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Saharan vegetation is generally sparse, with scattered concentrations of grasses, shrubs, and trees in the highlands, in oasis depressions, and along the wadis. Various halophytes (salt-tolerant plants) are found in saline depressions. Some heat- and drought-tolerant grasses, herbs, small shrubs, and trees are found on the less well-watered plains and plateaus of the Sahara.
The vegetation of the Sahara is particularly noteworthy for its many unusual adaptations to unreliable precipitation. These are variously seen in morphology—including root structure, a broad range of physiological adaptations, site preferences, dependency and affinity relationships, and reproductive strategies. Many of the herbaceous plants are ephemerals that may germinate within three days of adequate rainfall and sow their seeds within 10 or 15 days of germination. Sheltered in the Saharan massifs are occasional stands of relict vegetation, often with Mediterranean affinities.
Prominent among the relict woody plants of the Saharan highlands are species of olive, cypress, and mastic trees. Other woody plants found in the highlands and elsewhere in the desert include species of Acacia and Artemisia, doum palm, oleander, date palm, and thyme. Halophytes such as Tamarix senegalensis are found along the western coastal zone. Grasses widely distributed in the Sahara include species of Aristida, Eragrostis, and Panicum. Aeluropus littoralis and other salt-tolerant grasses are found along the Atlantic coast. Various combinations of ephemerals form important seasonal pastures called acheb.
In the 21st century, recognition that the Sahara and its border region to the south, the Sahel, were creeping southward owing to desertification led to efforts to stall that movement; most notable was the Great Green Wall for the Sahara and Sahel Initiative. The idea that led to the initiative—planting a “wall” of trees along the edges of the Sahara that would stretch across the African continent in order to halt further desertification—was first conceived in 2005 and was later further developed with the assistance of the African Union and other international organizations. It involved plans to plant drought-resistant native trees in a 9-mile- (15-kilometre-) wide swath of territory from the western to the eastern edges of the continent, creating a barrier to keep the desert from further encroaching on the lands to its south.