Great Green Walls: Hype, Myth, and Science

Abstract

Visions of planting walls of trees to block the expansion of the desert have long been promoted but never fully 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 The green wall myth persists today even though it is premised on outdated understandings of desertification.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 tree establishment, particularly in drier areas and 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. Expected final online publication date for the Annual Review of Environment and Resources, Volume 48 is October 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Restoring deforested drylands for a wetter future – harnessing trees for credits, climate and water

“…Thus, many reforestation activities focus exclusively on carbon capture; trees are considered to mitigate climate change solely by removing CO 2 from the atmosphere (Pan et al, 2011;Davis, 2016;Mo et al, 2023). This selective focus, the conflicting reforestation methods associated with maximizing CO 2 capture and what is required for improving hydrological functioning, and the fact that many reforestation projects are in drylands have resulted in many failed projects where carbon capture targets are not met, landscapes are further desiccated, and biodiversity and livelihoods are impacted (Hopkin, 2005;Andersson et al, 2011;Holl and Brancalion, 2020;Vetter, 2020;Rohatyn et al, 2022;Turner et al, 2023).…”

Rangeland afforestation is not a natural climate solution

“…For example, rangelands may be considered as unproductive wastelands and as such are ideal candidates for afforestation to achieve climate-change mitigation goals. Implementation of afforestation initiatives can be influenced by the political goals of governments, which may result in the resettlement of local populations, land privatization, and transformation of livelihoods (Elkin 2022;Turner et al 2023). Even when not overtly coercive, afforestation initiatives are often inconsistent with the interests and livelihood requirements of local peoples and create adverse ecological and socioeconomic impacts (Panel 3; Figure 2; Davis and Robbins 2018;Malkamäki et al 2018).…”

“…Afforestation initiatives are diverse, including stabilization of sandy soils in Senegal, rehabilitation of hardpan areas in Burkina Faso and Niger (Figure 2), and improved management of coppiced trees and shelterbelts on cropland in Nigeria. However, afforestation programs have experienced low rates of tree survival and irrigation is often necessary to enhance tree establishment, which diverts water from human and livestock needs (Turner et al 2023). In the Ferlo region of Senegal, a sandy area with a deep water table, increased water demand for GGWI nurseries has forced families to reduce their water use and delayed livestock access to water.…”

“…In contrast, the adverse outcomes can be immediate, including water scarcity and reduced access to grazing land, especially on communal and open-access rangelands that do not possess well-defined property rights. Marginalized populations that are highly dependent on natural resources, such as pastoralists and subsistence farmers, are most negatively affected by afforestation (Yeh 2009;Turner et al 2023). Consequently, rather than producing a win-win outcome, afforestation may increase the vulnerability of rural populations to climate change (Panel 3).…”

A paleoecological context for forest distribution and restoration in Grootbos Nature Reserve, Agulhas Plain, South Africa

“…In Africa, the proposed targeted landscapes are primarily centered around the semi-arid drylands of the Sahel (The Great Green Wall: Turner et al, 2023) and the grasslands and savanna grasslands of sub-Saharan Africa (The African Forest Landscape Restoration Initiative, 2023) . These landscapes are often perceived to be degraded or to have become deforested over time (Zaloumis and Bond, 2016;Bond et al, 2019;Turner et al, 2023). However, many landscapes with lower tree cover are not always linked to landscape degradation.…”