Do Forests Remove CO2?

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Robert Pavlis

Part of our pollution problem is the production of too much CO2. Burning fossil fuels, driving cars and manufacturing all contribute to the problem. Trees and other plants absorb CO2 and convert it to oxygen and carbon. The carbon is converted into plant parts such as wood, leaves and roots. A solution to the CO2 problem seems fairly simple – maintain and expand our forests. It seems natural to ask the question – do forests remove CO2 from the air? The answer will surprise you.

Do forests remove CO2
Do forests remove CO2

 

Forests Sequester Carbon

Trees do absorb CO2 and convert it to oxygen and carbon – this fact is accepted as true.  What happens to the carbon? While the tree is alive the majority of it is stored as wood in the trunk and branches of the tree. When the tree dies, the wood starts to decay. As part of the decay process, microorganisms like bacteria and fungi help to digest the wood. Some of the digested carbon results in a growth of microorganisms, so now they are storing the carbon. That is all good so far.

You see references all the time about how much carbon forests can absorb each year. For example, this reference says trees sequester 2,000 – 6,000 lbs of carbon per year, per acre.

Other soil organisms such as arthropods (spiders and insects) and worms eat and digest the microorganisms, and these in turn are eaten by higher order animals.

Compost Science for Gardeners by Robert Pavlis

Respiration Produces CO2

As part of  the digestion and living processes of most microorganisms and all arthropods, worms and higher animals, carbon is taken in as a food source. It is then combined with oxygen, and respired as CO2. This process is called respiration. In a nut shell, the carbon that is stored (ie sequestered) in wood  is eventually released back into the atmosphere.

Mature Forests do Not Sequester Carbon

Once a forest is mature, the amount of live vegetation reaches a steady state – it does not change year to year. An acre of soil can only accommodate a certain number of trees. Once the forest is mature, an old tree needs to die before a new tree can grow. If the amount of vegetation remains the same, then how can the forest keep absorbing more carbon each year? It can’t.

The reason for the misunderstanding is that people only look at the amount of carbon that is absorbed by living trees. They don’t consider the CO2 being produced by the decay process. Most estimations of carbon sinking by forests is based only on tree growth and even then most studies don’t even look at the root system. The scientific  data is incomplete.

A study was made of a 150 year old mature forest in Manitoba, Canada. The aim was to measure the amount of CO2 being absorbed or produced by the complete forest. They concluded that over a long-term the forest was neutral with respect to carbon sinking (ie absorbing CO2). In summer when new leaves were active, it tended to absorb more CO2 than it produced, but in fall the reverse happened.

In conclusion, destroying our forests adds to the CO2 problem since the removed wood eventually decays. Converting bare land back to forests will reduce CO2 in the atmosphere for a relatively short time while the forest matures – it is not a long-term solution. Mature forests have little effect on the amount of CO2 in the air and as such they can’t help us with our global warming problem.

You might also be interested in my post called Lawns Reduce CO2 Levels.

References:

1) Photo Source: Moyan Brenn

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

44 thoughts on “Do Forests Remove CO2?”

  1. For future discussions about forests and carbon storage it might be really helpful to distinguish between “sequestration”, which is a process of addition of carbon, and “storage”, which is already there. In old forests, the net sequestration is very low, but the storage is high. When the forest is cut, sequestration will increase with the new growth and reach a peak, and then drop back. But it is the storage that is important. When the “mature” forest is cut, much of the storage is lost, and while sequestration rates go up, they are working against a deficit. If we talk about forests and forest soils, it is helpful to realize that we are working against a deficit, and that every time we harvest, we increase that deficit, only to have it rebound somewhat before the next harvest.

    Much of the forest carbon is stored in the soils as well, which will release their carbon when the land is disturbed by harvesting, when the ground is exposed to sunlight, and when erosion cuts open the ground. Various biological and chemical oxidation processes rapidly increase the release of CO2. (This applies to plows and rototillers as well) Mature forests do not represent a good solution for increasing storage because net sequestration is low. But reducing mature forests by repeated harvesting adds to the problem of reducing storage and increasing atmospheric CO2.

    While it is easy to understand that increased CO2 in the atmosphere is the result of adding it by our various human activities, it is sometimes useful to look at it as a storage deficit. Then our goal becomes one of preserving and increasing stored carbon.

    Reply
  2. Yet another way a mature forest can be an effective carbon sink is to harvest trees for fuel on a rotating basis to prevent decay of dead trees. The harvested trees replace fossil fuels so do not create more carbon and the new growth absorbs carbon.

    Reply
    • But burning trees releases most of their CO2 into the air. In fact you probably release more than you would by letting them decay since some of the carbon from decay adds humus to the soil.

      Reply
  3. It’s sad to see this kind of misinformation being spread with all the available literature available out there. The fact that there wasn’t a single mention of the formation and role of carbonic acid shows that very little research was done. The plants of a forest release CO2 into the soil when roots die or an organic matter is decayed and subsequently buried. When it rains, this CO2 combines with the water to form carbonic acid that breaks down even more organic matter as well as carbonate minerals in the soil and underlying bedrock. When the dissolved carbon comes into contact with reactive minerals they form strong ionic bonds that can remained stored in the soil for many years. Eventual weathering transports these ions to the oceans where they are taken up by a plethora of organisms for use in shell/skeleton formation. After they die they and sink to the ocean floor, they are eventually buried and the carbon gets stored in oceanic crust. This is one of, if not the, most permanent way to store atmospheric carbon as it takes millions of years for the carbon to eventually be re-released into the atmosphere through tectonic and volcanic activity. So, in other words, all forests absolutely play a large role in carbon sequestration and your article is incorrect in many ways. If you want more evidence, I suggest you read Ruddiman’s journal that shows a direct correlation between early civilization deforestation and the drastic increase in atmospheric carbon ~8,000 years before the industrial revolution. The worlds forests at the invent of agriculture were as old as can be yet look at the permanent effects that cutting them down had on the world.

    Sources:

    Balal, et al; (2016). “Investigating the biochar effects on C-mineralization and sequestration of carbon in soil compared with conventional amendments using the stable isotope (δ13C) approach”. GCB Bioenergy. 9 (6): 1085–1099.

    Ruddiman, W. F. “The early anthropogenic hypothesis: Challenges and responses.” Rev. Geophys 45 (2007).

    I am a geology graduate student and have taken multiple paleoclimate and climate system courses.

    Reply
    • Based on what you are describing, the plants release CO2 into the soil, which forms carbonic acid, which dissolves rock material, which eventually ends up in the oceanic crust. I can agree that this happens.

      How effective is this process? How much CO2 is moved from the air to the ocean and at what speed? How much of this CO2 is lost back into the air during this process? As a geology student you should not have a problem finding some references to support this.

      Re: “direct correlation between early civilization deforestation and the drastic increase in atmospheric carbon” – nobody doubts this. This post does NOT contradict this. What it says is that if the forests had not been cut down, they would not have a net effect on CO2 in the air. Maybe in geological time frames they would as you described above, but not in life time frames.

      Reply
  4. If organic sediment is the key to carbon sequestion then why recycle paper and wood? Surely the green option should be to landfill it and replace forests with fresh carbon sequestering growth.

    Reply
    • Your logic is only partially correct. Before man destroyed the forests, there were forests!

      If forest removes co2 from the air, even once it then by your logic establishes an equilibrium, the point you are missing is that when the forest initially absorbs the co2.

      Then that co2 is removed and as the forest matures, dies, gets reborn the initial removal still occurs, so if more forests are planted and each takes up co2 then there is a greater amount of co2 being removed.

      As it happens I dispute the logic that the mature tree no longer absorbs co2, because plants absorb and convert co2 and they exist in the forest, additionally the soil becomes more fertile from the dead wood, microorgaisms produce food and the carbons are no longer confined to just the trees.

      Reply
  5. “then how can the forest keep absorbing more carbon each year? It can’t.”

    It can and it does. When a tree decays, a portion of the carbon based material remains in the soil as organic sediment. This organic sediment, as it gets layered over and over become sedimentary carbonates. A small portion becomes kerogen. An even smaller portion of the kerogen becomes fossil fuel.

    In other words, as photosynthetic life grows, dies, and decays, it results in a net transfer of carbon from the atmosphere/ocean into the lithosphere reserve where it’s only likely to get released via geological events like a volcanic eruption.

    This can even be demonstrated mathematically.

    A = Amount of CO2 used during photosynthesis.
    B = Amount of Carbon brought in as other forms (like organic and mineral compounds brought in via the roots).
    C = Amount of CO2 given off by the tree’s respiration.
    D = Amount of Carbon that becomes part of the tree.

    It should be obvious that A + B = C + D.

    When considering the decaying process, D can be broken down into two parts.

    X = Amount of carbon released as CO2 via decaying.
    Y = Amount of carbon remaining as sediment.

    So the equation is A + B = C + (X + Y).

    Now what if a new tree grows from the last one? Wouldn’t B of the new tree simply be the Y from the last tree? A portion of it, yes but not all of it. A portion of Y from the last tree will remain behind as the carbon based sediment. Therefore, Y > B.

    By recognizing that the parenthesis can be dropped, and knowing that Y > B, we can do the following.

    A + B = C + X + Y
    A + B – Y = C + X
    A – Z = C + X where Z is the net amount of carbon left behind in the soil.

    A = C + X + Z

    Which means that A > C + X. Which means that the amount of CO2 removed by a tree over the its lifespan is greater than the sum of the amount of CO2 released from respiration over its lifespan and the CO2 released from its decay.

    Reply
    • How quickly does sedimentary carbonate form?

      What amount is formed per acre each year?

      Do you have references with this data.

      Reply
      • How quickly is irrelevant. The statement was “then how can the forest keep absorbing more carbon each year? It can’t” and that is simply untrue because the trees of a forest is not frozen in time. In a forest, even a mature forest, trees grow, they live, they die, and new growth takes their place. My ‘reference’ is the carbon cycle. Wiki has a good article on it. It even employs a photo depicting what I’m talking about.
        https://upload.wikimedia.org/wikipedia/commons/d/d5/Carbon_cycle.jpg

        123 removed from the atmosphere by photosynthesis meanwhile plant respiration + microbial respiration and decomp = 120 with a net of 3 as what they call “terrestrial uptake”. I literally represent this as A > C + X and showed how I derived it taking into consideration all variables of where carbon is transferred/stored. To use their numbers it would be 123 > 60 + 60.

        Aside from people having a habit of forgetting about the existence of the carbon cycle, is that the transfer values are not static. Best example is with plants. Greenhouses supplement the air inside with CO2 to aid in plant growth. The effects are well proven. Plants will grow bigger and more photosynthesis takes place. It’ll also mean that more of the carbon pulled in as CO2 will end up as sedimentary carbon when the plant inevitably dies and decays. This means the rate at which carbon gets pulled out of the air will increase. So if we currently hold steady on our industrial output, it won’t cause the CO2 levels to endlessly increase. Levels will increase, the biosphere will change, and a new equilibrium will be reached. Studies are showing that the Earth is already greening. We would have to continue to increase our annual output so that the biosphere is playing constant catch up. This is inevitably impossible as fossil fuels are a finite resource and will eventually deplete. Would allowing it to get that far cause catastrophic ecological collapse? No.

        Based on the estimated amount of carbon as fossil fuels, if we released all of it at CO2 in the atmosphere we would cap out at roughly 2400 ppm. That’s ignoring the carbon cycle, ignoring that we’ll never be able to reach all of it, and ignoring that not all of it ends up as CO2. Factoring it all in and a better estimate would be 1500 ppm which the Earth was at as little as 50 million years ago.

        The Earth was full of life back then. More so than now. Remember that it was just little over 20,000 years ago that we were at a glacial maximum.

        This should all come as no surprise. Plants have a minimum amount of CO2 needed to support enough photosynthesis to even live. While not all plants are the same in terms of exact level, and finding info on this is scarce, I’ve been able to find anywhere from 120 ppm to 150 ppm. Meanwhile, after some searching, found that plants suffer from too much CO2 at levels of 10,000 ppm. Well, we do too at that level. 10,000 ppm is 1% and that is pretty much our own cut off. Meanwhile, according to many greenhouse sites, complimenting CO2 up to levels of 1500 ppm can increase crop yields up to 30%.

        https://fifthseasongardening.com/regulating-carbon-dioxide

        So our current CO2 levels are on the low side. They are much closer to the minimum levels to support photosynthetic life than they are to optimum levels to support it.

        Now the rate of increase and thus the rate of change to our climate may be too fast and many ecosystems may not be able to keep up, but in the long run and regarding the big picture, the biosphere of our planet will benefit to be at 1500 ppm.

        Reply
        • The speed of formation is relevant. If it is very slow, which it seems to be, then in the short term – say a 100 years, the effect is negligible.

          We don’t even know how much carbon is in forests, and yet you seem to be pulling out numbers. Do you have some research that show how quickly sedimentary carbonates form?

          I don’t disagree that over geological time frames there might be a change, but in the short term of a few centuries, I don’t see how the mature forests will affect carbon levels.

          Reply
          • “We don’t even know how much carbon is in forests, and yet you seem to be pulling out numbers. Do you have some research that show how quickly sedimentary carbonates form?”

            Take that up with wikipedia.

            “The speed of formation is relevant.”

            Again, speed is irrelevant. Your statement, “then how can the forest keep absorbing more carbon each year? It can’t”, is wrong.

            In order for what you say to be true, the trees would have to stop aging and never die. In other words, A + B = C would have to be the whole equation and a true one. That’s simply not the case. Also, trees continue to grow until they die. I grew up in the country and the trees always had new growth each spring. Once a tree matures, its rate of growth slows but never stops until it dies. Don’t confuse getting taller with growing.

            https://earthsky.org/earth/what-makes-a-tree-stop-growing

            A tree stops getting taller but will still grow. That is, they will continue to accumulate mass.

            Then there are the needles/leaves that are constantly shed and get replaced.

            Honestly though, forests are a small player regardless if new or old. Over 50% of photosynthesis takes place in our oceans where those organisms fall down into the deep ocean when they die. Even non-photosynthetic life helps in a similar manner. Watch a few deep ocean videos and you see what looks like snow constantly falling. It’s call Marine Snow and is actually carbon based compounds (organic sediment) falling to the deep ocean floor.

      • You are failing to consider the effect of photosynthesis and only focusing on carbon. While also forgetting that for every tree planted in the forest more co2 is being converted, your logic is that a tree starts at 0, but each tree that is planted reduces co2 by 1.

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        • Not sure of the point you are making. I am not talking about planted trees. This post is about a mature forest. Clearly planting new trees in areas that are not mature forests will sequester carbon.

          Reply
  6. There is about 2 billion hectares of degraded soil. 1 hectare of forest can absorb about 15 tons of CO2 per uear. If we would reforest all of these 2 billion hectares we would absorb 30 gigatons of CO2 per year. Annual emmissions worldwide are currently about 38 gigatons. I think this clearly indicates that reforesting these lands could at least buy us some time to work towards efficient renewable energy sources, which we are already in the proces of. I don’t think it should be seen as a permanent solution, but rather a transitional solution.

    As for the growing population argument. People are increasingly living in cities, meaning less hectares of land are needed to house the same amounts of people (high rises and sich). Most of our active land use goes towards agriculture. Increased crop yields can still improve food production to meet demands in many places (not that that’s happening yet). Also we produce much more food today than is needed to feed all humans (but our system does not facilitate efficient distribution) and I agree with one of the earlier comments that moving to a vegetarian or less meat diet could solve part of that problem as well.

    All things considered there are many potential solutions, just like technological solutions already present. The main hurdle are that financial markets are not made to work towards solving this problem. The free market as a solution climate change as the problem are imo not compatible at this point in time.

    Reply

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