Many people wonder if it makes sense for the climate to plant trees. After all, doesn’t a tree that dies release the CO₂ that has been absorbed again? In this article we look at the effects trees have on the climate, both in the short and long term. We focus on climate, and not on the environmental benefits trees provide.

How trees grow

To answer the questions above, we first look at the effect of a single tree on climate. The amount of CO₂ absorbed by a tree is proportional to its growth rate. A small tree (seedling) absorbs very little CO₂ in its first few years. Depending on the tree species, it can take five to fifteen years before the growth spurt begins. This growth then decreases slowly. This resembles an S-curve:

Looking at a forest, we see the same effect. A young forest absorbs a lot of CO₂, then the uptake weakens year by year:

The decrease in CO₂ uptake is slower than for a tree because the leaves that fall from the trees become part of the soil as humus (biomass). Thus, CO₂ is stored not only in the trees, but also in the soil.

What happens when a tree dies?

If a tree is burned, or if it rots completely, the carbon will return to the cycle in the form of CO₂. Yet this does not mean that planting trees is of no benefit. In fact, there are numerous reasons why the net climate impact of trees is positive:

1. In a healthy forest, a felled tree will provide space for seedlings to take their place;
2. Some of the dead tree is retained in the forest as biomass, which is good for biodiversity;
3. Wood can be used as a building material, storing CO₂ and preventing emissions;
4. Saw residues can be used as a source of energy, reducing dependence on oil.

Space for new trees

Trees need light to grow. Because existing trees absorb this light, seedlings cannot grow. Once a tree dies, its place will be taken by a new tree. At the same time, the dead tree becomes a source for biodiversity by providing food and shelter for numerous insects. This stimulates biodiversity and thus the robustness of the forest. A prerequisite for this is that the soil and environment are healthy and robust enough.

Trees as a sustainable building material

Although not obvious, small-scale and responsible tree harvesting in a biodiverse and healthy forest is the most climate-friendly option. This makes wood available as a building material. By felling trees responsibly and on a small scale, the impact on the environment is minimal. In fact, a robust, well-maintained forest regenerates itself. The wood can then be used to make products necessary for society, in which the CO₂ remains stored. As wood is used, the use of, for example, plastic (and thus our dependence on oil) can be reduced. Moreover, because old trees absorb less and less CO₂, the average CO₂ uptake per hectare can be kept higher than when a forest remains untouched.

A University of California study looked at the CO₂ effect of responsible logging in privately managed forests. De conclusie was dat deze bossen 30% meer CO₂ opnemen dan bossen die onaangeroerd bleven:

Large-scale and irresponsible tree felling has a major effect on the environment. Think of fields of the same trees and acres of forest being cut down simultaneously with large machines. To prevent such practices, it is important for our economy to switch to climate responsible business practices. In this way, we bring the need for natural resources down and in line with the regenerative capacity of healthy forests.

Is planting trees always positive?

No. Many tree planting projects focus only on absorbing as much and as cheaply as possible CO₂ in as short a time as possible. This often results in a monoculture, where lots of the same kind of trees are planted. These trees absorb a lot of CO₂ on paper, and the lack of diversity results in low costs. It is also common that the planted trees are not suitable for the climate or do not belong in the environment (invasive species), or need more water than the environment can provide. The result of these choices is a weak forest that is susceptible to disease and often dies en masse within the first year. Care must also be taken to ensure that special ecosystems are not lost and that local people benefit from planting the trees (to avoid illegal tree cutting).

How it can be done

A great example of a good tree planting organization is Trees for the Future. Their premise is that the tree is not an end in itself, but a means for the farmers they work with to live healthier lives and generate more income. As a result, the trees are planted voluntarily and properly maintained. This involves looking at what type of trees fit the local climate. Depending on the purpose of the tree, the type is chosen. For example, trees are planted as a natural wooded wall on the outside of the plot, providing shade and prickly enough to serve as a natural separation. Trees planted between crops are species that use their roots to improve the quality of the soil (for example, by fixing nitrogen). There are also fruit trees planted or companion planting, where the presence of one species provides protection for another. The result is a robust and resilient Forest Garden.

Conclusion

Does it make sense to plant trees for the climate? Yes, trees reduce CO₂ in the air in various ways. However, it is important to pay close attention to how the project used to plant the trees works. In this way, environmental problems and massive tree mortality can be prevented. The trees should be planted together with the people who live there, and they should benefit from the trees staying. Finally, thought must be given to how to create a robust and biodiverse ecosystem in the long term, and how to deal with the wood from trees that die or are cut down. It is therefore important to choose the right tree planting projects.