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Do Forests Remove CO2?

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.

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

Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

21 Responses to 'Do Forests Remove CO2?'

  1. Sparafucile says:

    A tree can’t respire anymore carbon than it has taken in. The question is do Forests remove CO2, not do mature Forests reduce CO2. Only a mature forest supposedly is carbon neutral. So renew the forests and use the wood for structural purposes. Replace the synthetics that have been replacing wood products. This will further reduce CO2. Using the wood for structure doesn’t just do a little to sequester the carbon, it does a lot. Humus will eventually decay, but that’s a long process and in the meantime that carbon is sequestered while a young forest somewhere else will sequester the carbon eventually given off somewhere in the future, so take that out of the equation. The tree roots and humus increase the depth of topsoil. Topsoil is a carbon sequestering agent so as the topsoil gets richer and deeper more carbon is sequestered.it will happen anywhere we have vegetation so take that out of the equation. Look at a tree. That’s how much carbon has been sequestered. Look at the growth rings. Even if at some point if a forest starts releasing more CO2 than it captures, how many year does it take to release all the carbon it collected over 50 plus years? Over a hundred, so it collects much more carbon than it releases. The only time a forest will not, is in the event of a forest fire, caused mainly by unmamaged forests.

  2. The recycling of CO2 by a mature forest as you describe could just as well be applied to the burning of carbon fuel for electricity. If a power station chopped down a tree as fuel, and they then planted another tree in place of it, that new tree would absorb as much CO2 as the power plant released.

    Thus, ban the burning of fossil fuels. Use wood instead, and force the power company to replace the trees. Whether that is practical on the scale that fossil fuels are being burned, I have no idea. Alternatively power companies need to store tree logs equivalent to the coal they use.

    Deserts have increased over a long time. The Sahara Desert used to be green and lush. Domesticated animals apparently changed grasslands into scrublands. If deserts could be replanted, that would absorb some CO2.

    • In theory making electricity from wood would work. But a tree will take 50-100 years to grow full size, so the electricity produced needs to last that long too. We use too much electricity.

      Storing wood does help to some extent. Some furniture manufacturers consider the process of making their product environmentally sound since the furniture will last 100 years (the good stuff) and in the mean time a new tree is grown.

  3. Brian Stiner says:

    Yes another great reason that Canada should grow Cannabis to produce our paper products and anything else that science will allow us to extract from this God given plant….it grows annually and can absorb lots of Co2 every crop grown…

  4. Josh says:

    Does the humus created by the decomposition of organic matter in forrests not store some of the carbon that would have been release back into the atmosphere?

    • Humus does store carbon, but in a mature forest, it is only increasing the amount stored if the amount of humus keeps growing. That does not seem to be the case in forests. If it were you would expect deep deposits in untouched forests that have accumulated over thousands of years.

  5. Mike says:

    This does not change the fact that mature forests are the best natural carbon sinks outside large bodies of water like the ocean. Mature forests may stop sequestering (keyword:) additional carbon once mature, but up to the point of maturity, they take lots of carbon out of the air and store it, and at the point of maturity, ultimately store the most carbon that they are going to store (i.e., carbon cycles through that semi-closed mature forest system at that point). Thus, in order to take additional carbon out of the air, we must reforest human-deforested lands (i.e., farmlands and land used for other human purposes). This will help defeat climate change on multiple fronts: less animal husbandry (since most farmland now is currently devoted to producing feed for livestock animals), less total farmland used more efficiently (primarily vegetarian diet for people worldwide), and more mature forests simultaneously existing to sequester additional carbon. This will ultimately create a 2 prong attack of simultaneously lowering co2 output and upping co2 sequestration.

  6. Dennis says:

    Mr Pavlis. Interesting stuff… Thanks for contributing to the public dialogue. That said I couldn’t disagree more with your conclusion. The fallacy in your argument is your premise that the tree is gonna mature die and rot….thus releasing carbon dioxide. What if some wonderful organization called a lumber company harvests it and helps turn it into a wonderful carbon sink called a house. Then they replant at say 2 to 3 times the rate of harvest . The price of lumber eventually goes down housing prices decline and more and more people buy them. And we have ever larger growing forests sequestering more and more co2? Why are not our highways and interstates crammed with trees?????

    • A friend of mine who has a company making furniture made a similar statement. They claimed they were environmentally friendly since their furniture was a carbon sink. By making furniture they prevented trees from rotting.

      There is some validity to this, but making furniture or homes is just a delay in the process. At some point both get discarded and start to decompose.

      You also have to factor in the energy it takes to harvest the trees, make lumber, and get the lumber to market. There is also the waste wood and wood product. You don’t get 100% lumber from a tree. The waste is mostly rotting somewhere.

      To build more homes you need more land. That land needs to come from wild areas – many of which now have trees. More homes means less places to grow trees.

      It is quite possible that a properly maintained wood lot, where only older trees are removed, could continue to be a carbon sink over the long time. But a forest as discussed in my post does not meet this requirement.

    • Matt Pearson says:

      The discussion is moving toward sequestration as the topic, which I happen to believe is the real temporary solution until we learn to emit less. I long ago stumbled into this neutral forest issue and see no evidence that it is wrong. Forests are great, but net neutral once mature. The real solution is sequestration though I think the scale of construction is also rather steady state and we are losing. Maybe you could see marginal increase from mandating more use of wood. I doubt it. I always thought storing massive amounts of wood might work. Sea worms exist at every depth so the ocean is not a good place. Think Lake Baikal and Superior – fresh water. I am currently working out the cost / benefit on this, but at first glance it looks promising. (I thought of this when I read about them pulling logs out of Lake Superior that were perfectly preserved after almost 200 years.) The best place is probably the desert. Some 30% of land is desert. I’ve been playing with the math, but somewhat stymied. BTW, a forest conservation guy told me less than 1% of the C is lost in logging a forest. Maybe you finance this by way of carbon credits or crowd sourcing.

      • I’ve thought of placing logs under water as well. The desert sounds like a better place. but who would be willing to pay for this?

        Another option which may be viable is converting wood to biochar. It seems to remain a long time in soil, and can be very beneficial to soil. I don’t know what the energy inputs are for making it.

  7. Ultima says:

    The argument in this article implies a plant absorbs the same amount of CO2 as it emits from decaying. i.e. the plant takes CO2 uses the C for growth and emits O2. Then as the plant decays the C is recombined with O2 and emitted as CO2.

    It sounds plausible if you assume all the other chemical processes between these stages have no effect. The evidence against this article’s assumption is that the author has plenty of oxygen to breath. It is accepted that before plant life on earth little oxygen existed. If the exchange rate of CO2 was equivalent, O2 could not have mounted to 1/5 of the air content we currently have.

    A reference related to plants being the source of oxygen: http://www.livescience.com/39938-earth-had-oxygen-earlier.html

    • Very interesting comment – thanks for posting.

      I disagree with your comment “this article implies a plant absorbs the same amount of CO2 as it emits from decaying”. The article does not make any conclusion about a single plant or a single chemical process. What it says is the whole forest reaches a point where no more net growth takes place. A new tree can’t grow unless an old one dies. Since the carbon from CO2 is what leads to growth ie it is converted to wood, it is true that once this steady state is reached, the net conversion of CO2 to wood, is the same as the conversion of wood to CO2.

      You can think of it this way. A forest has a certain volume – length, width and height of tree. A young forest has empty space, so the forest can take in CO2 and convert it to wood – at this point it is a carbon sink. This is repeated each year as the forest accumulates more and more wood. At some point, the space occupied by the forest is full – it can’t make more net wood. New trees do grow, but at the expense of old trees dying.

      All of the other reactions are not relevant since they don’t produce a net sink for carbon. They are just short term transient movements of carbon.

      The last part of your comment is interesting. Can the building of all the forests in the world, before man interrupted them, absorb enough CO2 to generate all of the oxygen we now have? Probably not. So where did the extra oxygen come from? Long before we had trees we had algae and it too produced oxygen. We also have vast stores of oil and other carbon deposits. Oil is essentially wood that has been prevented from being converted back to CO2 – until we burn it in our cars.

      Peat fields are another source of stored carbon, from plants, that is decaying so slowly that it is a sink for carbon. Humus in soil is similar. Humus is very stable and if we leave it alone, the carbon in it remains sequestered.

      It is an interesting question, but I don’t think the current levels of oxygen say very much about what happens to carbon in a mature forest. I don’t believe they invalidate the position taken in the article.

  8. sander says:

    Since you’re highly ranked on Google for eco myths, I must amend your dangerously short-sighted article.

    All the carbon that is not in the soil or in the trees will be in the air, contributing to the greenhouse effect. Therefore, forests, whether or not they continue to fix carbon after maturing, still take all that carbon out of the air and will therefore mitigate climate change.

    In fact, the EU could cut its carbon emissions by one third(!) by integrating trees in existing farmland (as well as benefit from a host of other bonuses provided by trees). See:

    http://www.researchgate.net/publication/235407281_Valuing_the_carbon_sequestration_potential_for_European_agriculture

    Furthermore, plants including trees do sequester carbon in several ways. Part is increased microbial activity that work to put carbon in the soil, part is organic matter (i.e. carbon) that stays below-soil and therefore isn’t released back into the atmosphere.

    So trees _are very much_ a part of the solution to reducing global carbon emissions.

    • I agree growing trees on vacant land would sequester carbon.

      But I think you missed the point about a mature forest. Once the forest is mature, trees die and as they die they give back the carbon they sequestered. Without trees dying you can’t have new growth in trees – in a mature forest. So at best this is a short term solution.

      Trees are only a solution to carbon emissions if we have vacant land we do not need for other purposes. What may be much more valuable as a solution is to find ways to icorporate carbon into a stable form in the soil. Could cchar or humus be the solution?

      I had a look at the reference you provided. I think they make the same mistake so many people make. It is true that planting more trees and shrubs would sequester carbon short term. But they do not evaluate the value long term. Once fully grown the system stops or at least slows down drastically in it’s ability to sequester carbon. Granted we need a short term (100 years) solution, but that is all it is.

  9. Joan Casas says:

    Despite the fact the only source in witch you base your conclusions is a 150 year old study and you do not provide a link to that, you are cherry picking facts, a forest is a very complex organism. Just to point a fact I do not know if you are willfully ignoring, there are processes of integrating Carbon in the soil in witch Carbon is then not released back into the atmosphere. Check carbon-fixing plants.

    • Robert Pavlis says:

      There is no 150 year old study in my post??? There are two references, not just one. One of the references, and i did include a link to the reference, is referring to the study of a forest which is 150 years old. NOT a 150 old study!! I don’t have an exact date for this work, but it was started in the 1990’s and the reference is dated Nov 2006. It is important to study old forests since the whole point of the post is that old forests don’t sequester CO2.

      Much of the rest of the information comes from a variety of sources and is accepted by the scientific community. Nobody disputes facts such as plants absorb CO2.

      I don’t understand your comment about carbon-fixing plants. All plants, that use photosynthesis are carbon fixing and that was explained in the post?

      The idea that there is a “processes of integrating Carbon in the soil in witch Carbon is then not released back into the atmosphere”, is not supported by facts. The amount of organic matter is soil is fairly constant at about 5-10%. However, scientists do not fully understand what happens in soil with respect to CO2.

      • Jussi Luukkanen says:

        Late reply, but I cannot leave you without opposing your views. If you go out in a forest, you will see that the layer of soil is not constant, it grows by a layer every year. The thickness of that layer depends on the productivity of the area where the tree grows. In some areas this means that the organic soil layer with quite plenty more than 5-10% organic matter is several decimetres thick (some soils contain more than 60% organic matter, for example peat soils) and it is growing,

        • Peat bogs are not forests – so i don’t see how that relates to the discussion.

          If forest soils increased their organic matter content every year and continued to do so, then you would have a point. But they can’t. Even if we take the extreme case where organic matter is now at 100%, how does the forest increase the organic matter? It can’t. So at that point the forest stops sequestering more carbon.

          Show me a study that shows a mature forest that still sequesters carbon.

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