Planting trees can shift water flow

Ana Belluscio in Nature News: 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 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.

Jobbágy'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 Jobbágy, 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….

Beech and oak trees at Appley Park, Ryde, Isle of Wight, shot by Naturenet, Wikimedia Commons, under the Creative Commons Attribution ShareAlike 3.0 License