Afforestation / Reforestation
Description
As trees grow they capture CO2 from the atmosphere through photosynthesis. Trees store the carbon in their biomass – so mainly in the trunk and branches but also in their roots. A single tree can remove around 22kg CO2 from the air per year although this number is very different for different types of trees (source). Planting enough trees to create a forest can therefore remove a substantial amount of CO2 from the atmosphere. Planting trees on previously woodless land is referred to as afforestation whereas reforestation refers to planting trees on land where there used to be trees. There are different types of af- and reforestation beyond simply planting trees on woodless land. Examples include agroforestry, where some trees are planted on pastures or croplands; and restocking which means increasing the amount of trees in an already existing forest.
Reversal
The risk of CO2 reversal in af- and reforestation is quite high. This is because trees only store carbon as long as they are unharmed. Trees and forests are exposed to several risks such as droughts, wildfires, logging, all of which can release the previously sequestered carbon back into the atmosphere. In order for af- and reforestation projects to reliably store carbon for a long time, they have to come with ecological and social safeguards, such as anti-logging policies, that ensure that planted trees and forests remain unharmed and the carbon storage is not reversed.
Readiness and scale
Af- and reforestation is a very mature method of carbon removal and most carbon removal projects that currently exist globally are af- and reforestation projects.
Trees can safely grow in many areas globally, which makes af- and reforestation methods that can be deployed worldwide. However, the amount of land globally available for planting forests is limited. To store large quantities of carbon, a great amount of land is required, this can often compete with other important land-uses such as food production. Experts suggest that the global potential of af- and reforestation is 3.5 Gt of CO2 per year in 2050 (Fuss et al. 2018).
Co-benefits / limits
+ If planned and implemented correctly, af- and reforestation projects can be very beneficial to increase and preserve biodiversity. To achieve this, re- and afforestation projects have to plant a variety of native species and not monocultures and foreign species.
+ Forest regeneration can improve local soil quality, water balance and local air quality.
+ Well implemented projects can offer socio economic benefits to local communities.
– Large areas of land are required to store the carbon. This high land requirement therefore competes with other land uses, especially food production.
– Planting forests makes land darker which decreases the albedo (the earth’s reflectivity of sun). If land that is currently or recently been covered in light material such as snow and ice, is replaced with dark forests, the land would absorb more heat and actually contribute to climate change.
– Ill-managed af- and reforestation projects are actually harmful to the environment. For example, planting non-native species or monocultures (only one or a few species of trees) can harm biodiversity and can decrease the resilience of trees – thereby decreasing the permanence of carbon storage.
– Trees only capture excess CO2 while they are growing. Once a tree and a forest has reached maturity, there is still an exchange of CO2 and O2 between the trees and the atmosphere but it is balanced, so the trees do not function as a carbon sink anymore. This process is known as saturation. This means any single plot of forest is a time limited carbon removal project.
Extra info
There is no universal agreement on how long a land would have to be woodless for planting trees to qualify as reforestation or afforestation. We consider the threshold to be 50 years as suggested by The Economist.
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