Forest Carbon Accounting

This webpage was adapted from a bulletin by the Wisconsin Initiative on Climate Change Impacts Forestry Working Group. You can download and print Bulletin #9: Forest Carbon Accounting (PDF).

How is forest carbon measured?

Estimating the amount of carbon in a forest is a complex and important task. Every forest carbon project must use a specific, approved methodology for measuring forest carbon and calculating carbon credits. Although the majority of forest carbon is stored in the soil, carbon credit programs generally focus on carbon in live biomass (especially the trunks and branches of trees) because the live biomass is easier to measure and change through management practices.

As a rule of thumb, about half of the dry mass of a tree is carbon. On-site measurements of the carbon stored in trees therefore rely on estimates of the trees’ weight. The most common approach is to use an equation based on the trunk diameter at breast height, taking into consideration the species and stand density. After measuring all the trees in several sample plots within a stand, a forester can estimate the total carbon in a stand’s live aboveground biomass.

However, tree-by-tree measurements take a lot of time and money. An alternative approach is to use standardized estimates for different ecosystems, regions, and stand ages such as those provided by the US Forest Service’s Forestry Inventory and Analysis (FIA) program to model how much carbon is in a forest. FIA data come from permanent research plots in every state—Wisconsin has more than 6,000 forested FIA plots, each of which is measured every 7 years.

A third approach is remote sensing, in which satellites, airplanes, or other aerial sensors measure the light reflected by trees and their surroundings. With remote sensing, tree heights, canopy structure, and the general health of trees can all be measured. These metrics are then used to model and estimate forest biomass and carbon. This method enables frequent monitoring of forests to track changes in carbon over time but requires a lot of technical expertise. Remote sensing models do rely on some on-the-ground measurements for calibration and verification.

All these sources of data and more can be used to create models that predict future changes to the carbon stored in forests. Both current-day measurements and model predictions are crucial for carbon markets to function.

Key terms for carbon accounting

The goal of forest carbon projects is to generate carbon credits to be sold in carbon markets. Starting from the
aboveground tree biomass, carbon credits are calculated through a series of equations and modeled assumptions. One carbon credit is defined as one metric ton of carbon dioxide that either was not emitted or was removed from the atmosphere:

• Baseline (avoidance) credits represent an estimate of the carbon that would have been emitted if the forest were aggressively harvested.
• Growth (removal) credits represent the carbon captured through tree growth. Currently, markets have more confidence in growth credits than baseline credits.

Here are some other important terms for measuring forest carbon in the context of carbon markets.

• Baseline: A baseline represents the amount of carbon that a forest would have stored without enrollment in a carbon program. Different projects take different approaches to determining a baseline, and each has strengths and weaknesses.
• Additionality: Additionality refers to the amount of carbon stored as the result of a specific intervention (such as sustainable management practices or a harvest deferral). Since forests naturally grow and store carbon over time, carbon credits can only be claimed for additional storage beyond the baseline. Additionality is demonstrated when management practices go beyond what is legally mandated or locally expected and would present a barrier to the landowner (financial, logistical, etc.) without the assistance of the carbon project.
• Permanence: In order to represent a true carbon credit, the carbon stored needs to remain out of the atmosphere for a long time (ideally, 100+ years). Carbon programs typically require landowners to enter long-term contracts to monitor the permanence of the carbon credits they generate.
• Reversal: A reversal happens if a carbon project has an unexpected loss of carbon stocks or sequesters less carbon than expected. Potential causes of reversals include disasters like wildfire, windfalls, or insect outbreaks; overharvesting of timber; and inaccurate model predictions (for example, if actual tree growth rates are slower than predicted).
• Buffer pool: As carbon credits are generated by a project, some are placed in buffer pools instead of sold. These pooled credits can then be used as a form of insurance to make up for projects that experience unintentional reversals.

• Leakage: Leakage occurs when carbon that should have been sequestered due to a carbon project in one location still “leaks” into the atmosphere somewhere else. For example, one landowner reducing their intensity of timber harvest may result in another landowner increasing the intensity of harvest on their land to meet market demands for lumber. Most improved forest management projects assume that there will be zero leakage, while others assume that some leakage will occur and issue fewer carbon credits to account for leakage. To generate valid carbon credits, a project must not have extra leakage.

If you have questions about forest carbon or other topics related to the changing climate, or if you want to provide feedback on this webpage, contact:

Page last updated March 2025.