Reforestation can cause rivers to disappear

Preface. We’ll need a lot of forests in the future when wood once again becomes our may source of energy and infrastructure as it was before fossil fuels. But care needs to be taken on where trees are planted.

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Bentley, L., et al. 2020. Partial river flow recovery with forest age is rare in the decades following establishment. Global Change Biology.

River flow is reduced in areas where forests have been planted and does not recover over time, a new study has shown. Rivers in some regions can completely disappear within a decade. This 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.

Planting large areas of trees has been suggested as one of the best ways of reducing atmospheric carbon dioxide levels, since trees absorb and store this greenhouse gas as they grow. While it has long been known that planting trees reduces the amount of water flowing into nearby rivers, there has previously been no understanding of how this effect changes as forests age.

The 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%. By 25 years, rivers had gone down by an average of 40% and in a few cases had dried up entirely. The biggest percentage reductions in water availability were in regions in 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 University of Cambridge Conservation Research Institute, who led the study.

Published in the journal Global Change Biology, the research showed that the type of land where trees are planted determines the degree of impact they have on local water availability. Trees planted on natural grassland where the soil is healthy decrease river flow significantly. On land previously degraded by agriculture, establishing forest helps to repair the soil so it can hold more water and decreases nearby river flow by a lesser amount.

Counterintuitively, the effect of trees on river flow is smaller in drier years than wetter ones. When trees are drought-stressed they close the pores on their leaves to conserve water, and as a result draw up less water from the soil. In wet weather the trees 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.”

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2 Responses to Reforestation can cause rivers to disappear

  1. Don Stewart says:

    I suggest that anyone worried about this should read Albert Bates and Kathleen Draper’s book, Burn: Using Fire to Cool the Earth. The basic plan is a community furnace which converts biomass into energy (electricity and liquid fuel) and useful products, including biochar. The biochar can be included in many industrial products and also used in agriculture. The result is useful products plus carbon sequestration. The authors call it a ‘carbon cascade’.

    The forest is not allowed (at least not always allowed) to proceed to the relative stasis of maturity, but is harvested while it is still growing vigorously. The result is much more carbon produced and used and sequestered.

  2. Goran says:

    I think this research is an important piece of the puzzle. There is a lot of naivete around tree planting.

    Trees often help with the hydrological cycle, by buffering seasonal rains in the root area and help to moderate seasonal variations in streams. Tree roots are important to hold the soil and prevent erosion. There is plenty of research showing that upland tree planting is often useful.

    However, not every location is the same. I have seen several failed tree planting projects in north of China, where hybrid poplars were planted in arid areas. They grow like crazy and die. These trees even sucked the soil dry and made it more difficult for other plants. The wrong tree in the wrong place.

    I recommend the work of Mr Phiri in Zimbabwe and the work of Brad Lancaster in the US for examples of small- and mid-scale successful rainwater harvesting.

    I love trees. Especially nut trees. And I plant plenty of trees.
    We have to keep observing and learning what works in each location.