One thing that helps keep water in the soil is trees. The two regions in the world that have lost the most forest cover are Africa and Latin America, according to the State of the World's Forests 2007, released on Tuesday. Whereas many regions are reversing centuries of deforestation, Africa lost nearly 10 percent of its forests in the last 15 years.

Some countries, however, are bucking the trend. Farmers in Niger have saved their land from the encroaching Sahara desert, and local researchers are amazed to find that trees have spread to over 3 million hectares in just two or three decades. And to boot, the process has cost practically nothing. In an excellent article published in the New York Times last month, Lydia Polgreen explains how the farmers transformed their lives and land.

Ibrahim Danjimo, in his 40s, tells her how 20 years ago the people in his village realised that all the trees were disappearing. Fierce winds were carrying off precious topsoil, sand dunes threatened to swallow their huts and wells ran dry.

So they decided to make a small but radical change: they would no longer clear saplings from their fields before planting. Instead they would care for them and plough around them before sowing their millet, peanuts, beans and sorghum crops.

More recently the government helped by changing the law to allow farmers to own the trees, giving them added incentive to care for them. Since colonial times all trees were regarded as the property of the state.

"The benefits are so many it is really astonishing," Mahamane Larwanou, a forestry expert at the University of Niamey in Niger says in the NYT article.

Farmers can sell the branches, feed pods to their animals, sell or eat the leaves and fruit. And better still their roots keep the vital topsoil in place, and hold water in the ground so it doesn't flood villages and destroy crops.

Parts of Ethiopia also tell a success story. Once denuded land has been restored in some areas where the government allowed communities to manage their own land and introduce by-laws to protect the environment. In one region trees and grass which had disappeared grew back, crops improved, and even a spring appeared which could be used to irrigate the land.

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