Local water availability is permanently reduced after planting forests

Shallow river bed in Buderim Forest Park, Queensland, Australia.        Credit:

Shallow river bed in Buderim Forest Park, Queensland, Australia. Credit: Laura Bentley

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    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.


    River flow does not recover after planting trees, even after many years.

    David Coomes


    "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." 

    Reference 

    Bentley, L. et al: ’Partial river flow recovery with forest age is rare in the decades following establishment.’ Global Change Biology, Jan 2020. DOI: 10.1111/gcb.14954

    In the Mediterranean woodlands of Spain, trees have regrown naturally over the last 50 years. Laura Bentley is trying to understand how much carbon these trees are storing, and how this process is affecting water availability in the region.

    "Previous research has shown how important restoring forest cover will be in combatting climate change," says Bentley. "I want to inform how we use landscapes over the next century to meet environmental challenges. My goal is to help build a better understanding of what a world with increased forest cover will be like."

    Bentley developed an interest in the complex natural processes that support human needs while a masters student at Imperial College London. She subsequently moved to Cambridge for her PhD, where she is supervised by Professor David Coomes at the University of Cambridge Conservation Research Institute. 

    Her research encompasses processing data from satellites and other remote sources, and visiting her collaborators in Madrid - a Spanish National Research Council team led by Professor Fernando Valladares - for fieldwork.

    "My field sites in Spain’s Alto Tajo Natural Park are part of a truly beautiful landscape - but very punishing in the summer heat," says Bentley. "The most exciting part of my PhD so far was my first field season there. From driving through gorgeous remote landscapes, to getting caught in an intense hailstorm in the middle of summer, the experience will stay with me for a long time." 

    Bentley’s interest in the natural world has led to a budding passion for photography, and she tries to capture new images whenever the opportunity arises. Her observations have convinced her that there is a way to make environment work for both people and nature in the long term. "I believe science can help us find that goldilocks zone."

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