Planting trees has been a practice spanning thousands of years, driven by various motivations. Trees hold economic value, growing into wood for cutting or burning. They offer shade, act as windbreaks, and, undeniably, contribute to the beauty of landscapes.
In the 1970s, amid rising concerns about carbon and climate change, scientists discovered yet another incentive for tree planting: more trees equate to less carbon in the air. This is because, like all plants, trees absorb carbon dioxide from the atmosphere through photosynthesis, converting it into carbohydrates. Much of this carbon becomes stored within the tree, often constituting more than half of its dry weight.
A 2019 study published in the journal Science aimed to assess the available space worldwide for new trees. The findings captured global attention: over 2 billion acres, surpassing the landmass of the continental U.S. This extensive area could absorb more than two-thirds of the carbon emitted from all human activities since the Industrial Revolution. The study boldly declared reforestation as "our most effective climate solution to date." This assertion garnered the interest of a Silicon Valley entrepreneur who conceptualized the platform launched at Davos.
Other scientists swiftly criticized the Science paper. Some raised concerns about the estimates and calculations presented in the paper, while others emphasized that successful reforestation efforts need to address both social and technical challenges to be truly effective.
The world is like a complex patchwork of land ownership, making it challenging for trees to be planted just anywhere. Even in seemingly suitable areas, tree planting can bring about adverse impacts on nearby communities. For instance, a reforestation initiative in China ended up absorbing a significant amount of water, leading to a reduction in water availability for local communities. Moreover, since trees were planted in former agricultural lands, it resulted in the clearance of natural forests to make way for new crops.
Trees in inappropriate locations can lead to a decrease in biodiversity. For instance, if they replace undisturbed grasslands or savannas, it can have negative effects. In such cases, it would be considered afforestation, where trees are introduced to areas that were treeless before. Reforestation, on the other hand, involves replacing trees in areas where they were removed. Both afforestation and reforestation are part of the One Trillion Trees Initiative. However, some reforestation projects result in the creation of tree plantations—large areas dominated by a single species of tree for food or timber production. While economically productive, these plantations do not store as much carbon as natural forests and fail to provide extensive habitat.
Everyone agrees that trees are beneficial, but it's crucial to plant them in appropriate locations and manner. Planting trees in tropical forests near the Equator is a priority because their rapid growth allows them to absorb carbon more efficiently than in other parts of the world. On the contrary, snowy regions are not ideal for tree planting as trees can obstruct the reflectivity of snow, preventing it from bouncing sunlight away from the earth and contributing to the warming effect.
The United Nations has announced a "Decade on Ecosystem Restoration" for the next ten years, with support from the One Trillion Trees Initiative. The UN's guidelines for reforestation emphasize planting in areas where forests have thrived before, specifically avoiding historical "grasslands, peatlands, or wetlands." The guidelines also promote the use of native trees, as planting non-native species can potentially lead to increased carbon release from forest soils.
Scientists have recently raised concerns about the potential risks of relying on forests to store carbon, given the increased likelihood of wildfires and pest infestations due to climate change. Successful reforestation will require effective communication between scientists and policymakers, along with stringent protection measures for the planted forests.
Even the scientists who authored the 2019 Science paper, promoting the idea of planting a trillion trees, acknowledge that tree-planting has its limitations. The full benefits of such massive reforestation efforts may not be realized for a hundred years, according to Tom Crowther, one of the authors. Crowther emphasized to the New York Times that if tree planting is used as an excuse to avoid reducing greenhouse gas emissions or to weaken environmental protection, it could lead to significant problems.
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