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 Jobbágy, 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.
However, planted smartly -- choosing the right species, planted at the right densities -- trees can help protect communities from flooding and erosion:
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.