Our methodology for estimating forest carbon emissions associated with logging in BC
WHEN AN AREA OF FOREST is clearcut, a premature release of carbon to the atmosphere commences. If that area had not been logged, it likely would have continued to sequester carbon for many years, in many cases for centuries. In this era of a declared climate emergency, in which a meaningful response must be made within a time frame that is much shorter than the expected life of a forest, it's necessary that the provincial government reshape forest policy to reflect the urgent need to use forests to store and sequester carbon. As the price put on carbon emissions increases, the monetary value of standing forests as carbon sinks will also increase. Depending on the price of carbon, standing forests may have higher monetary value as sinks than as sources of building material or energy.
To determine how much carbon is released by logging, we determine how much biomass was in a forest before it was logged. To do that we work backwards from the volume of merchantable wood that was taken from the stand—which is recorded by the ministry of forests Harvest Billing System and other ministry records.
Our re-creation of the original biomass of an area is based on an account of forest biomass given in a 2020 study by Dr Suzanne Simard and others: Harvest Intensity Effects on Carbon Stocks and Biodiversity Are Dependent on Regional Climate in Douglas-Fir Forests of British Columbia. Simard et al made a distinction between "humid" and "arid" forests. That account is shown below:
To reconstruct the original biomass volume from the merchantable log volume, additional details about the "Live trees (above ground)" category are needed. This involves estimating the relative volumes of all the categories of above-ground live tree parts that result from clearcut logging. The proportion of each part is based on a synthesis of data from several sources. This is a work in progress. At this time we are using a conservative estimate of decay, waste and breakage. Our current breakdown is shown below:
We can now refine Simard's breakdown so that it includes all the different fates for the live trees, along with the other pools of carbon that study identified. That reconstruction is shown below:
From the above we observe that the merchantable volume removed from the forest accounts for around 40.9 percent of the biomass in a live humid forest and 39 percent in an arid forest.
Now we are able to work backwards from the merchantable volume taken from an area to determine how much dead biomass, killed by logging, was left behind. Once dead, this biomass is in a state that will allow it to pass to the atmosphere relatively quickly.
Simard et al predicted that little to none of the carbon in the mineral soil is released as a result of logging. They estimated 28.5 percent of forest floor carbon was lost. Other researchers have observed that mineral soil carbon falls significantly over time after logging, but for our estimates we are using Simard's finding of little or no immediate loss of carbon from minerals soils. All other pools are assumed to be returned to the atmosphere over time, some more rapidly than others. Once we include these considerations of carbon loss from soils, we can restate the table immediately above as follows:
From this breakdown, we see that in both humid and arid forests, the merchantable stems account for ~50 percent of the total carbon that is released to the atmosphere sometime after logging. This means the total carbon released to the atmosphere will be approximately 2.0 times the carbon contained in merchantable logs reported by the Harvest Billing System.
How much carbon is released per cubic metre?
We calculate the emissions associated with an average cubic metre of wood harvested in BC as follows:
• About one-half (.5) the dry weight of a tree is carbon.
• The decay of one tonne of tree carbon will result in the release of 3.667 tonnes of carbon-dioxide-equivalent emissions.
• The average density of timber felled in BC (based on 2018 MoF numbers for relative volumes of tree species cut, and their known dry-weight densities) was about .447 tonnes/cubic metre.
So, carbon-dioxide-equivalent emissions released by decay of one cubic metre of average wood = 1/2 x 3.667 x .447 = .82 tonnes/cubic metre.
Our estimates of carbon-dioxide-equivalent emissions released by an area, or by a mill, are based on the volume (V) taken from that area, as recorded by the Harvest Billing System, or the annual volume processed by a mill, and are determined by the following arithmetic:
CO2-equivalent emissions = V x .82 tonnes/cubic metre x 2.0 = V x 1.64
The value of the multiplier, 1.64, will no doubt change as further science helps to refine our estimates.