The global tree restoration potential

Bastin et al. (2019) misrepresent the current status and potential achievability of commitments to restore vast areas of land under the Bonn Challenge. Their statement that "Approximately 10% of countries have committed to restoring an area of land that considerably exceeds the total area that is available for restoration" is incorrect, and arises from an oversimplified understanding of the restoration options available under Bonn Challenge commitments. We are concerned that these misunderstandings will further undermine the Bonn Challenge, rather than promote improved implementation of restoration pledges.

Bastin et al. (2019) assume in their analysis that all Bonn Challenge area commitments are to restore non-forest, non-agricultural land to forest cover. However, a variety of potential restoration options are available to Bonn Challenge program participants that are not included in this assumption, including silviculture, agroforestry, and planting mangrove forests (Dave et al., 2017).

These alternatives make up large proportions of the commitments from many countries. For example, silviculture (e.g., forest thinning and prescribed burning in the American West) constitutes 96% of the US restoration pledge of 17 Mha. Likewise, agroforestry (e.g., planting trees in agricultural fields) makes up >70% of El Salvador's restoration pledge of 1 Mha (MARN, 2017).

The potential restoration maps utilized in Bastin et al. (2019) deliberately exclude agricultural areas and are created using satellite data that cannot detect ecological restoration opportunities in forests with intact overstories. By excluding these two enormous land areas that are available for restoration (agriculture and intact forests), they dramatically underestimate the potential restoration area available to Bonn Challenge participants. Therefore, it is false and misleading to claim that countries will not be able to meet their Bonn Challenge commitments because they cannot restore sufficient areas of new forest. They never committed to doing so.

While our research suggests that many countries have taken on large and potentially impractical commitments (Fagan, Reid, Holland, Drew, & Zahawi, 2020), it is too soon to count them out, especially by misreading the rules of the game. If the Bonn Challenge and other volunteer global restoration programs are to succeed, they need to be supported by the best available science, and judged on their actual progress towards achieving their stated commitments.

Literature Cited:
Bastin, J. F., Finegold, Y., Garcia, C., Mollicone, D., Rezende, M., Routh, D., … Crowther, T. W. (2019). The global tree restoration potential. Science, 365(6448), 76–79. https://doi.org/10.1126/science.aax0848

Dave, R., Saint-Laurent, C., Moraes, M., Simonit, S., Raes, L., & Karangwa, C. (2017). Bonn Challenge Barometer of Progress: Spotlight Report 2017. Gland, Switzerland. Retrieved from https://infoflr.org/sites/default/files/2017-12/2017-060.pdf

Fagan, M. E., Reid, J. L., Holland, M. B., Drew, J. G., & Zahawi, R. A. (2020). How feasible are global forest restoration commitments? Conservation Letters, 13(3), e12700. https://doi.org/10.1111/conl.12700

MARN. (2017). Strengthening the National Restoration Strategy. San Salvador, El Salvador. Retrieved from https://www.iucn.org/sites/dev/files/content/documents/2017/strengthenin...

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 ramosissima, Alhagi sparsifolia, and Calligonum mongolicum have been chosen to replace the young olded-trees in these degraded plantations.
Tropical and subtropical dry broadleaf forests and grasslands will lose ~50% of their tree restoration potential in the mid-century owing to the fast approach of their tropical climate tipping points (7, 8). Shrub introduction into the tropics would also be advantageous and allow for more time to reach global reforestation goals. Many countries will implement forest reconstruction projects (9) and planting shrubs rather than large-statured trees may be necessary for effective forest management in a drying world (10).
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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 rigorous consideration of the practicality of their proposal. Rather than an increase in forest cover, recent evidence suggests that the rate of deforestation is increasing in many parts of the world (Hansen et al. 2013), including Brazil, which had slowed its deforestation rate but where deforestation has increased in the past year under the Bolsonaro administration (Phillips 2018). And, many areas where forest cover has started to regrow are being re-cleared for human uses (Reid et al. 2019). A primary focus of land-based carbon sequestration efforts should be to keep the existing forests standing since it is much more challenging and costly to restore them.

Third, the areas of land proposed for reforestation (https://www.crowtherlab.com/maps-2/) include areas that were historically grasslands. Afforesting such areas would not only destroy native biodiversity (Veldman et al. 2015) but is unlikely to be successful.

Restoring forests is certainly part of the climate solution, but it needs to be done thoughtfully and is not the silver bullet that Bastin et al. (2019) describe. We should be undertaking the challenging steps needed to reduce greenhouse gas emissions and to protect existing forests.

Literature Cited
Bastin, J.-F., Y. Finegold, C. Garcia, D. Mollicone, M. Rezende, D. Routh, C. M. Zohner, and T. W. Crowther. 2019. The global tree restoration potential. Science 365:76-79.
Hansen, M. C., P. V. Potapov, R. Moore, M. Hancher, S. A. Turubanova, A. Tyukavina, D. Thau, S. V. Stehman, S. J. Goetz, T. R. Loveland, A. Kommareddy, A. Egorov, L. Chini, C. O. Justice, and J. R. G. Townshend. 2013. High-resolution global maps of 21st-century forest cover change. Science 342:850-853.
Phillips, D. 2018. Brazil records worst annual deforestation for a decade. The Guardian https://www.theguardian.com/environment/2018/nov/24/brazil-records-worst....
Reid, J. L., M. E. Fagan, J. Lucas, J. Slaughter, and R. A. Zahawi. 2019. The ephemerality of secondary forests in southern Costa Rica. Conservation Letters 12:e12607.
Veldman, J. W., G. E. Overbeck, D. Negreiros, G. Mahy, S. Le Stradic, G. W. Fernandes, G. Durigan, E. Buisson, F. E. Putz, and W. J. Bond. 2015. Tyranny of trees in grassy biomes. Science 347:484-485.

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"

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.