The carbon that working forests remove from the atmosphere is stored in multiple ways. It can be stored in tree branches, stems, and roots, as well as in the soil. Once a tree is harvested, the wood products manufactured from it holds that carbon until it is replaced or decays. The trees we plant then grow, renewing the cycle and growing net carbon storage.
We have divided our forest carbon stocks into three pools that allow us to track our carbon inventory and to follow and account for stored carbon when timber is harvested. The three pools are: 1) merchantable portions of trees, 2) above ground non-merchantable portions of merchantable trees and pre-merchantable trees, and 3) below ground portions of all trees (excludes soil carbon).
At the end of 2021, our forests stored a total of 114 million metric tons of CO2e in all three pools. Merchantable above ground portions accounted for 69 million metric tons of CO2e, 26 million metric tons of CO2e were in pre-merchantable above ground portions, and 19 million metric tons were in below ground portions of trees.
The carbon stored in forest soils accounts for approximately 56% of forest carbon stocks and is a major component of the contribution of forests as a natural climate solution. Soil carbon pools can be dynamic over long periods of time, but they do not flux in predictable, reportable ways like above ground carbon does in response to annual tree growth and harvest. The US Forest Service Forest Inventory and Analysis program is continuing to develop methods to accurately measure soil organic carbon (SOC) and include estimates in their reporting. Utilizing the most recent regional estimates of SOC measured to a depth of 100 cm, our Idaho soils are storing ~105 million metric tons CO2e, our Gulf South soils are storing ~108 million metric tons CO2e, and our ownership is storing ~53 million tons CO2e in the understory and dead wood. For our ownership, the combined total tree carbon, soil carbon, and understory and dead wood is 380 million metric tons CO2e and is a result of our lands being maintained in working forest conditions.
Carbon from harvested wood remains in wood and paper products until the end of their use and eventual decay. The rate of decrease in storage is dependent on the specific product end use. Approximately 68% of total tree carbon in a sawlog-sized tree is transported to a sawmill and 55% of that amount is captured in solid wood products, such as lumber and plywood, as a long-lived carbon vault. The total tree carbon that is used for pulp and paper products has a shorter life span. Pulp and paper products have a rate of decay or release that is high initially as products are used, recycled, or disposed of, then the rate slows substantially after initial use because of the portion that is stored in landfills.
When wood-based products are used in place of fossil fuel-intensive products like steel or concrete, there is a permanent benefit to our atmosphere. For example, researchers have found that the CO2 intensity of lumber production is 50% less than steel and 25% less than that of cement. By building with wood, additional carbon is stored in everyday products and buildings. A wood house will store carbon until it decays or is replaced. If the wood house stands for 108 years, the forest will have regrown two to four cycles resulting in compounding carbon storage. When trees are sustainably harvested, wood continues to store carbon in the thousands of products we use every day, from paper products to lumber. Trees then regrow, repeating the cycle.
The continuing cycle of active forest management, including planting, growing, and harvesting, optimizes a forest’s ability to sequester and store carbon and improves resiliency, maintaining the ability to sequester carbon in the future. The life cycle of managed forests and the production of long-lasting wood products have a significant climate benefit, with relatively low emissions associated with the production of lumber. Over multiple cycles of wood products production and forest renewal, total carbon storage increases.
Forest products store carbon for an extended period spanning multiple harvest cycles, and the net impact is a continually increasing amount of carbon stored related to each acre of timberland. The first graph below illustrates this cumulative increase in the storage of carbon in wood and paper products over 108 years (net of the storage loss as products reach the end of their lives). The graph assumes that a fixed set of timberlands are harvested in the base year and then four more times in the South and two more times in Idaho. This reflects our 27-year harvest cycle in the South and our 54-year harvest cycle in Idaho.
Forest products store carbon for an extended period and each year additional carbon from harvested trees is added to forest products storage. The net impact is a continually increasing amount of carbon storage in forest products. The second graph below shows the accumulating amount of carbon storage with the addition of each successive year of harvest from our timberlands. The analysis includes the decay associated with the paper and wood products in use.