink, but a source of carbon.
- The regrowth forests are important carbon sinks, but the replanting operations are a very important source of carbon because of the energy spent for the cultivation of the plants, the use of mechanical machinery, the construction of roads, etc.
- When conditions are favorable, it is much better to leave room for natural regeneration, which is generally very favorable to biodiversity and does not require replanting.
- Rather than replanting, priority should often be given to protecting large areas of forest (especially in the tropics and Russia), where soil moisture conditions are favorable for tree growth (and natural regeneration) and where carbon stocks are decreasing due to lack of protection.
This article (in french) was originally published in December 2021 by Planet Vie under a Creative Commons license: Rôle des forêts dans le bilan de carbone de la planète. It was edited by Mr. Pascal Combemorel, Editorial Manager of Planet-Vie.
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To cite this article: WIGNERON Jean-Pierre, CIAIS Philippe (April 27, 2023), Role of forests in the planet’s carbon balance, Encyclopedia of the Environment, Accessed September 26, 2023 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/life/role-forests-planet-carbon-balance/.
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