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  • Do new forests or old ones capture more carbon dioxide from the atmosphere?

    Evergreen Alliance Staff

    A comparison of the carbon sequestration capacity of younger and older forests was posted by CBC and was read as a question on Quirks and Quarks:

    With all the recent attention being paid to climate change and decarbonizing our atmosphere, I am curious, which takes more carbon dioxide out of the atmosphere —100 hectares of mature old growth forest, or 100 hectares of young forest?”

    Gregory Paradis, a forester, engineer, and assistant professor of forest management in the Faculty of Forestry at the University of British Columbia has an answer.

    Trees capture carbon from the atmosphere by converting sunlight to cellulose through photosynthesis. When trees die and fall to the ground, they gradually emit most of this captured carbon back into the atmosphere. Young vigorous stands grow and sequester carbon at maximum speed. As stands get older, the tree canopy closes and individual trees begin to die off from self-thinning and other causes. 

    Very old forest stands can reach a sort of carbon neutral equilibrium state where trees are dying and decaying at approximately the same rate as they are growing back.

    So, taking into account both growth and mortality, 100 hectares of young forest will generally speaking have a higher net carbon capture rate than older but otherwise identical stands.”

    Paradis said that research has shown that the optimal landscape-level carbon sequestration policy may be to harvest and replant stands when they reach their peak growth rate. This is typically between 80 and 120 years old for most Canadian forest ecosystems, much younger than what is typically called old growth. 

    Ideally we would use the harvested forest material — wood and fibre — to displace as much fossil fuel, steel, and concrete as possible, to reduce carbon dioxide releases.

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    The answer presented on the CBC website is oversimplified and dramatically unhelpful, because it fails to address the big picture as well as omitting several new findings that counter the traditional view of many foresters.

    Most importantly the answer omits that old-growth forests, particularly coastal and temperate rainforests in BC with record high biomass per hectare, are like a carbon bank and destroying them releases massive amounts of carbon that can never be restored. That’s why scientists mapping these ecosystems describe them as holding ‘irrecoverable carbon’.

    They have accumulated carbon in soil, trees, and organic matter over millennia (for more information and references see Dr. Jim Pojar’s forest and carbon report. Comparing the sequestration rate of trees of different age doesn’t make a lot of sense without highlighting the urgency to reduce the skyrocketing carbon losses caused by clearcutting and climate impacts in BC and Canada that are now much higher compared to what our forests still absorb.

    Shockingly, clearcutting old-growth results in the loss of about half of the carbon stored in these ecosystems from exposed soils and large amounts of wood left behind. This huge loss must be considered in the context of claims by industry that young trees sequester more carbon than old trees. The impact of a clearcut on the carbon stored in the forest is like a business hit by bankruptcy. The business experiences big, short-term losses (equivalent to losing all the money in the bank) plus the loss of the profits the business would have made, if it had continued to exist.

    When a forest is clearcut, large amounts of carbon are released into the atmosphere by decomposing organic matter and exposed soils. The forest also loses its potential to capture carbon for many years, until young trees reach a certain size. During this time, they are “carbon sequestration dead zones”: clearcut lands that emit more carbon than they absorb.

    Research shows that clearcutting forests in the Pacific Northwest creates “sequestration dead zones” that emit more carbon than they absorb for 13 years. This is the typical time span in the Pacific Northwest required for young trees to reach a size where their ability to sequester carbon matches the ongoing high carbon losses that result after clearcut logging.

    Other reports found that it takes even longer before Canadian forests become a net carbon sink after clearcutting. A 2008 report concluded that forests less than 20 years old generally show low levels of carbon capture or a net carbon release due to decomposition. Based on a review of several science papers, a Natural Resources Defense Council boreal forest report concluded that it takes up to three decades following a clearcut before the regrowing forest can capture more carbon than is being lost during that time.

    A global review of research on old-growth and carbon sequestration in 2008 found that the majority of old-growth forests continue to sequester additional carbon. A new more recent study found that the older a tree, the better it absorbs carbon. Almost 70 per cent of the carbon stored in a tree is accumulated in the second half of its life. The new findings suggest that old trees store more carbon in proportion to their size. It makes no sense to cut down carbon-storing older trees at a time when scientists are madly trying to invent carbon-capture technology.

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    Assistant Professor Gregory Paradis’ answer withholds the larger truth about carbon storage in plantations compared to primary forests. He fails to tell listeners and readers that a primary forest has stored way more carbon than would an equivalent area of short-rotation plantation on the same site.

    Therefore, Paradis ignores telling two important points about logging primary forests:
    (1) Substantial carbon emissions from above and within the soil are immediately released into the atmosphere through the act of clearcut logging of primary forests; and,
    (2) The lost carbon capacity created by logging primary forests and replacing them with plantations is huge, a carbon debt that will never be repaid under short-rotation industrial logging. 

    In his answer, Paradis is focusing on the wrong metric to mitigate against climate change in times of climate crisis. The key to understanding what Paradis is saying is the word "rate". The rate (i.e., how fast) at which a plantation captures carbon is not what is important. What is important is the absolute amount of carbon captured, which is much higher in primary forests than it is in plantations.

    Essentially, Paradis is saying that the City of Langford outside Victoria has the highest rate of population growth in B.C.; what he is not saying is that Vancouver has a much higher absolute population but a lower rate of population growth than does Langford. If the object is to populate the province densely as much as possible, it would not make sense to raze Vancouver to the ground and put a "Langford" in its place because a "Langford" has a higher rate of population growth than does a “Vancouver". In this analogy, it is the absolute amount of population that is important, not the rate at which population grows; the same is true for carbon.

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