• Open Access

On the forest cover–water yield debate: from demand- to supply-side thinking

Authors

  • David Ellison,

    Corresponding author
    1. Department of Forest Ecology, Swedish University of Agricultural Sciences, 90183 SE Umeå, Sweden
    2. Department of Social and Economic Geography, 90187 Umeå University, Umeå, Sweden
    • Institute for World Economics, Hungarian Academy of Sciences, 1014 HU Budapest, Hungary
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  • Martyn N. Futter,

    1. Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, 75007 SE Uppsala, Sweden
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  • Kevin Bishop

    1. Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, 75007 SE Uppsala, Sweden
    2. Department of Earth Sciences, Uppsala University, 75236 SE Uppsala, Sweden
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Correspondence: David Ellison, tel. + 36 30 929 5246, fax + 36 1 224 6761, e-mail: EllisonDL@gmail.com

Abstract

Several major articles from the past decade and beyond conclude the impact of reforestation or afforestation on water yield is negative: additional forest cover will reduce and removing forests will raise downstream water availability. A second group of authors argue the opposite: planting additional forests should raise downstream water availability and intensify the hydrologic cycle. Obtaining supporting evidence for this second group of authors has been more difficult due to the larger scales at which the positive effects of forests on the water cycle may be seen. We argue that forest cover is inextricably linked to precipitation. Forest-driven evapotranspiration removed from a particular catchment contributes to the availability of atmospheric moisture vapor and its cross-continental transport, raising the likelihood of precipitation events and increasing water yield, in particular in continental interiors more distant from oceans. Seasonal relationships heighten the importance of this phenomenon. We review the arguments from different scales and perspectives. This clarifies the generally beneficial relationship between forest cover and the intensity of the hydrologic cycle. While evidence supports both sides of the argument – trees can reduce runoff at the small catchment scale – at larger scales, trees are more clearly linked to increased precipitation and water availability. Progressive deforestation, land conversion from forest to agriculture and urbanization have potentially negative consequences for global precipitation, prompting us to think of forest ecosystems as global public goods. Policy-making attempts to measure product water footprints, estimate the value of ecosystem services, promote afforestation, develop drought mitigation strategies and otherwise manage land use must consider the linkage of forests to the supply of precipitation.

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