Annual Review of Environment and Resources Volume 48, 2023
Visions of planting walls of trees to block the expansion of the desert have long been promoted but never realized. The green wall myth persists today even though it is premised on outdated understandings of desertification. We review the history of the idea of green walls and focus on two sets of contemporary initiatives to assess their outcomes: peri-Saharan programs (Algeria’s Green Dam and Great Green Wall in sub-Saharan Africa) and China’s Three Norths Shelterbelt Program. This review reveals a mixed record of technical success with low rates of the establishment of monocultures of fast-growing trees vulnerable to disease. While there is evidence for reduced wind erosion in some areas, afforestation is also associated with reduced soil moisture and lowering of water tables. Social impacts include increased water scarcity for people and livestock in some cases, and resource enclosures that particularly work against pastoralist livelihoods.
Green walls refer to continuous bands of planted trees stretching across single or multiple countries in dryland regions. As socioecological projects, green walls invoke powerfully attractive images of maintaining life over the assumed sterility of the desert through human ingenuity and effort. A wall of planted trees is envisioned to hold back an expansionary desert from degrading the productivity of the lands behind the wall. Despite the persistent discursive power of this image, its scientific foundation rests on now discredited understandings of desertification. 1 Desertification, scientifically understood as land degradation due to human mismanagement, is less a regional phenomenon across a broad front than a highly localized phenomenon in areas of greater and more persistent human pressures. The power of the green wall vision is illustrated by its persistent use in promotions of a diverse array of dryland afforestation initiatives with different afforestation patterns (e.g., shelterbelts, scattered woodlots and larger afforested blocks, and agroforestry) and goals (ecological rehabilitation, carbon sequestration, and improved climate resilience). In short, green wall rhetoric mobilizes support and empowers certain actors rather than describing actual dryland afforestation practices. 2 This article focuses on the vision, practices, and effects of these programs.
This review was written during a period of global enthusiasm for trees and mobilization for mass afforestation, with the World Economic Forum’s Trillion Trees initiative the most prominent example ( https://www.1t.org/ ) but also a wide range of other afforestation efforts across the globe (3). While the arborocentrism of this moment has a long history (see below), concerns about climate change and an interest in increasing the sequestration of industrial carbon has led to a rapid rise of tree planting initiatives. The world’s drylands could be seen as “empty” and thus important “untapped” landscapes to store carbon. Through a consideration of the empirical record of green wall programs, this article outlines reasons to be cautious. Trees are often not suited for the arid zones where they are planted and, even if they are established, may have negative ecological and social impacts (4). By ignoring the need for ecological and social monitoring of afforestation impacts, green wall enthusiasm has often worked to hide the mixed record of these initiatives.
CVD 