Regrowth of trees explains why grasslands in western Africa known as the Sahel have recovered after devastating droughts in the 1970s and 1980s, according to South Dakota State University professor Niall Hanan. The Sahel is a semiarid region with grasslands and scattered trees sandwiched between the Sahara Desert to the north and tropical savannas to the south.
Hanan, a senior scientist at the Geospatial Sciences Center of Excellence, and colleagues postdoctoral research associate Armel Kaptué and assistant research professor Lara Prihodko examined satellite images from 1983 to 2012 to evaluate the vegetation in 260 watersheds in four regions of Senegal, Mali and Niger.
"Studies in the past have suggested that the Sahara Desert was marching southward," noted Niall, but the study refutes this notion. "Our results show the resilience of the Sahel, with much of the area getting greener and responding better to rainfall."
Their findings were published in the Sept. 29 issue of the Proceedings of the National Academy of Sciences. The research is part of a National Science Foundation project to examine the hydrology, ecology and pastoral societies in the Sahel.
The Sahel is used primarily for grazing livestock. Livestock-owning communities, known as pastoralists, herd their cattle, goats and sheep through seasonal migration routes that their families have been following for hundreds of years.
The short grasses that grow in the Sahel during the rainy season are more nutritious than the tall grasses of the tropical savanna to the south, Hanan pointed out.
The researchers identified a greening trend in 84 percent of the watersheds with 17 percent showing significant improvement during the rainy season, June through October, within the 30-year time frame.
"Vegetation began to recover as rainfall came back," he explained. "The grass came back immediately, proportional to the rain."
The regreening that the researchers documented is largely due to increases in tree communities, according to Hanan. "It takes a few years of drought to kill most Sahelian trees," he explained. "but the tree population cannot recover immediately in wet years -- it takes time for new seedlings to establish and for us to see more trees in the landscape."
However, the greening trend did not encompass the entire region. The researchers found that 16 percent of the watersheds had experienced decline, with 1 percent showing a significant decline.
"The problems of food insecurity and poverty are very real, but the notions about desertification and overgrazing are misplaced," Hanan explained. "Ecologically, a majority of the Sahel has recovered."
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