How should we go about accounting for the many ways a tree continues to sequester carbon after a harvest? We know forest carbon accounting systems must account for:
- Forest inputs such as fertilizer and herbicides
- Moisture and material loss
- Electricity used to transform raw wood material to wood pellets
- Fuel (diesel) used to harvest trees and transport materials from the forest to the boiler
Methods to interpret other variables that considerably affect forest carbon – forest composition, geography and past land use, for instance – are less clear. Environmental stressors such as hurricanes or pest outbreaks and economic pressures like conversion to crop land and urbanization contribute to imbalanced carbon accounting ledgers. Add to this the difficulties of applying the same set of measurement criteria to different wood markets, and we have a complex problem indeed.
Carbon Sequestration in Long-Lived Wood Products
One particular question that deserves an answer is how forest carbon accounting schemes account for the carbon sequestered in end use products. Changes in forest carbon stocks occur whenever forests are harvested, yet these changes do not mean all of the carbon sequestered by the harvested trees is immediately released into the atmosphere. In fact, it is quite the opposite.
Hundreds of products made from trees – lumber, OSB, plywood, paper, packaging and furniture, to name a few – continue to sequester carbon for decades. The table below displays the percentage of primary wood products that remain in an end use product after a specified number of years following harvest and production.
For example, column two indicates 57.9 percent of softwood lumber remains in an end-use product like lumber or furniture 25 years after the initial harvest. This means that as of today 57.9 percent of the softwood lumber manufactured 25 years ago remains “in service” sequestering carbon in the studs, joists and floors of our homes. After 100 years, nearly a quarter (23.4 percent) of softwood lumber harvested a century earlier remains in products that sequester carbon.
When considered in terms of a typical 25-year sawtimber rotation, the ongoing sequestration benefits of a harvest are staggering. Forests in the US South produced 47 million tons of Southern Yellow Pine lumber in 2012. Converting this to tonnes and taking moisture content and carbon percentage into account translates into carbon sequestration, expressed as its CO2 equivalent, of 59,190,390 tonnes.
By 2038, when 57.9 percent of lumber from the original harvest remains in service in products, 34,271,236 tonnes remain sequestered.
Now let us consider the CO2 emissions from wood pellets produced from the same harvest. If we estimate 6,000,000 tonnes of pellets were produced from sawtimber harvest byproducts, we find 9,122,571 tonnes of CO2 are released to the atmosphere from those pellets.
When we subtract the 9,122,571 tonnes of CO2 emitted from those pellets from the 59,190,390 tonnes of CO2 sequestered in the lumber, we have a net gain of 50,067,819 tonnes CO2 sequestered (column 2, sequestered v. released, below).
Assumes 47,000,000 green short tons of southern yellow pine harvest goes toward lumber production.
To consider these benefits across time, the amount of lumber taken in an initial harvest is multiplied by the percentage of lumber that remains in an end-use product in the years following that harvest. Immediately after a harvest, 100 percent of the lumber produced sequesters carbon. Within one year, three percent of that lumber is no longer in use. As a result, only 97 percent of the lumber produced in the initial harvest remains in use to sequester carbon in year two.
The above chart shows these cumulative affects at the time of harvest and one, five, 10, 15, 20 and 25 years into the future. Keep in mind, this sequestration benefit is from the softwood lumber produced from just one harvest. These numbers do not account for the carbon that remains sequestered in other wood products such as OSB or paper.
Carbon Accounting for Wood Pellets
Just as it is illogical to believe 100 percent of the carbon sequestered by a harvested tree is immediately released to the atmosphere, it is irrational to include the carbon from a whole tree when accounting for carbon in the wood pellets used to generate electricity. Pellets are produced from harvest byproducts such as the unmerchantable tops and limbs of whole trees, and a good portion of the carbon stored by those trees remains sequestered in lumber and other forest products. Likewise, carbon accounting for wood pellets sourced from whole pulpwood trees (the come-along products of a sawtimber harvest) must count only the carbon sequestered by those small trees that go into the pellet, not the large sawtimber-sized trees used for lumber and other long-lived products.
An accurate forest carbon accounting system must consider both forest carbon and carbon that remains in harvested wood products. A failure to account for carbon stores in long-lived wood products considerably inflates emissions estimates in the harvest year. We simply cannot declare wood-to-energy is a problem until we account for and explore the many challenges inherent in complex systems of carbon accounting.
Calculations in this piece are based on logic presented in Dovetail Partners Carbon in Wood Products – The Basics.
Your article: Role of Wood Products in Carbon Accounting; was absolutely excellent. It was written in language the layman can understand; was reasonable, logical; founded on research and in toto : the best discussion I’ve read of the debt our species owes the small family forester as well as mega forest companies for Gobal Warming issues. thank you…