New forests mean permanently lower river flows

Planting trees helps to combat the climate crisis by cutting greenhouse gases. But the price can be permanently lower river flows.

LONDON, 20 January, 2020 − New forests are an apparently promising way to tackle global heating: the trees absorb carbon dioxide, the main greenhouse gas from human activities. But there’s a snag, because permanently lower river flows can be an unintended consequence.

A study by scientists at the University of Cambridge, UK, has found that river flow is reduced in areas where forests have been planted − and, significantly, it does not recover over time. Rivers in some regions can disappear completely within 10 years.

This, the researchers say, highlights the need to consider the impact on regional water availability, as well as the wider climate benefit of tree-planting plans.

“Reforestation is an important part of tackling climate change, but we need to carefully consider the best places for it. In some places, changes to water availability will completely change the local cost-benefits of tree-planting programmes”, said Laura Bentley, a plant scientist in the University of Cambridge Conservation Research Institute, and first author of the report.

Age effect missed

Planting large areas of trees has been suggested as one of the best ways of reducing atmospheric carbon dioxide levels, because trees absorb and store the gas as they grow, although uncertainties about the strategy persist. Science has known for a long time that planting trees reduces the amount of water flowing into nearby rivers, but no-one had realised how this effect changes as forests age.

The Cambridge study looked at 43 sites across the world where forests have been established, and used river flow as a measure of water availability in the region. It found that within five years of planting trees, river flow had reduced by an average of 25%.

But 25 years after the trees were planted, rivers had gone down by an average of 40%, or in a few cases had dried up altogether. The biggest percentage reductions in water availability were in parts of Australia and South Africa.

“River flow does not recover after planting trees, even after many years, once disturbances in the catchment and the effects of climate are accounted for,” said Professor David Coomes, director of the Conservation Research Institute, who led the study.

“In some places, changes to water availability will completely change the local cost-benefits of tree-planting programmes”

Published in the journal Global Change Biology, the research showed that the type of land where trees are planted determines the impact they have on local water availability.

Trees planted on natural grassland where the soil is healthy decrease river flow significantly. But on land previously degraded by agriculture, establishing a forest helps to repair the soil so that it can hold more water, and therefore decreases nearby river flow by a smaller amount.

Strangely, the effect of trees on river flow is smaller in drier years than in wetter ones. When trees are drought-stressed they close the pores on their leaves to conserve water, and as a result take up less water from the soil. In wet weather, though, they use more water from the soil, and also catch the rainwater in their leaves.

“Climate change will affect water availability around the world,” said Bentley. “By studying how forestation affects water availability, we can work to minimise any local consequences for people and the environment.” − Climate News Network