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.

Soil Science for Gardeners book 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.

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