Planting forests in areas that currently don't have trees can reduce the local availability of water, by shifting water flow, determine two new studies.
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 percent, Esteban Jobbagy, an ecologist at Argentina's national scientific council (CONICET) and the National University of San Luis, told Nature News.
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," said 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 Cordoba, 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.
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 percent 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," said Skaggs.
Besides reducing base flow, afforestation can affect how water filters through the ecosystem. "Afforested sites are not as 'splashy' as pastured sites," said Skaggs.
Jobbagy said 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 said.
Also, choosing tree species wisely might help, Binkley and Jobbagy suggest, as different species use water at different rates.