YACOUBA Sawadogo murmurs advice to his sons as they press a sapling into the red earth using a centuries-old technique that he has adapted to conjure a forest from Burkina Faso's rain-starved soil. The farmer who is well into his 70s is hailed across his province as "the man who stopped the desert". He won that title after tweaking a method of growing plants in pits to trap water - essential in the hardscrabble region fringing the Sahara. After a terrible drought ravaged the Sahel in the 1970s and 1980s, many of Sawadogo's neighbours abandoned their farms in northern Burkina…