Carbon Accounting in Forest Management

Forest owners who want to be paid for sequestering carbon need to understand which carbon qualifies as additional and permanent.

Measuring Carbon Over Time

How much carbon is in living woody biomass varies over space and time, depending on tree species, stand density (e.g., number of trees per acre), silvicultural management (e.g., fertilization), and tree age. The USDA Forest Service has an open-access database called the Forest Inventory Analysis (FIA). This database can be used to estimate the average amount of carbon stored in a typical acre in the United States, based on tree species and location. The "carbon lookup tables" offered by the US Forest Service were created using FIA data and describe forest carbon values based on region, forest type, and stand age.

A planted pine stand provides a good illustration of how carbon storage changes over time. FIA data were used to develop Figure 1, which shows the amount of carbon sequestered over time for a planted slash pine stand in Mississippi. The concepts presented here can also be applied to an uneven-aged or mixed hardwood forest as well.

In this figure, given a relative scale, the grey line for "above ground living" biomass increases over time compared to the thin black line for "below ground living" biomass which remains flat. This means each year the amount of carbon stored per acre tends to increase meaningfully above ground, but not much below ground.

After the trees are first established, the rate of carbon sequestration increases, but then it eventually starts to level off as the trees mature. You can see this between years 5 to 15, where the grey line is slightly steeper in slope compared to years 25 to 45. These changes are often referred to as the rate of carbon sequestration, or change in carbon (pounds or tons) over time (year). Rates can change depending on the age of the stand.

In most cases, only the above-ground living biomass is considered in a carbon offset scheme, because living biomass can be easily managed to provide additional storage. Managing carbon in the soils and below-ground living biomass, however, is also an important part of climate-smart forestry.

Setting the Baseline

Setting a baseline is critical for determining when carbon storage is "business-as-usual" and when it is "additional" and can be sold in a market. Reforestation, or planting trees, is a common strategy for gaining additional carbon sequestration. The baseline amount of carbon in a reforestation project is zero, because there were no trees to begin with. It is assumed that the forest was unlikely to occur unless planted, and after planting, the land would remain forested into the future. If slash pine trees were planted, the total amount of carbon would approach 100,000 pounds per acre at year 45 (Figure 1). The carbon stored in a planted forest is typically not accounted for during the first five years, due to higher rates of tree mortality and measurement error.

Delaying harvest is another common strategy and is often referred to as an Improved Forest Management (IFM) practice. The baseline in a delay harvest scheme is the average amount of live woody biomass per acre just before harvest. If the starting point in Figure 1 was set at 25 years (when southern pine stands are financially mature for harvest), then the baseline amount of carbon would be 80,000 pounds of total carbon. Delaying harvest another 5 years is predicted to provide an additional 8,000 pounds of carbon stored beyond the baseline amount. Keep in mind that when a stand has been cut (business-as-usual), another forest may start to grow after harvest. Any carbon sequestered after a harvest would also have to be accounted for when estimating how much additional carbon was stored by changing management activities.

There are several key challenges to determining an appropriate baseline for offset projects that include multiple properties of land.

• If the average amount of living woody biomass across all the properties is used as the baseline, then some properties may already exceed or be below average at the time of enrollment. This can be problematic when trying to determine which properties will provide more benefits and what is fair compensation.
• There will always be some proportion of properties that do not fit the business-as-usual assumptions about harvesting. In other words, some owners' original intentions about harvesting can change due to circumstances. Carbon accounting procedures tend to be more accurate when baselines are more representative of these kinds of changing conditions.
• In cases where the period of contract is shorter than the window of optimal harvest, payments for delaying harvest may not cover the whole period of "harvest risk". Longer contracts can offer more assurances that a harvest was truly delayed. 

Since working with multiple properties can be complicated, how much error is acceptable when measuring the baseline is of continuous debate.

Permanence

During respiration, up to half of the carbon taken in by a forest is stored to create new woody biomass. Wood that is buried underground, where there is little oxygen, can hold onto carbon for centuries. Increased use of fossil fuels has allowed the carbon from ancient forests to reenter the atmosphere, enhancing the greenhouse effect. A true carbon offset should provide the same benefits as never extracting fossil fuels from the ground in the first place. This is why the concept of permanent storage or "permanence" is fundamental to the role of a carbon offset.

Permanence in a forest carbon offset project has been defined by the California Air Resources Board as carbon emissions avoided for at least 100 years. However, climate models show that even when a carbon dioxide molecule has been removed from the atmosphere, a portion of the effects on temperature can still linger for thousands of years. This means that the impacts of fossil fuel emissions can never be fully erased, only mitigated.

The challenge with using living forests as a carbon offset is that the carbon is temporarily locked up. Most trees have life spans of less than 100 years, either because the species is short-lived or due to forest management. Carbon is gained and lost on a regular cycle, as trees grow, die, decompose, and grow back again. Because of this, the carbon accounting in living biomass is really the quantification of total carbon "gains versus losses" within the project area over time. Forest management activities help carbon gains persist into the future by maintaining a certain amount of woody biomass above a prescribed baseline. Unfortunately, forests are also vulnerable to unplanned disturbance (e.g., wildfire), which can reverse the carbon storage benefits. This is why forests are sometimes considered a leaky reservoir for carbon. Moving forest carbon into certain kinds of wood products can help slow down the leak by extending the carbon storage time.

Harvesting and Carbon Accounting

Working forests are an important source of sustainable wood products, which means that they are occasionally harvested. Currently, there is more wood grown than harvested in the US (i.e., tons of living woody biomass). Between 1990 and 2016, up to 15% of US greenhouse gas emissions were offset due to voluntary and unintentional slowdowns in harvesting on public and private lands. When harvesting does occur, the impact on forest carbon often depends on the type of harvesting that is being conducted.

Clear-cutting a forest resets carbon storage in the living above-ground biomass to almost zero. Vegetation and woody debris may still have carbon, but this is typically not included in some carbon offset projects. Clear-cutting is an uncommon management practice on family forest lands in the US except when the forest is a planted stand, the species being managed (e.g., white pine) benefits from clear-cut treatments, or the owners want to clear the land for other uses. Forest lands that are cleared and converted to other uses typically become a permanent carbon source. Forests that are cleared, but new trees grow back, are considered a temporary carbon source. That land eventually becomes a carbon sink again when the new forest exceeds the age of the original forest.

A study published in the Journal of Forestry shows that wood products, such as biofuels, can help working forests restore important climate benefits at a faster rate after harvesting. In this example, the climate benefits provided by biofuels are included in the carbon accounting. Figure 2 shows an increase in forest carbon stocks over time (panel A), but carbon stocks decrease temporarily when the stand is harvested (Panel B). If the wood from the harvest is used to make biofuel or wood pellets, a portion of greenhouse gas (i.e., GHG) emissions can be offset by displacing the use of fossil fuels (Panel C). The benefits of displacing the use of fossil fuels allow the carbon debt from the harvest to be repaid sooner (Panel D).

Selection harvesting methods, or partial cuttings, are generally more common in natural forests on family forest lands. This is when only certain trees are harvested. Sometimes the trees are selected for their commercial value, and sometimes to help improve stand health and wildlife habitat. The harvested trees are eventually replaced through planting or natural tree regeneration. If done correctly, species composition and tree age classes remain relatively the same, and the volume of carbon remains relatively stable over time. Reducing or delaying a selection harvest allows the average age of the forest to increase, thereby increasing the total amount of live woody biomass and carbon stored.

Annual Rates and Carbon Accounting

In a carbon offset project, total carbon gains and losses within a designated area are quantified in order to generate an offset. However, calculating an average rate of carbon gains for different types of forests can be useful for planning and making predictions about the value of a project. The average annual rate of carbon sequestration for the slash pine forest in Figure 1 can be calculated by dividing the total amount of carbon stored by total number of years. This is illustrated in the following equation,

100,000 pounds carbon per acre ÷ 45 years = 2,222 pounds of carbon per acre per year.

In other words, a carbon project that includes slash pine forests will sequester an average of 2,222 pounds of carbon per acre per year.

The economic value of additional carbon storage is based on how much greenhouse gas emissions are avoided by preventing carbon from entering the atmosphere. When forest carbon is released through decomposition, it binds with oxygen molecules to form carbon dioxide (CO₂), a common greenhouse gas. Carbon dioxide has about 3.6667 times the atomic weight of carbon alone. So, when converting forest carbon into atmospheric CO₂, use the following equation,

2,222 pounds of forest carbon × 3.6667 = 8,147 pounds of CO₂ emissions avoided

A carbon credit sold in a market is typically traded using metric tons. Converting pounds of emissions avoided to metric tons can be done using the following equation,

8,147 pounds of CO₂ × 0.000453592 = 3.69 metric tons of CO₂ emissions avoided

In this final equation, it is determined that on average, one acre of slash pine forest offsets about 3.69 metric tons of CO₂ emissions per year. In reality, there can be a lot of variation from site to site due to differences in stand age, site quality, silvicultural management, and planting density. To estimate total carbon gains and losses at the site level, one would first need to determine the total amount of woody biomass per acre as the baseline, and then track changes in the amount of woody biomass resulting from changes in forest management.

Closing Thoughts

• The total value of a forest is greater than its carbon storage benefits. For example, trees located near surface waters help protect stream health by controlling water temperatures and streamside erosion. Forests in urban areas help provide recreational benefits and provide corridors for wildlife to pass through. Working forests provide wood products that help displace the use of other, more fossil-fuel-intensive products (e.g., plastics, concrete). The sustainable management of forests under climate change requires that all the values associated with that particular forest also be taken into consideration.
• The slash pine illustration in this article is an example of a tree species that is highly efficient at storing carbon because it is a densely planted, fast-growing species. In reality, most forests in the US are not as efficient and offset about 1 to 2 tons of CO₂ emissions per acre per year. This is because over the landscape, forests differ in tree species, age classes, stand density, and silvicultural management. You can learn more about the average rate of carbon sequestration in different tree species and in your state by checking out the links below.

This article was produced by the Forest Owner Carbon and Climate Education (FOCCE) program. What do you think? Please take this short survey.

If you have any questions or are interested in collaborating with FOCCE, please reach out to Melissa Kreye at mxk1244@psu.edu.

Related FOCCE Articles and Resources

• How to Manage Forests for Carbon
• Conversions Commonly Used When Comparing Timber and Carbon Values
• The Economic Value of Private Forests and Climate Change Mitigation
• Carbon calculator for Loblolly Pines and Guidebook.
• Quick Estimates of Carbon in Loblolly Pine Plantations Using Carbon-Basal Area Ratios

Article Information Sources

• Lenhart, J.D., T.L. Hackett, C.J. Laman, T.J. Wiswell, and J.A. Blackard.  1987. Tree content and taper functions for loblolly and slash pine trees planted on non-old fields in East Texas. South. J. Appl. For. 11:  147-151.
• Norris Foundation. 2019. Timber Mart-South quarterly price data. University of Georgia, Athens. Data was retrieved March 25, 2022.
• Ter-Mikaelian, M. T., Colombo, S. J., & Chen, J. (2015). The burning question: Does forest bioenergy reduce carbon emissions? A review of common misconceptions about forest carbon accounting. Journal of Forestry, 113(1), 57-68.
• US Forest Service. 2021. FIA DataMart 1.9.0; last accessed May 25, 2021.
• US Forest Service: Carbon lookup tables in Standard Estimates of Forest Ecosystem Carbon for Forest Types of the United States.
• Woodall, C. W., Coulston, J. W., Domke, G. M., Walters, B. F., Wear, D. N., Smith, J. E., ... & Wilson, B. T. T. (2015). The US forest carbon accounting framework: stocks and stock change, 1990-2016. Gen. Tech. Rep. NRS-154. Newtown Square, PA: US Department of Agriculture, Forest Service, Northern Research Station. 49 p., 154, 1-49.