Researchers at University of Copenhagen have developed a method to map the carbon stock of individual trees by collaborating with Rwandan authorities and researchers. Together, they have created a national inventory of tree-level carbon stocks in Rwanda.
Manually mapping the trees of an entire country would be a huge endeavor and excessively costly. Thus, the new method constitutes a breakthrough since no other method would realistically be able to provide the same information at the level of individual trees.
It was paramount that the method could distinguish individual trees as the relationship between the extent of the crown and the total carbon content of a tree is very different depending on the size of the tree. One very large tree will have a much higher carbon content than a group of trees with the same joint crown extent. So, if the group was mistaken for one tree, the carbon content would be significantly overestimated.
The team trained a deep neural network to detect individual trees using a set of 97,500 manually delineated tree crowns, representing the full range of biogeographical conditions across Rwanda. The study also used publicly available aerial and satellite images of Rwanda at 0.25 x 0.25 m resolution. These images were collected from June to August 2008 and 2009 and were provided by the Rwanda Land Management and Use Authority and the University of Rwanda. More than 350 million trees were mapped.
"It is important to take a holistic approach and also include trees which are outside forests," said University of Copenhagen researcher Ankit Kariryaa, noting that 72% of the mapped trees were in farmlands and savannas, and 17% on plantations.
At the same time, the relatively small proportion of trees which are found in natural forests—11% of the total tree count—comprise about 51% of the national carbon stock of Rwanda. This is possible mainly because natural forests have a very high carbon content per tree volume, thanks to very low human disturbance secured through national legislation.
"This suggests that conservation, regeneration, and sustainable management of natural forests is more effective at mitigating climate change than plantation," said University of Copenhagen Ph.D. researcher Maurice Mugabowindekwe.
Once the study results were presented to Rwandan authorities, they immediately asked Mugabowindekwe and team to update the mapping based on newer aerial images acquired in 2019—a project the Danish team is now undertaking.
Additionally, the method has already been tested for a handful of other countries, including Tanzania, Burundi, Uganda, and Kenya.