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Published on 14th May, 2024

In recent years, we have witnessed a significant increase in NbS (Nature-based Solutions) carbon projects, particularly those focusing on reforestation, commonly known in the market as ARR (Afforestation, Reforestation and Regeneration).

These initiatives offer a wide span of possibilities in terms of technical itineraries, ranging from agroforestry, commercial timber plantations, tree planting for ecosystem restoration, to mangrove restoration and assisted natural regeneration. We can combine several itineraries inside the same project, providing flexibility and adaptability to suit various project scopes within different local contexts.

Regardless of the technical itinerary, we must follow procedures to issue carbon credits. But how do we do this? How do we calculate the quantity of carbon stored in each tree throughout time?

The most accurate and precise method for calculating the amount of carbon stored per tree would be to cut the tree and weigh all its roots, trunk, branches and leaves. After that, we would need to do lab analyses to determine the amount of carbon in each portion of the tree, resulting in the total amount of carbon in one tree. Then, we would have to do the same for all the other trees in the project. Seems quite impracticable, right?

This approach is expensive, time-consuming, destructive and impractical, especially when dealing with projects involving thousands of hectares of reforested land. Hence, a critical question arises: How can we accurately quantify how much carbon is stored in a tree without cutting it down?

Fortunately, scientists have made significant progress in this area developing non-destructive methods that yield precise carbon calculations.

Typically, the data collected in the field for carbon calculation are quite simple: **DBH** (diameter at breast height), often measured using a tape measure, **tree height**, commonly determined using a clinometer, and **species identification**, typically performed on-site.

With just these parameters, carbon content in a tree can be calculated through two distinct approaches: either by computing wood volume and subsequently converting it to carbon, or by directly utilizing allometric equations that yield carbon content as an output.

However, not all species have readily available allometric equations. So, in such cases, the first method—calculating wood volume and then converting it to carbon—comes to the rescue as a versatile solution that works across the board.

Now, let’s roll up our sleeves and dive deeper into exploring the step-by-step of this method.

To start the calculation of wood volume, we should apply **allometric equations** from literature tailored for each species. These equations use DBH and height as inputs to determine stem volume. Many allometric equations have been created for different species and under different environments, so finding a representative one is critical at this point. Fortunately, resources like www.globallometree.org compile thousands of allometric equations from literature, making them incredibly useful tools for our task at hand.

This step is straightforward and involves using **wood density**, readily available in literature, to convert stem volume measured in cubic meters to stem biomass in tons of dry matter.

At this stage, we begin to employ factors and ratios developed over time. As we only have the biomass weight from the stem, we utilize what’s known as **biomass expansion factor (BEF)**. This factor accounts for the total aboveground biomass weight that includes also branches and leaves. BEF values vary considerably depending on the species and age, but a conservative estimate, when data is lacking, is 1.1 (as recommended by Gold Standard). Additional values can be found in table 4.5 of IPCC guidelines for further reference.

Because wood contains elements beyond carbon, such as oxygen and hydrogen, we rely on a factor known as **carbon fraction**. The IPCC suggests a standard value of 0.47 for this factor, though it may slightly differ across species. By multiplying the weight of aboveground biomass by the carbon fraction parameter, we obtain the total carbon content within the aboveground biomass.

Now that we have the total aboveground biomass (in tons of C) calculated, our focus shifts to estimating the belowground biomass, which represents the carbon content within the tree’s root system. Since digging up the entire tree to weigh its roots is impractical, we rely on the **root-to-shoot ratio**, derived from various scientific studies. These values, also available in the IPCC documentation, vary depending on factors like vegetation type and tree age. Overall, however, we can consider the ratio to be around 0.20. This means that for every ton of carbon in the tree components that we can see (trunk, branches and leaves), there is approximately 0.20 ton below ground. Once we calculate the belowground biomass, we’ll obtain the total biomass of the tree (in tons of C) as our final outcome.

As the carbon emissions are measured in CO_{2}equivalent, the final step in determining the amount of carbon credits generated by a tree involves using the **C to CO**_{2}** factor**. Given that the molecule of CO_{2} has a molar mass of 44 (with carbon having a molar mass of 12 and oxygen a molar mass of 16, totaling 1 atom of carbon and 2 atoms of oxygen), while the molecule of C has a molar mass of 12, we simply multiply the total biomass in tons of C by 44/12. This gives us the number of tCO_{2}eq that a tree has.

After completing this step-by-step for every tree measured, we obtain the total amount of tCO_{2}eq in a given area (in m^{2}), so we can transform it to tCO_{2}eq/ha.

Voilà! It appears quite manageable, right? This image summarizes everything mentioned above:

As the step-by-step illustrates, carbon calculation in forests involves numerous nuances influenced by various parameters. Therefore, it is paramount to proceed with meticulous care at every stage, from measuring the trees to selecting the correct equations, ratios and factors. By doing so, we uphold the integrity of carbon credits, ensuring they are of high quality, transparent, and accountable.

If you have questions, suggestions or insights, feel free to share them with us in the comments below. Let’s keep the conservation going!

Stay tuned for the next carbon post in this blog!