Many countries are wasting millions of dollars planting trees because of myths that forests always help improve water flows and offset erosion, a British-led study said on Friday.

Many trees, especially fast-growing species like pines and eucalyptus favored by the paper industry, suck more water from the ground than other crops, it said. The water transpires from the leaves and so the trees dry out the land.

“Trees on the whole are not a good thing in dry areas if you want to manage water resources,” said John Palmer, manager of the tropical Forestry Research Programme run by the British Department for International Development.

“When it comes to wet areas, trees may be beneficial or no worse than pasture and crops,” he told Reuters of the study of plantings in India, Costa Rica, South Africa and Tanzania in a four-year project led by British and Dutch researchers.

Forests have many other benefits — ranging from habitats for birds, insects or animals to human sources of building materials and firewood.

But the report said it was a myth that forests acted as sponges that soak up rain, releasing it throughout the year and ensuring more regular flows in rivers. Instead, trees’ deep roots often aggravate water shortages in dry seasons.

It also said it was wrong to believe forests attracted more clouds and rainfall or that tree roots helped slow erosion more those of short plants. It said the myths had been anchored in cultural history since at least the 17th century.

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“We don’t want to be seen as against forests or trees,” said Ian Calder, a lead researcher who is director of the Center for Land Use and Water Resources Research at England’s University of Newcastle.

“But there is a need to be careful when you plant forests in the belief you are promoting water resources,” he said. “We need policies based more on scientific evidence. Hundreds of millions of dollars are being spent, if not billions.”

By Alister Doyle

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