Leaves on trees intercept rain before it hits the ground, allowing it to evaporate back into the atmosphere at much higher rates than from short vegetation. Just as clothes peg-ged onto a line dry more quickly than those lying on the ground, tall trees with many leaves are better evaporation surfaces than grass or crops.
Studies of upland evergreen forests by scientists at the Government's Centre for Ecology and Hydrology in Wallingford have found an increase in evaporation rates in forested areas of up to 100 per cent compared with treeless catchments. In some upland areas of Scotland, researchers have found this leads to a 20 per cent reduction in run-off to reservoirs.
Now hydrologists are concerned about the impact on water supplies of the likely large increase in lowland woodlands over the next few decades. ''Community forests'' are being planted on the edge of big cities, a large new National Forest has begun to grow in the Midlands and "energy plantations" of coppiced woodland are being considered. These and other policies came together last autumn when the Government announced that it wanted a doubling of England's forest cover by 2045.
Most of the new trees in the lowlands are broad-leaved species which drop their leaves in autumn, so they evaporate less water through the winter than coniferous trees. But during dry spells, they take up water from the soil at far higher rates than grassland or crops.
Recent research by the centre suggests that water use of some broad-leaved species may be significantly higher than previously thought. In one forest, recharge to the underground chalk aquifer was found to be reduced by 40 per cent, according to Dr Ian Calder, a hydrologist. "Planting forests now might significantly affect water resources in the long term, particularly if climate change becomes a reality," he says.
Few policy-makers appear to have recognised the implications of the combined effects of tree planting and global warming. The most recent Department of Environment report on the impact of climate change in Britain, released last week, makes no mention of the implications of afforestation.
Given the economic and aesthetic benefits of planting forests, any potential water shortages should be dealt with by an "emphasis on reducing leakage and reducing demand, rather than altering vegetation," says Dr Tom Nisbet, a hydrologist at the commission.
Our national tree, the oak, is the baldest in the country, according to the latest Forestry Commission survey of tree condition. One in nine is missing at least half of the leaves of a tree in peak condition, and just over two-thirds are missing at least a quarter. Winter moths, frost, and a mysterious degenerative condition affecting oaks in the south and east of England in the early 1990s are to blame.Reuse content