Conversions Commonly Used When Comparing Timber and Carbon Values

Introduction

You may be wondering why some measurements for forest carbon are based on units used for merchantable or harvested wood. In a delay-harvest scheme, only the timber most likely to be harvested is included in the carbon accounting. This ensures that delaying harvest results in additional carbon storage or a change from the "business as usual" outcome.

If you decide to delay a timber harvest, then you may want to be compensated for forgoing expected timber revenues for a short time. For the choice to be economically efficient you would need to compare the value of the timber "today" with the value of the carbon and the timber "tomorrow". The following conversions will help you keep track of the units describing carbon and timber values.

Conversions for Conventional Forestry Measurements

Foresters traditionally used measurements such as cords or thousand board feet (mbf) to describe the volume of harvested wood. Today foresters are more likely to use English units such as cubic feet (a volume measure) and tons to quantify wood. Tons and cubic feet are also useful for converting to metric units and calculating the amount of carbon in wood.

A typical rule of thumb for converting a cord of softwood logs (e.g., pine species) to tons and pounds (lb.) is as follows:

1 cord = 2.68 tons = 5,360 lb.

A typical rule of thumb for converting a cord of hardwood logs (e.g., oak or maple species) to tons and lb. is as follows:

1 cord = 2.90 tons = 5,800 lb.

Why are there more tons in a cord of hardwoods than softwoods? The reason for this is the specific gravity of wood. Specific gravity is the measure of a wood's density in comparison to water. If wood were the same density as water, the specific gravity would be 1.00.

This is an important consideration since tree species with different specific gravity (along with tree size) will have different amounts of carbon for the same volume. Pines and other softwoods store less above-ground carbon per unit of volume compared to hardwoods like oak, hickory, and beech, which store more carbon per unit of volume. Regardless of the species, the amount of carbon stored will always increase as tree size increases.

Greenwood Weight and Carbon

The term "green" is wood that has been recently cut and therefore has not had an opportunity to dry out. Typically, the weight of wood is reduced by half when dried in a kiln. Kiln drying of wood is simply the process of removing moisture from the wood. This means that if a slash pine tree that weighs 989.2 lb. (wood and bark) was cut down and dried, it would only weigh 437.8 lb.

After estimating a dry weight, most foresters use another convention that assumes that 50% of the wood’s dry weight is solid carbon. This means the 437.8 lb. of dried out slash pine wood is expected to contain approximately 218.9 lb. of carbon. While the 50% rule for drying works very well for softwood species, it can undervalue hardwoods species since the wood is denser.

Converting Carbon to Carbon Dioxide

Forest carbon remains in a somewhat solid form until disturbed by decomposition or combustion. When oxygen is added the carbon transforms into carbon dioxide (CO₂), which acts as a greenhouse gas in the atmosphere. If a forest owner wants credit for carbon storage, it is important to know how much CO₂ is being prevented from entering the atmosphere by delaying harvest.

Carbon has an atomic weight of about 12, and the two oxygens each have an atomic weight of about 16. Adding the weights together (12 + 16 + 16) results in a total atomic weight of 44. In other words, CO₂ has about 3.667 times the atomic weight of carbon alone. So, when making this conversion, 1 unit of carbon equals 3.667 units of CO₂. In the example for a slash pine tree, the storage of 218.9 pounds of carbon equals 802.7 pounds of CO₂ being prevented from entering the atmosphere.

Converting Carbon Dioxide to Carbon Credits

Carbon credits are sold in metric tons of CO₂ emissions avoided from entering the atmosphere. If you are working with lb. (pounds), it must be converted to metric tons in order to calculate prices and conduct trading. Keep in mind that an English ton is slightly less than a metric ton. One metric ton of CO₂ emissions avoided for 100 years is generally equal to one carbon credit.

Innovation in carbon trading schemes have led to some alternative mechanisms for carbon accounting. For example, a harvest deferral credit (HDC) is generated by the forest owner and represents a certain amount of carbon emissions that have been temporarily reduced by delaying harvest for one year. The Verra registry recently determined a one-year HDC provides about 1% of the climate mitigation benefits compared to a verified carbon credit.

Putting it Together

Below are a few examples of how to convert a predicted amount of additional woody biomass into CO₂ emissions avoided can be found in Table 1.  Keep in mind that all conversions need to consider: (1) the transfer of values from English to metric units, (2) whether or not the wood is dry, and (3) the final carbon value represents avoided emissions of CO₂.

In scenarios (a) and (b) the cubic foot volume of wood is slightly different because one scenario includes the space between the logs and the other does not. This doesn't affect the weight, they are both 5,360 lb., but if you intend to calculate weight based on cubic feet, you need to account for the gaps between logs (solid weight).

Since we know the weight of a green pine cord in pounds, we can calculate the number of tons by dividing 5,360 lb. with 2,000 lb. (the weight of one English ton), to arrive at 2.68 tons. Green wood weighs about twice as much as kiln dried wood, so to calculate the dry weight, the green value is then divided by 2 to arrive at 1.34 tons of dry wood.

The English ton (2,000 lb.) is somewhat less than a metric ton (2,205 lb.). To convert from English to metric units, 1.34 tons is multiplied by 0.90 to get 1.20 tons. To estimate carbon, the total weight of the dry wood is divided in half, so that 1.20 tons of dry wood is estimated to have 0.60 tons of carbon. Finally, to determine how much the carbon stored represents avoided CO₂ emissions, the carbon value is multiplied by 3.67 (the atomic weight of CO₂) to get 2.20 tons of CO₂ avoided.

Moving on to scenarios (c) and (d), we understand the weight in one short green ton is always 2,000 lb. The cubic foot volume of wood in these scenarios is slightly different, however, because one contains pine species and the other contains mixed hardwood species. 

The dry weight of the pine is expected to be about 50% of the green weight. However, the dry weight of hardwoods is a little more, with the wood sometimes representing 60% of the green weight and water presenting 40% of the green weight.

To convert from English to metric units, the ton value is multiplied by 0.90 to get 0.45 and 0.54 tons, respectively. The amount of carbon in dry wood is assumed to be half the weight of dry wood, at 0.22 and 0.27 tons, respectively. These values are then multiplied by 3.67 (the atomic weight of CO₂) to get 0.81 and 0.99 tons of CO₂ respectively. Notice that one ton of green hardwood can sometimes equal almost one ton of CO₂ emissions avoided.

Closing Thoughts

• Delaying harvest means a delay in payments for timber. Moreover, payments for forest carbon only occurs after the carbon is sequestered. So, taking part in a carbon program means a real change in expected income. Since money is more valuable today than in the future, it is necessary to use discounting to "bring back" potential revenues from the future and compare them with other options today using the same baseline. Be sure to read FOCCE article "Long-Term Financial Planning for Timber and Carbon".
• The CO₂ emissions avoided represents only part of the value of a carbon credit sold in a carbon market. The price of a carbon credit also depends on guarantees that the offset will be permanent (100+ years) and will not result in leakage (i.e., cause harvests in other regions to increase). Forest owners may find carbon payments tend to be higher for projects with longer contracts.

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

Related FOCCE Articles and Resources 

• Carbon Accounting in Forest Management
• Forest Carbon Pools
• How Much Should I be Paid to Manage Forest Carbon?
• Long-Term Financial Planning for Timber and Carbon

Article Information Sources

• Self, B., Dicke, S., and Parker, R.  2019. Pine Timber Volume-to-Weight Conversions.
• 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.
• Verra Ton commitment letter.
• USDA Forest Service. 2021. FIA DataMart 2.0