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The global tree restoration potential

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Science  05 Jul 2019:
Vol. 365, Issue 6448, pp. 76-79
DOI: 10.1126/science.aax0848

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  • RE: Planting shrubs is more sustainable in a drying world

    J. F. Bastin and colleagues (“The global tree restoration potential,” 05 July, p. 76) predict that there are currently 0.9 billion hectares (ha) of forest with restoration potential, but ~223 million ha may be lost by 2050, and ~174 million ha will be lost despite aggressive CO2 reduction. Although tree planting can significantly reduce CO2 and help reverse climate change, another crucial vegetation type should not be overlooked—shrubs.
    Global drylands will increase by 7% by the end of the 21st century (1) and planting trees in arid lands may be inopportune. The reasons: (i) trees are more susceptible to drought-induced embolism (2) and are more likely to die owing to hydraulic failure (3) than shrubs; (ii) large-scale tree planting can adversely affect local water resources due to the presence of denser canopies and higher transpiring water loss (4) and (iii) the maintenance costs are high (5). Therefore, tree planting in expanding arid and semi-arid regions is not sustainable. For example, the Three-North Shelter Forest Program that launched in 1978 in China resulted in indiscriminate tree planting without consideration for physiological limits and has led to 3 million ha of degraded plantations comprising young olded-trees (e.g., Populus simonii & Robinia pseudoacacia). In northern China, ~16% of coniferous forests and ~10% broadleaved and mixed conifer-broadleaved forests in plantations are not suitable for tree planting (6) and shrubs, such as Tamarix ramosi...

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    Competing Interests: None declared.
  • RE: Tree planting is not a silver bullet to address climate change
    • Karen D Holl, Professor, University of California, Santa Cruz

    Bastin et al. (2019) highlight the potential role that restoring forests can play in sequestering carbon and thereby help combat climate change, along with dramatic and rapid efforts to reduce greenhouse emissions. As somebody who has studied tropical forest restoration forest two decades, I concur that restoring forests, in areas previously covered by forests, provides important services beyond just carbon sequestration, such as conserving biodiversity and improving water quality, but I contend that they grossly overestimate the climate change mitigation potential of forest restoration.

    First, they base their assessments of carbon sequestration potential on relatively intact forests. Extensive research shows that the rate and extent of forest recovery is strongly affected by the intensity of past human disturbance, invasive species, increasing wildland fires, and numerous other factors. In other words, it is unlikely that much of the heavily disturbed lands that they propose for forest restoration will recover to the extent predicted by their model and certainly not over the time frame of a decade or two that will be needed to keep warming to below 2°C or even the short, 30-yr duration they use in their model.

    Second, they only briefly note that “much of the land that could potentially support trees across the globe is currently used for human development and agriculture, which are necessary for supporting an ever-growing human population” overlooking a ri...

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    Competing Interests: None declared.
  • RE: Soving Climate Crisis Today with Trees for Tomorrow

    from Greta Thunberg (July 5 at 11:30 AM):
    Yes, of course we need to plant as many trees as possible.
    Yes, of course we need to keep the existing trees standing and rewild and restore nature.
    But there’s absolutely no way around stopping our emissions of greenhouse gases and leaving the fossil fuels in the ground.
    “The only way we can keep below 1.5C or 2C, is to stop emitting fossil fuels.” says Glen Peters, research director at Norway’s Center for International Climate Research.
    “It could take hundreds of years to add enough mature forests to remove what we will emit in 20 years at the current rate of 40GtCO₂/yr”

    Competing Interests: None declared.
  • RE: Important Omission in This Article

    The calculations in this historic article omit the undeniable cooling effects of the shade cast by one trillion trees.

    As such, the projected benefits of planting one trillion trees is understated by a non-negligible amount in this work.

    Competing Interests: None declared.

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