Trees will be a key component to fighting climate change this century. They are a natural ally in absorbing carbon and storing it. To be effective, however, we need to get a better understanding of how many trees there are on Earth and plan to strategically plant trees to increase carbon absorption.
Using Satellite Imagery to Map Trees in West Africa
One area that challenges this is West Africa. It is a diverse region that has very dry to wet, even tropical environments.
A challenge has been mapping trees in very dry areas where large spaces exist that have minimal trees and even sand. Usually dry regions appear as very white or monotone in color as satellites map these regions. More recent higher resolution satellite imagery, provided by commercial companies such as Digital Globe, have enabled much higher resolution to be widely available.
Using 50,000 images available in areas covering West Africa, a research team from the University of Copenhagen, University of Bremen, and NASA’s Goddard Space Center was able to show that even dry deserts or semi-arid regions are important regions for tree cover and could even aid in the fight against climate change.
In their paper, they were able to show that there is about 0.7 trees per hectare in the driest regions, 9.9 trees per hectare in relatively arid regions, about 30 trees per hectare in semi-arid areas, and 47 in the sub-humid regions. In other words, it is not just tropical rainforests but also semi-arid and sub-tropical regions that are important areas for trees.
The team manually marked 90,000 trees in training imagery and then created a deep neural network model, applying a convolutional neural network model, to then estimate the number of trees. About 1.8 billion trees were found, indicating the importance of the region for the total number of trees.
Globally, it is estimated there are over 3 trillion trees, or roughly 422 trees per person. Not long ago, it was shown that about 4.4 billion hectares of tree canopy could potential exist in the world and about 0.9 billion hectares can be added for increasing tree cover.
What the West Africa paper shows is that the picture is not only potentially more complex but tree cover can be added even in lands that might be deemed less productive. While a lot of attention has focused on tropical regions, particularly tree loss there, we also need to focus on semi-arid and sub-tropical regions that also can hold a surprisingly high number of trees and can play a crucial role in climate change because these less wet regions are more prevalent.
Agroforestry Mitigates Deforestation
One issue that arises affecting tree coverage is land use and commercial agricultural operations, such as crop farming and pastoralism, which could destroy forested lands. In a study using various satellite imagery for West Africa, it was found, however, that agroforestry, where trees are planted around or within agricultural regions, could be a way to solve the problem of losing trees to land use change.
Instead of removing large tracts of forested ground, trees can be planted along with agriculture. It was found that currently in West Africa there has been about a 17% reduction in forested regions since the 1990s. However, tree coverage in agricultural regions increased by about 2.4 million hectares, or about by 5% since 1992.
This means agricultural regions have contributed more to carbon capture since the 1990s. It also shows that many regions could be reoriented to apply agroforestry than currently practiced, which would help the globe’s reforesting goals while also helping West Africa retain its trees.
What is clear is that tree cover needs to be both preserved and more trees are likely to be needed in many regions to reach climate goals and mitigate climate change. We are now improving in how we can measure the number of trees, even in regions where trees are more sparse but still play a very important role in protecting against erosion and mitigating climate change through carbon capture.
A lot of attention has focused on tropical and wet regions, but protection of trees in semi-arid and even arid regions will be needed. Recent work in West Africa shows we can better measure trees in this region as well as derive ways to protect them better.
 For more on how deep learning methods were used to better count the number of trees in West Africa, see: Brandt, Martin, Compton J. Tucker, Ankit Kariryaa, Kjeld Rasmussen, Christin Abel, Jennifer Small, Jerome Chave, et al. 2020. “An Unexpectedly Large Count of Trees in the West African Sahara and Sahel.” Nature 587 (7832): 78–82. https://doi.org/10.1038/s41586-020-2824-5.
 For more on tree cover around the world and its potential, see; Bastin, Jean-Francois, Yelena Finegold, Claude Garcia, Danilo Mollicone, Marcelo Rezende, Devin Routh, Constantin M. Zohner, and Thomas W. Crowther. 2019. “The Global Tree Restoration Potential.” Science 365 (6448): 76–79. https://doi.org/10.1126/science.aax0848.
 For more on agroforestry in West Africa and what can be done to improve the situation for tree cover, see: Tschora, Héloïse, and Francesco Cherubini. 2020. “Co-Benefits and Trade-Offs of Agroforestry for Climate Change Mitigation and Other Sustainability Goals in West Africa.” Global Ecology and Conservation 22 (June): e00919. https://doi.org/10.1016/j.gecco.2020.e00919.