July 29, 2005
‘Myth’ that forests improve water flows – study
By Alister Doyle, Environment Correspondent
OSLO (Reuters) - 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.
"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."
The report said Panama was seeking hundreds of millions of
dollars from the World Bank to back a project to plant trees on
the apparently mistaken belief that it would attract more
rainfall to help feed the Panama Canal.
Other countries from China to Mexico also had costly
afforestation schemes at least partly based on misconceptions
In the Indian states of Himachal Pradesh and Madhya
Pradesh, the study said conversion of agricultural land to
forests had damaged water supplies, cutting flows by 16-26
Availability of fresh water is a constant problem.
The World Commission on Water has estimated that demand for
water will increase by about 50 percent in the next 30 years
and that around four billion people, or about half of the
world's population in 2025, will have problems with supplies.
The study said trees often showed the "clothes line"
Just as wet clothes dry quicker if hung out rather than
left lying on the ground, the enormous combined surface of
trees' leaves combined with their deep roots meant they
transpired more water into the air than other crops, it said.