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  • Greater conservation of forests is needed to mitigate climate change


    David Broadland
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    Favourable biogeoclimatic conditions on the Discovery Islands make this area well-suited for sequestration of atmospheric carbon and long-term safe storage of forest carbon, an important strategy in mitigating climate change.

     

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    Douglas fir veterans on the peninsula at Hummingbird Lake, Quadra Island

     

    THERE ARE TWO MAIN WAYS in which clearcut logging on the Discovery Islands is resulting in increased concentration of carbon in the atmosphere, thereby contributing to climate change.

    First, when a forest stand is converted to a clearcut, the process of carbon sequestration—the removal of carbon dioxide from the atmosphere and the subsequent long-term storage of carbon in the forest—is brought to a sudden halt. Modern agroforestry, which liquidates natural forests and turns them into short-rotation plantations, results in a profound decline in the level of carbon sequestration capacity, the magnitude of which is described below.

    Secondly, logging a forest initiates a premature return to the atmosphere of most of the carbon sequestered by that forest. This occurs both rapidly (intentional slash pile burning and combustion of the residual biomass by forest fires) and more gradually (decomposition of the dead biomass left in the clearcut). It also occurs through decomposition of manufactured wood products over time, which BC’s forests ministry has shown occurs much more rapidly than if the forest had been left to store the carbon.

    To understand why we need to conserve more forests just to reduce logging’s considerable impact on climate change, we need to understand why modern forestry practices cause such a large loss of carbon sequestration capacity. We also need to comprehend the vast quantity of carbon that’s being prematurely returned to the atmosphere as a consequence of logging. Let’s start with modern forestry’s plan to limit the maximum age of forests to around 60 years and the impact that has on carbon sequestration capacity.

    Through photosynthesis, trees remove carbon from the atmosphere and store it in their trunk, branches, roots and foliage. On the Discovery Islands, forests are able to continue this process of sequestering and storing carbon for hundreds of years. The widespread presence of large stumps on these islands—and a few remaining primary forests with large trees in them (we have found trees greater than 500 years of age on Quadra)—are proof that the islands’ forests have great potential for safe, long-term carbon sequestration and storage.

    When the idea of managing BC’s forests on the basis of “sustained yield” was first promoted in the middle of the 20th century, forest managers were promising that the period of time between consecutive cuts would be 100 to 120 years. Now, though, it is not uncommon in BC for plantations of only 45 to 60 years of age to be logged. That rotation period will result in a much lower level of carbon sequestration than would have occurred naturally. How much lower?

    This project’s analysis of ministry of forests’ growth and yield data shows that allowing forests to grow to 300 years of age would result in approximately 3.7 times as much tonne-years of carbon storage as would be the case if those forests were cut every 60 years over that 300-year period.

    It’s necessary to understand that the dimension of time—how many years a tonne of carbon is kept out of the atmosphere—must be taken into consideration when comparing the long-term climate impact of a tonne of sequestered carbon. If it is kept out of the atmosphere for 10 years, the impact will be 10 times greater than if it is only sequestered for 1 year.

    The four graphs below are based on ministry of forests’ yield curves for Douglas fir stands of various growing conditions (site index). The first illustration shows a typical yield curve used by the ministry to project expected growth in cubic metres per hectare. The exact numbers and units don’t really matter in this exercise; we are simply going to give you a sense of how the impact of carbon sequestered by forests that will be logged every 60 years compares with the impact of carbon sequestered in a forest that is left to grow for at least 300 years. So we start with a typical yield curve:

     

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    The solid green area in the graph below illustrates the tonne-years that would be sequestered on a hectare of forest over 300 years where the site index is 30. This is what would happen if a hectare of newly planted Douglas fir was left to grow. It’s also a rough approximation of what happens naturally in areas that have low levels of natural disturbance, such as forest fires.

     

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    The graph below illustrates the tonne-years of carbon that would be sequestered on that same hectare, but logged 5 times over those 300 years, with each cut 60 years apart.

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    The graph below compares the tonne-years of carbon sequestered over 300 years without logging and the tonne-years of carbon sequestered over 300 years with logging every 60 years.

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    As mentioned above, logging every 60 years results in only about one-quarter of the tonne-years of carbon that would be sequestered by a given area of forest if it was left to grow. Over millions of hectares, this is a very large loss in carbon sequestration capacity, and a significant threat to climate stability. A caveat about this comparison: It assumes there would be no loss in soil productivity as a result of removing much of the biomass from the hectare of forest every 60 years. More likely, there would be a loss in site productivity and so the loss in carbon sequestration capacity would likely be greater than 80 percent.

    This same degree of loss occurs over the full range of site index.

    One argument offered by the logging industry to temper this loss is the idea that manufactured wood products would result in the safe storage of carbon. But BC ministry of forests’ own research, as illustrated by the graph below, has shown that only 20 percent of the carbon “stored” in manufactured forest products is still “stored” after 100 years. The rest has been released to the atmosphere.

     

     

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    In the future, if trees are harvested at a much younger age than in the past, as is planned, a much smaller volume of the tree will be suitable for long-lived products and a greater volume will be used for ephemeral uses such as wood pellets for burning in thermal electricity generating plants. 

    The Discovery Islands Forest Conservation Project is tracking the volume of logging on the islands and its impact on the carbon sequestration capacity of island forests.

    Now lets consider the second way that the concentration of carbon in the atmosphere is increased by logging.

    When forests are logged, not only is there a sudden halt to carbon sequestration, but almost all the carbon that was stored in the biomass of that forest begins to be released to the atmosphere. Although this decomposition occurs over an extended period of time, by international convention the date on which emissions are considered to have been released is on the date the forest was logged.

    The image below illustrates the problem in a nutshell. These piles of wasted biomass, created by TimberWest on Quadra Island, were burned soon after this photo was taken. If the piles had been left unburned, they would have constituted an even higher fire hazard than does the fuel-laden clearcut and plantation that exists there now.

     

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    Logging slash piles on TimberWest-Mosaic clearcut on Quadra Island in 2020. These piles were later burned.

     

    As mentioned above, the shorter the period of time between successive cuts, the smaller the trees and the greater the percentage of each tree that is wasted because it is too small and uneconomic to be removed from the clearcut and processed into something other than short-lived pellets.

    The Discovery Islands Forest Conservation Project is tracking the volume of forest logged on publicly-owned land on the Discovery Islands each year and is calculating the forest carbon emissions and loss of carbon sequestration associated with that logging. We are using the methodology developed by the Evergreen Alliance.

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