353 - The Great Green Wall: how to check the advancing desert
The desert has been nibbling away at the Sahel’s fertile land for several decades. To halt this advancing desertification, 11 African countries( 1) have come together to build a “Great Green Wall”. This immense project, initiated in 2005, aims to replant with trees a 15 km wide strip stretching 7000 km across the Africa, from Dakar to Djibouti.
A community of international specialists, including IRD researchers, has been mobilized to ensure that this wall of greenery will be as effective as possible. Their objectives are to choose the revegetation techniques and the best adapted species. These experts( 2) are focusing on a natural process which functions in most plant species: symbiosis between the plant and a fungus. Favouring this process would improve plant growth in degraded soils and increase their drought resistance. A tree with remarkable properties( 3) is one of the various recommended species that will be used: the filao tree, which can fix nitrogen from the air and hence colonize impoverished land.
It remains to be determined how the project can be integrated into an environment which is already used and how to enable the local communities to benefit from it.
In the Sahel, climate change combined with demographic-pressure driven intensive agricultural exploitation, are causing the forested areas to retreat. This forest erosion is leading to almost unrelenting soil degradation and desertification. Two-thirds of Africa have now become desert or are severely degraded.
Stopping the advancing desert
In reaction to this menace, 2005 saw 11 African countries( 1) initiate a continental-scale project, steered by the African Union: the Great Green Wall. Like a line of resistance to counter the desert’s advance, this 15 km wide wall of greenery will be planted, from Dakar to Djibouti, a distance of 7000 km.
A committee of specialists on trees and arid environments, including IRD scientists, has been engaged to implement the scheme. They are to determine the most appropriate techniques and choose the best adapted species for the Sahelian situation (drought resistance, ability to develop in nutrient-deficient soils, fertility restoration, etc.) in order to optimize the effectiveness of reforestation operations.
Fungi, ideal allies
To win back the degraded environments, the IRD scientists and their partners( 2) recommend that use should be made of a natural process, established for 400 million years and now functioning in 80% of plant species. This is mycorrhizal symbiosis, an association between a plant and a fungus,. The fungus plays a vital role for the host plant’s water supply and mineral nutrition. It takes up practically immobile nutrient elements, mainly phosphorus, from the soil and transports them to the plant.
Two biotechnological procedures can be envisaged: the massive introduction of a highly effective strain of fungus (known as “controlled mycorrhization”) or the use of nurse plants, which increase the mycorrhizal potential of the soil. The research team’s studies in Senegal and Morocco have shown that these practices improve plant growth in nutrient-deficient soils and in arid environments. They brought decreased plant mortality, significantly higher growth in height, better root development, optimized use of water resources and therefore better resistance to hydrological stress. Currently there are very few studies on these biotechnologies in real situations, particularly in the arid and semi-arid regions.
The filao tree well adapted
The other objective is that the plants selected for the Great Green Wall, whether local or imported, must prove to have a strong ability to adapt to the Sahelian environment and possess the ecological properties sufficient for the fight against desertification. Among the species that will be used, the experts( 3) recommend the tropical Casuarina , better known a the filao tree. In the course of its evolution this species has acquired remarkable qualities which allow it to alleviate nutritive deficiencies in degraded soils and regenerate vegetation. It is a pioneer tree, in other words able to colonize soils very poor in nutrient elements. It owes this exceptional property to a symbiotic relationship with a bacterium, called Frankia , present in the soil. This-provides the tree with the special ability to fix nitrogen from the air when the soil is deficent in this essential element. In order to do this, the bacterium forms special organs, called nodules, on the filao roots, which change atmospheric nitrogen into ammonium, which can be directly assimilated by the plant.
To understand how this tree is adapted to depleted soils and can modify the architecture of its root system, a research team3 is seeking to decipher the molecular mechanisms that govern the formation and functioning of nitrogen-fixing root nodules. They have characterized one of the genes essential for the symbiosis between the filao and Frankia .
Originating from Australia, the filao tree is also commonly called Iron tree, for the hardness of its wood. It is already used in various areas of the world to produce biomass and firewood, for restoring soil fertility and for erosion control.
These investigations show the advantage to be gained by harnessing mycorrhizal symbiosis and such species as filao, well adapted to the conditions in the Sahel, for the Great Green Wall. The community of experts now must establish the ways in which this monumental project can be integrated in an environment already occupied and exploited for farming. The reforestation techniques recommended must be compatible with the current land uses and practices in the countries the wall is to cross and the species chosen must have potential for aiding the development of the local economies concerned.
2. This research was conducted by scientists from the Laboratoire des Symbioses Tropicales et Méditerranéennes (UMR IRD, CIRAD, INRA, Montpellier SupAgro, University of Montpellier 2) working jointly with Cadi Ayyad University, Marrakech in Morocco and Cheikh Anta Diop University in Dakar, Senegal.
3. These investigations are being made by researchers from the research unit UMR Diversité et Adaptation des Plantes Cultivées (IRD, CIRAD, INRA, Montpellier SupAgro, Université Montpellier 2) jointly with scientists from the Chinese Academy of Forestry at Guangzhou and CSIRO Plant Industry at Canberra in Australia.
Redaction DIC – Gaëlle Courcoux
Translation – Nicholas Flay