By Ana Belluscio
Planting trees, which can significantly help to reduce atmospheric carbon dioxide, nevertheless comes with potentially damaging side effects. According to two new studies, planting forests in areas that currently don't have trees -- a process called afforestation -- can reduce the local availability of water.
One key measure of water flow is 'base flow', the proportion of a stream or river not attributable to direct run-off from precipitation or melting snow. Base flow is often seen as the minimum supply of water on which people can safely rely. But in basins that contain small rivers, afforestation can reduce base flow by up to 50%, says Esteban Jobbagy, an ecologist at Argentina's national scientific council (CONICET) and the National University of San Luis.
Less base flow means less water for local populations. "It's a concern especially in drier regions, where the differences in base flow may be more noticeable," says Dan Binkley, a forest ecologist at Colorado State University in Fort Collins who was not involved in the research.
Jobbagy's team conducted a two-year study on seven paired basins -- seven with native grasslands and seven that had been planted with forests -- in the province of Córdoba, Argentina. With their deep roots and tall canopies, trees absorb and transpire more water than do grasses, resulting in drier streams. According to Jobbagy, reductions in base flow are less pronounced in sloping or rocky basins, as water can escape from the tree roots and travel through the rocks.
Jobbagy presented his team's results last month at the World Forestry Congress in Buenos Aires.
A second study presented at the conference, and conducted in Uruguay, came to similar conclusions. A team led by Wayne Skaggs of North Carolina State University in Chapel Hill, in collaboration with the US Forest Service, afforested one of a pair of watersheds. The researchers observed an 18-22% drop in base flow in the afforested watershed compared with the watershed that had been left as grassland. "As the trees get larger, the effect will be somewhat greater," says Skaggs.
The differences between Jobbagy's and Skaggs's figures are mainly due to differences in the number of trees per hectare in the two studies -- greater in Jobbagy's basins -- and the trees' age. In Córdoba, the studied areas were afforested in the 1970s, whereas Skaggs and his team planted their trees in 2003.
Besides reducing base flow, afforestation can affect how water filters through the ecosystem. "Afforested sites are not as 'splashy' as pastured sites," says Skaggs. Tree roots help to filter water into the soil, thus slowing the rate at which water levels rise after rain. "This is actually a good thing," he says. "It could reduce flood flows, particularly from small watershed areas." According to his team's observations, the afforested parts of watersheds also prevent the erosion and sediment-leaching that were seen in their grassland counterparts.
Jobbagy says that, at least for his study areas, the ideal balance between afforestation and water needs is for one-quarter of the river basin to be planted with between 400 and 500 trees per hectare. "It is possible to prevent drastic effects" on water availability, he says.
Choosing tree species wisely might help, Binkley and Jobbagy suggest, as different species use water at different rates. Whereas fast-growing evergreen pines consume large amounts of water, deciduous trees such as poplar and walnut use less, especially in winter. Also, planting only some portions of the watershed "might achieve the balance of providing wood products for the people without the impact on the basin's water balance", says Binkley.