... Lesson plan 12 - Hµ Thu Hoµ - Chieng Sinh Upper Secondary School the plants die, and the land becomes desert In developing countries, 90 percent of the people use wood for cooking and heat They ... They cool the land under them and keep the sun off smaller plants When leaves fall from a tree, they make the land richer When the trees are gone, the smaller plants die, and the land becomes desert ... of the Sahara to stop the desert sand from spreading Mauritania planted a similar wall around its capital Iran puts a thin covering of petroleum on sandy areas and plant trees Other countries build...