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Severe drought in the 1970s and ’80s, coupled with a population explosion and destructive farming and livestock practices, was denuding vast swaths of land. The desert seemed determined to swallow everything. So Mr. Danjimo and other farmers in Guidan Bakoye took a small but radical step. No longer would they clear the saplings from their fields before planting, as they had for generations. Instead they would protect and nurture them, carefully plowing around them when sowing millet, sorghum, peanuts and beans.

Today, the success in growing new trees suggests that the harm to much of the Sahel may not have been permanent, but a temporary loss of fertility. The evidence, scientists say, demonstrates how relatively small changes in human behavior can transform the regional ecology, restoring its biodiversity and productivity.

In Niger’s case, farmers began protecting trees just as rainfall levels began to rise again after the droughts in the 1970s and ’80s.

Image In Tahoua, where women have regenerated once-barren fields by digging manure pits, women mill their grain by pounding it with wooden pestles. Credit... Michael Kamber for The New York Times

Another change was the way trees were regarded by law. From colonial times, all trees in Niger had been regarded as the property of the state, which gave farmers little incentive to protect them. Trees were chopped for firewood or construction without regard to the environmental costs. Government foresters were supposed to make sure the trees were properly managed, but there were not enough of them to police a country nearly twice the size of Texas.

But over time, farmers began to regard the trees in their fields as their property, and in recent years the government has recognized the benefits of that outlook by allowing individuals to own trees. Farmers make money from the trees by selling branches, pods, fruit and bark. Because those sales are more lucrative over time than simply chopping down the tree for firewood, the farmers preserve them.

The greening began in the mid-1980s, Dr. Reij said, “and every time we went back to Niger, the scale increased.”