Scientists may be about to upset one of the most cherished tenets of conservation: that trees save water.
The research, published yesterday 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 vapor 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 programs to conserve water.
For decades, conservationists have argued that forests serve as a kind of sponge, collecting water during the rainy season and releasing it throughout the year. But in many cases, trees may make things worse.
"Generally, forests evaporate a lot more water than other vegetation types," said Ian Calder, of the center for land use and water resources research at the University of Newcastle.
"There are two quite simple reasons. One is that they tend to have deeper roots than shorter crops, and they keep on transpiring in dry seasons. The other simple reason is that they are tall, and when they are wet, their surfaces evaporate off much more quickly ... This is, essentially, why we all hang up our washing on the line to dry rather than leave it on the ground," he said.
Calder and British colleagues worked with scientists at the Free University in Amsterdam, as well as colleagues in Colombia, Costa Rica, Germany, Canada, India, South Africa, Sweden, Switzerland, Tanzania and the US. Their report shows evidence of falling water tables and reduced stream flows where forests have been planted.
In Himachal Pradesh and Madhya Pradesh in India there have been falls of up to 25 percent in water yield.
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