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Trees and Carbon Dioxide: What Is the True Connection?

It’s not hard to find wacky ideas about science on the internet — bizarre concepts that stand out because they are so far removed from reality. However, when popular ideas about science are very nearly correct — but not quite — such subtle errors can be hard to spot. A fascinating example involves our popular explanations for the relationship between trees and carbon dioxide. It’s not that these popular explanations are completely wrong — in fact they are mostly correct — and yet the limitations of some of these models can lead to erroneous conclusions.

Below are six common mental models that we often use to explain the connection between trees and carbon dioxide. Many of us have been exposed to more than one of these concepts, although we might rely on a single model as our principal mental framework for the topic. All six of these models can be found in educational materials and on the internet. As we examine these six ideas, it may be useful for you to consider which model most closely aligns with what you have been taught.

The common thread among these models is that “trees absorb carbon dioxide”. This concept has recently grown quite popular — because forests help offset some of the human-caused increase in atmospheric carbon dioxide. In other words, there is a connection between forests — especially tropical rainforests — and global climate change. If we can slow down or reverse the worldwide reduction in the number of trees, then this will help to slow the increase in atmospheric CO2. But what does it really mean when we say “trees absorb carbon dioxide”? Each of the six mental models provides a concise — but different — explanation of what this phrase means.

By using the term “mental model”, we can focus on what happens in the mind of a person who learns and interprets a concept. This mental model might not exactly match what the author of the learning material had intended — in part because the learner is likely to “connect the dots”, drawing conclusions that are not explicitly stated in the learning material.

The first three models listed below are the simplest — which makes them quite popular — but they are also the mostly likely to lead to scientific misconceptions. The final three models are better at avoiding such misconceptions, but even these models can be slightly misleading if used in isolation. Therefore the strongest mental framework is a combination of models 4, 5, and 6:

Model 1 — Trees filter carbon dioxide from the air.

This mental model equates trees to an air filtration system, filtering out carbon dioxide and other “bad” substances from the air. Unlike some of the other models, this model offers no explanation as to what happens to the CO2 that has been removed. This can lead to the misconception that the extracted CO2 is completely destroyed. (Note: Any variation of this model that specifically mentions storing carbon dioxide is actually Model 2.)

One advantage of this model is that it is so easy to understand — and it is certainly true that trees remove carbon dioxide from the air, although the mechanism is different from that of a filtration system. However, there are two principal weaknesses to this model:

1) By failing to acknowledge that trees store massive amounts of carbon, this model suggests that the only harm in cutting down trees is that there are fewer trees to filter out carbon dioxide. There is no suggestion that destroying a forest can release huge amounts of carbon dioxide back into the atmosphere.

2) This model avoids the question of “Why do trees do this? What’s in it for the trees?” This oversight sharply limits the value of the model — because answering this question opens the door to several important insights about the relationship between trees and CO2.

Another weakness, common to all of the first three models, is the implication that trees are the only green plants that remove carbon dioxide from the air.

Model 2 — Trees absorb and store carbon dioxide.

This mental model equates a tree to a giant sponge that sops up carbon dioxide from the air. The underlying idea is that trees constantly absorb and store CO2.

Like Model 1, this model is very easy to understand — which is certainly an advantage. A second advantage is the acknowledgment that the carbon dioxide is not magically eliminated. And a third advantage is the implication that carbon dioxide will return to the atmosphere if the tree is destroyed.

Even though Model 2 is better than Model 1, it still has several weaknesses:

1) Like Model 1, this model avoids the question of “Why do trees do this? What’s in it for the trees?” Again this oversight sharply limits the value of the model.

2) It is just plain wrong to say that trees “store carbon dioxide”. Trees use carbon dioxide — they don’t store it. What is true is that a tree contains a large quantity of carbon-based compounds. In other words, a tree converts carbon dioxide into other carbon-based chemical compounds that are useful to the tree. The great mass of a tree consists primarily of just two things: carbon-based compounds (also called organic compounds) and water.

However, a tree does not actually store most of those organic molecules — at least not in the popular sense of the word “store”, which implies that unused material has been set aside for possible later use. On the contrary, most of those molecules have been turned into wood or leaves or other essential parts of the tree.

3) This model implies that any carbon dioxide “absorbed” by the tree remains locked away until the tree dies. (Some educational materials explicitly make this point, even though it is wrong.) In fact there are several mechanisms by which carbon dioxide is returned to the air even while the tree is alive — including the metabolism of sugars by plant cells, and the annual shedding of leaves by deciduous trees.

4) This model ignores the role of other plants in removing carbon dioxide from the air. It’s not just trees that do it! In fact, some non-forest ecosystems — such as peat bogs — are extremely good at removing carbon dioxide from the air.

Despite the weaknesses of this model, a person who learns this model will realize that destroying a forest has two negative effects connected to carbon dioxide. First, there are fewer trees to remove carbon dioxide from the air. And second, destroying a forest tends to release a lot of carbon dioxide into the atmosphere in a short period of time.

Model 3 — Forests are the lungs of the planet.

This mental model equates forests — especially tropical forests — to a set of lungs, allowing the planet to “breathe”. The idea is that a forest “purifies” the air by absorbing carbon dioxide and releasing oxygen. On a literal level, this is the opposite of what lungs actually do. Lungs take in “fresh” air and exhale the “stale” air — partially depleted of oxygen, but enriched in carbon dioxide. However, because the lung model is clearly a metaphor, it is easy to understand that trees do the opposite of what animal lungs do. Thus there is an implied balance between the forests of the world and the animals of the world. In fact, many educational materials contain graphics that illustrate such a balance.

The main strength of this model is its emphasis on gas exchange — the exchange of carbon dioxide and oxygen — which is an important concept. But if a forest has the equivalent of lungs, then where are these lungs? The answer is that most of the gas exchange occurs in the leaves. Pores on the surface of each leaf allow gases to move in and out. During the day, carbon dioxide enters through these pores, and oxygen escapes. This is consistent with the “reverse lungs” concept. But at night the opposite happens — oxygen enters through the pores, and carbon dioxide escapes — a phenomenon that Model 3 does not explain, or even acknowledge.

Despite the helpful emphasis on gas exchange, this model has several weaknesses:

1) Like Models 1 and 2, this model (in its simplest, most common form) avoids the question of “Why do trees do this? What’s in it for the trees?” A child who has been taught this model might answer this question by saying “Because people and animals need oxygen.” This answer confuses a benefit with a reason.

2) Like Model 1, the simplest version of this model fails to acknowledge that trees store massive amounts of carbon. There is no suggestion that cutting down a forest can release a huge amount of carbon dioxide back into the atmosphere.

3) Furthermore, by failing to explain what happens to the carbon, this model can promote the misconception that carbon dioxide is completely eliminated by conversion to oxygen.

4) This model diverts much of the attention away from the reduction of atmospheric carbon dioxide, shifting the attention to the production of oxygen. Indeed, some websites and educational materials suggest that if the world’s forests were to be cut down, then we would soon run out of oxygen to breathe. (“Forests are the lungs of the earth. If we destroy them, we destroy ourselves!”) Destroying the world’s forests would indeed be catastrophic, but it would not result in our suffocating.

5) Like the first two models, this model also undervalues the role of non-forest ecosystems in reducing atmospheric CO2.

Model 4 — Green plants use sunlight to convert CO2 and water into sugar.

This mental model explains the essence of photosynthesis quite succinctly. Unlike the first three models, this model provides a reason that plants remove carbon dioxide from air — to produce sugar. It also explains what happens to the carbon — it becomes part of the sugar (C6H12O6). This model also implies how green plants benefit from the process — they can use the sugar.

This model usually mentions that oxygen is given off as a waste product of photosynthesis. CO2 and water contain more oxygen atoms than are needed to make sugar, so the excess oxygen is released as a gas. That’s the reason that green plants give off oxygen — not because animals and humans need it. In fact, when early green plants began to pump oxygen into the atmosphere, the gas poisoned much of the existing life on earth — killing it off, but paving the way for the later evolution of oxygen-dependent creatures.

This simple mental model of photosynthesis — that green plants use sunlight to convert CO2 and water into sugar — provides a great foundation for understanding the relationship between trees and carbon dioxide. However, this model is incomplete without a second mental model that explains what happens to all that sugar. The simplest such model (although incomplete) is that the sugar produced by photosynthesis serves as food for the plant. This is a crucial concept. Every living cell needs energy to survive — and for most plant and animals cells, this energy is delivered as sugar. Therefore the sugar produced in the leaves of a plant must be transported to all the living cells in the plant — particularly the roots.

Once you fully grasp these two ideas — that every plant cell needs food in the form of sugar, and that a living plant must move sugar to where it is needed — it makes perfect sense that most land-based green plants have an internal water-based transport system. In fact there are two distinct transport systems. One system moves sugar water down from the leaves to the roots, and the other system moves mineral water up from the roots to the leaves.

So why do plant cells need energy? Cells use the chemical energy of sugar to drive the normal metabolic processes that keep the plant alive. When the cells use this energy, the sugar reverts to carbon dioxide and water — although oxygen is also consumed in the process.

The upshot is that every cell in a plant constantly consumes oxygen and gives off carbon dioxide — just as animal cells do. However, when the sun is shining, the chloroplasts in the leaves and other green surfaces do just the opposite — and they do it at a much faster rate. Thus, during the day, green plants are net consumers of carbon dioxide and net producers of oxygen. But at night, when photosynthesis shuts down, it is just the opposite.

Model 4 is therefore a powerful concept that is closely connected to several important details. But even if you remember all of these details, there is a crucial concept that is missing — the key concept underlying Model 5.

Model 5 — Green plants create biomass; animals and decomposers break it down.

The concept missing from Model 4 is that much of the sugar produced by green plants is not used to provide energy to the cells of the plant. Instead, the sugar is converted into other organic compounds that are useful to the plant. A surprisingly wide range of compounds are produced, including starches, fats, proteins, and many other classes of molecules. Some of these compounds, such as starches and fats, require nothing more than the atoms already present in sugar — carbon, hydrogen, and oxygen. But some compounds (such as proteins) require additional atoms (such as nitrogen) that arrive via the mineral water sent up from the roots. This wide range of molecules serves many different purposes in the life of a plant.

However, a very high percentage of the sugar is simply converted into cellulose — or in the case of woody plants, cellulose and lignin. These are the structural materials that give a plant its shape and allow it to stand upright. (Lignin, which is much stiffer than cellulose, is the compound that makes woody plants “woody”.) Therefore the dry mass of a woody plant is composed primarily of cellulose and lignin, and the dry mass of an herbaceous (non-woody) plant is usually composed primarily of cellulose. Humans cannot digest cellulose or lignin, so we tend to eat the parts of plants where the digestible compounds — such as sugars, starches, fats, and proteins — have been concentrated.

Biomass is any material that consists either of living tissue, or tissue that had once been living. In a forest ecosystem, most of the biomass consists of living trees or dead remnants of trees, such as the leaf litter on the forest floor. Some of the biomass is underground, including tree roots, fungus, other microorganisms, and the myriad little critters that live in the soil.

One component of biomass is water — embedded in living or dead tissue. But the rest of the biomass consists almost entirely of energy-rich carbon-based compounds. For that reason, dried biomass is flammable, and can be used as fuel. The most obvious example is firewood, but any dried plant material tends to burn easily. This fact reveals a key detail: that cellulose and lignin contain a lot of stored chemical energy. This energy was originally captured from sunlight and stored in sugar molecules that were later converted to other high-energy molecules. In fact, all the carbon-based compounds in a plant are high-energy, and this energy can be traced back to sugar created by photosynthesis.

The upshot is that green plants are the only organisms that can create biomass — because these are the only organisms that can use the energy of sunlight to manufacture sugar. (There is a minor exception for organisms that use the chemical energy of deep-sea hydrothermal vents.) Animals, like plants, can convert certain high-energy compounds into other high-energy compounds, but in doing so there is always a loss in biomass. In other words, when an animal eats biomass — plant or animal tissue — a small part of that biomass is often incorporated into the body of the animal, becoming muscle or other tissue. But a larger part of that biomass is simply metabolized for its energy. And a far larger part of the eaten biomass is wasted — especially if the animal is incapable of digesting cellulose. The key point here is that in a typical ecosystem, such as a forest or grassland, all of the biomass is originally created by plants.

When discussing the biomass of an ecosystem, it is helpful to consider how dense the biomass is. This can be expressed, for example, as tons of biomass per acre (or in metric tons per hectare). Not surprisingly, forests tend to have the densest biomass figures — especially tropical forests — because so much biomass is locked up in woody tree trunks, branches, and roots.

Model 6 — The forests of the world are a huge carbon sink.

Because all biomass consists of carbon-rich compounds — and the carbon in these compounds originated as atmospheric CO2 captured by green plants to create sugar — forests can be viewed as a major carbon sink. A “carbon sink” is anything that absorbs large amounts of carbon dioxide from the atmosphere, retaining the carbon in one form or another.

Of course, this is a two-way street — because carbon can move in either direction. The biomass of a forest becomes CO2 again whenever any of the following processes occur:

  • Sugars are metabolized by plant or animal cells in order to access the stored energy.
  • Dead biomass, such as fallen leaves or downed trees, decomposes into simpler compounds. (Decomposer organisms play a key role, consuming some of the stored energy while breaking down the organic compounds.)
  • Fire races through a forest, burning the dead forest litter — and in the case of a crown fire, then also consuming parts of living trees.

In a typical forest, far more carbon is captured than is released — although the amount varies according to the type of forest, the age of the forest, and other factors.

Because trees can be very large, it seems intuitive that a forest would store more carbon per acre than any other type of ecosystem. But is that really true? If you only consider the above-ground storage of carbon, then the tropical rainforests of the world are the clear winners in terms of carbon mass. Forests in temperate climates also store a lot of carbon, but less than tropical forests.

However, if you consider the organic carbon stored in soils, then the picture becomes more complicated. There are extensive areas of peatlands in the world, where the density of carbon storage is as great as in tropical forests. However, much of this carbon is stored in a thick blanket of peaty soil, not in living vegetation. The acidic, waterlogged soils prevent fallen organic matter from decomposing, so it builds up over a long period of time. Peatlands are especially common in the far north — such as Canada, Russia, Scandinavia, and Alaska — but the tropics also contain significant areas of peatland.

Destroying peat bogs is as bad as destroying tropical forests, when viewed through the lens of preserving our major carbon sinks. Peat bogs are easily destroyed by draining away the water, which exposes the soil to air, allowing the organic matter to decompose. However, peatlands are not the only ecosystem with high levels of organic carbon in the soil — other examples include grasslands and mangrove swamps. In fact, worldwide there is more organic carbon in the top meter of soil than in all the above-ground biomass, including tropical forests.

Despite the crucial role of vegetation and soil as carbon sinks, they are not the only carbon sinks in the world. The ocean is also a major carbon sink, because carbon dioxide is soluble in water. In fact, there is far more carbon dioxide dissolved in the ocean than there is floating in the atmosphere. Therefore vegetation, soil, and oceans are the three major carbon sinks — but each is capable of returning carbon dioxide to the atmosphere, depending upon current conditions.

To round out this picture, it is also helpful to think about the former carbon sinks of the world, now locked away deep in the earth. There are two such former sinks:

1) Our fossil fuel reserves — oil, gas, and coal — are the remnants of ancient swamps in which large amounts of plant material accumulated without decomposing. This organic matter eventually became buried under deep layers of soil, which hardened into rock. This pool of carbon has been locked away for hundreds of millions of years — but now humans actively seek out these reserves to burn them as fuel, returning the carbon dioxide to the air.

2) The vast amounts of limestone in the earth’s crust are a result of carbon dioxide dissolving in the oceans. CO2 combines with water to form carbonate, which remains dissolved in the water. Many forms of sea life extract carbonate to produce shells, reefs, and other hard structures. Additional carbonate interacts with calcium that has weathered from continental rocks and washed into the ocean. Both of these processes result in a steady rain of calcium carbonate settling to the bottom of the ocean, forming thick layers of marl that eventually become limestone and related rocks. When limestone is processed to create cement, some of the carbon dioxide returns to the air.

Summary

We have now examined six popular mental models that attempt to explain the relationship between trees and carbon dioxide — each model consistent with the basic concept that trees remove carbon dioxide from the air:

1. Trees filter carbon dioxide from the air.

2. Trees absorb and store carbon dioxide.

3. Forests are the lungs of the planet.

4. Green plants use sunlight to convert CO2 and water into sugar.

5. Green plants create biomass; animals and decomposers break it down.

6. The forests of the world are a huge carbon sink.

Each of these mental models can help the learner to draw useful insights. However, the first three models all have serious weaknesses — including a failure to address the reason that plants absorb carbon dioxide, and a tendency to produce scientific misconceptions. The final three models are far stronger, but each in isolation only paints part of the complete picture. When combined, these last three models can provide a powerful understanding of the relationship between trees and carbon dioxide.

Of course, the forests of the world provide far more benefits than just capturing carbon — and the wholesale destruction of forests does far more harm than just releasing carbon dioxide into the atmosphere. But with the current emphasis on trees as part of the solution for fighting the rising levels of atmospheric carbon dioxide, it is helpful to have a good understanding of the underlying scientific concepts.