CO₂ Storage Capacity

How do we know how many trees we need to plant to sequester one tonne of CO₂?

We plant 5 trees to sequester one tonne of CO₂ in the long term.

Trees absorb CO₂ from the atmosphere during their growth through photosynthesis, storing it in the form of carbon (C) in their cells. The amount of CO₂ that a single tree can sequester in 80 years has been determined using studies and current data, and we arrive at 1.76 tonnes of CO₂ per tree over 80 years.

At ForTomorrow, we plant 4 000 trees per hectare. After 80 years, approximately 520 trees of various ages remain, collectively storing 822 tCO₂. To calculate how many trees we need to plant to sequester 1 tonne of CO₂, we do the following: We start with the original 4 000 trees planted. We divide this by the 822 tCO₂ stored by the remaining trees after 80 years. This leads us to approximately 5 trees that we need to plant to sequester 1 tonne of CO₂.

What is the basis for our CO₂ calculations?

We want to carry out the afforestations made possible through your donations with the best possible quality and according to current scientific standards. To achieve this, we use our [planting code]/en/planting-code), which sets a qualitative baseline for the plantings themselves and explains what kind of projects we want to support and which we exclude. We have also enriched and further improved our calculation method for the storage capacity of trees with additional data sources.

For this purpose, we use data from the 2017 greenhouse gas inventory and analyze them independently. In addition, we refer to the data of the study on carbon sequestration of young afforested (German only) areas by Paul et al., in which again many different, partly international data sources are used.

How do trees store CO₂ anyway?

Trees do not actually store CO₂, but carbon (C). Trees absorb carbon dioxide (CO₂) from the air through their leaves to produce sugars during photosynthesis, which they use as food. The carbon is stored in the tree in the form of biomass, while the oxygen is released into the air. Biomass is wood, branches, roots, leaves, bark, etc. The absorbed CO₂ is broken down by the tree into its individual parts and thus rendered harmless to the atmosphere.

The amount of CO₂ that a tree can absorb depends primarily on its leaf mass and also on the age of the tree. A 20- to 40-year-old tree filters the most CO₂ (German only) from the atmosphere. However, the total amount of absorbed CO₂ increases with the age of the tree, as the biomass of a tree also increases over the years. This means that while productivity in terms of CO₂ storage is highest in trees when they are young, the total amount of stored carbon is higher in old, large trees because a tree can accumulate more biomass over the years.

Here’s a graph that illustrates this relationship of carbon storage:

CO₂ storage of trees by age

How to calculate the amount of CO₂ stored in a tree?

To calculate the amount of CO₂ that a tree has stored over the years, you first need to calculate the carbon content. This requires knowing the volume of biomass, the wood density of the respective tree species, and the carbon content of trees. The volume can be calculated from the circumference and height of a tree. However, the result only reflects the content of the wood trunk and not the entire biomass of a tree, including its roots and canopy. To account for this, we use a so-called expansion factor (1.4). This factor includes the roots and canopy in the calculation (“Handbuch für Waldökologen” by Burschel et al. (2017)). The wood density (in dry condition, also called kiln density) of trees varies depending on the tree species. On average, one can assume about 0.53 g/cm³ or 530kg/m³. The carbon content, on the other hand, is almost the same between the different tree species at 50 % (in the dry state).

This leads to the following equation:C = Volume × Expansion Factor × Wood Density × Carbon ContentNow that we have the carbon value of the tree, we need to multiply this value by 3.66 (german). This is the conversion formula from carbon to CO₂, based on the molar mass of the molecules.mCO₂ = mC × 3.66This way, we have calculated how much CO₂ the tree has removed from the atmosphere.

How we use the formula in a concrete example:

C = Volume × Expansion Factor × Wood Density × Carbon Content

Example values: A tree at the age of 80 years has a volume of 0.89 m³. The expansion factor is 1.4. The wood density is 0.53 g/cm³ or 530 kg/m³. The carbon content is approximately 50%, or as a factor, 0.5.

The calculation for the carbon content looks like this: C = 0.89 × 1.4 × 530 × 0.5 = 330.19

So, the amount of carbon in this example tree is 330.19 kg.

Next, we proceed to calculate the CO₂ quantity: mCO₂ = mC × 3.66 CO₂ = 330.19 × 3.66 = 1 208,5 kg CO₂

In total, this tree would have stored 1 208,5 kg CO₂ or 1.2 tCO₂ from the atmosphere in its biomass. Using this calculation method and the data from the 2017 greenhouse gas inventory, we have calculated the amount of CO₂ that an average tree at the age of 80 has removed from the atmosphere, namely 1.76 tCO₂.

How many trees survive after 80 years if we plant 4 000 trees?

On the areas we finance, we usually plant 4 000 trees per hectare. We arrive at this figure after consultation with many partners in the forestry sector, and it roughly corresponds to the average number of trees planted per hectare across different tree species. It’s possible to plant more than 4 000 trees, but this increases competition among the trees for light, water, and nutrients. Often, the goal in doing so is to encourage straight growth in the trees, with the aim of maximizing profits when selling the wood.

In the years and decades to come, however, many trees will die and only a certain proportion will survive. This is normal, natural selection. If we look at how many trees are still standing on a hectare after 80 years, there are about 520 trees of different ages - of which the majority, about 400 trees, are 80 years old. The other 120 trees have naturally regenerated from the seed of the original trees or from the seed of surrounding trees.

The initial 4 000 trees we plant already establish a thriving forest and provide not only protection but also enough light, water, and nutrients for additional young trees to naturally establish themselves.

How many trees do we plant for one tonne of CO₂?

So on the area where we initially planted 4000 trees, after 80 years, on average there will be about 520 trees of various age groups remaining. Together, these trees will have removed approximately 822 tCO₂ from the atmosphere. The majority, with just over 700 tCO₂, is stored by approximately 400 eighty-year-old trees (an eighty-year-old tree ≈ 1.76 tCO₂).

Additionally, there are about 95 sixty-year-old trees that store just over 100 tCO₂, as well as some younger trees that together store nearly 12 tCO₂. Naturally, there are no strict age classifications in a forest. We always work with average numbers.

In so-called permanent forests, which we aim to establish in the long term, ideally all age classes are present. However, to keep the calculation from becoming too complicated, we use average values and the age groups mentioned.

Now, if we take the number of initially planted trees (4000 trees) and divide it by the amount of CO₂ sequestered after 80 years (822 tCO₂), we arrive at approximately 4.9 trees per tonne of CO₂. Rounded, this is 5 trees to offset one tonne of CO₂.

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