Scientists may be about to upset one of the most cherished tenets of conservation: that trees save water. The research, published today by Britain's Forestry Research Programme, could mean that billions spent on forest projects in the developing world have been wasted. New measurements suggest that forests soak up water from the ground and discharge it into the atmosphere as vapour at least twice as fast as grasses, low-lying scrub or most food crops. By 2025, around 4 billion people - half the world's population - will be short of water. India, China, Costa Rica and Panama have invested in huge forestry programmes to conserve water.