Abstract Bastin et al.’s estimate (Reports, 5 July 2019, p. 76) that tree planting for climate change mitigation could sequester 205 gigatonnes of carbon is approximately five times too large. Their analysis inflated soil organic carbon gains, failed to safeguard against warming from trees at high latitudes and elevations, and considered afforestation of savannas, grasslands, and shrublands to be restoration.

Bastin et al. (1) used remote sensing and machine learning to estimate that global “tree restoration” could sequester 205 gigatonnes of carbon (GtC). If accurate and achievable, this would constitute an astounding accomplishment, equal to 20 times the current annual fossil fuel emissions (10 GtC/year) (2) and about one-third of total historical anthropogenic emissions (660 GtC) (2). Unfortunately, key assumptions and data underlying Bastin et al.’s analyses are incorrect, resulting in a factor of 5 overestimate of the potential for new trees to capture carbon and mitigate climate change. We show that Bastin et al. (i) overestimated soil carbon gains from increased tree cover by a factor of 2; (ii) modeled new tree cover in regions where trees reduce albedo and increase climate warming (3, 4); and (iii) relied heavily on afforestation of grasslands and savannas—biodiverse ecosystems where fires and large herbivores have maintained low tree cover for millions of years (5, 6).

Bastin et al.’s inflation of soil carbon gains resulted in a ~98 GtC overestimate of potential carbon sequestration (Table 1). They mistakenly assumed that treeless areas have no soil organic carbon (SOC) and that SOC increases in direct (1:1) proportion to tree cover. The contribution of SOC to total carbon stocks is substantial in most terrestrial ecosystems. In humid tropical savannas, for example, 86% of all carbon is in soils (174 tonnes of SOC per hectare) (7). In boreal forests, 64% of carbon occurs in soils (8). North American grasslands can store as much carbon in soil (9) as tropical forests store as biomass (8). In Table 1, we display SOC-corrected carbon sequestration estimates that use more realistic (literature-derived) values for the changes in SOC that occur with afforestation and reforestation.

Table 1 Corrected estimates of the potential for increased tree cover to sequester carbon and mitigate climate change. We corrected Bastin et al.’s estimate (205 GtC) to represent realistic gains or losses of soil organic carbon (SOC) that occur with increased tree cover in each biome [based on (9, 16–21)]. We then excluded biomes (assigned a value of 0 GtC) where tree planting for climate change mitigation should not occur because of unintended consequences (e.g., net warming from reduced albedo or loss of biodiversity). Although we disagree with several of the carbon density values used by Bastin et al. [e.g., they applied values for intact tropical forests (8) to estimate second-growth forest biomass, and applied values from humid tropical savannas (7) to deserts and tundra], we retained these values to demonstrate the magnitude of the SOC and biome corrections. View this table:

In addition to the SOC overestimate, Bastin et al. did not account for the warming effect of trees due to decreased albedo (3, 4). Trees, particularly evergreen conifers, are less reflective than snow, bare ground, or grasses, and thus absorb more solar energy, which is ultimately emitted as heat. At high latitudes and elevations, the warming effect of trees is greater than their cooling effect via carbon sequestration (3, 4). Similarly, trees planted in low-latitude, semi-arid regions can produce net warming for decades before carbon sequestration benefits are realized (10). Because, at a minimum, carbon from trees planted in boreal forests, tundra, or montane grasslands and shrublands should not be counted as climate change mitigation (Bastin et al. counted a SOC-corrected 17 GtC), in Table 1 we provide a corrected estimate that excludes these biomes.

The carbon sequestration estimate of Bastin et al. is also dependent on the false assumption that natural grasslands and savannas with fewer trees than predicted by their statistical model are “degraded” and in need of restoration (11). Ecological restoration of savannas and grasslands rarely involves planting trees, and more often requires tree-cutting and prescribed fire to promote biodiversity and ecosystem services (12). Yet after correcting for SOC, 46% of the carbon sequestration estimate of Bastin et al. comes from increased tree cover in grasslands, savannas, and shrublands (Table 1). Among all biomes, tropical grasslands are the largest contributor to Bastin et al.’s estimate of potential carbon sequestration (SOC-corrected 40 GtC or 37% of the global potential; Table 1).

Although Bastin et al.’s model, developed with climate and soil data in protected areas, may be reasonable in some of the driest and wettest places on Earth, any statistical approach to predict tree cover at intermediate precipitation (500 to 2500 mm annually) must include the effects of fire and, where they still exist, large grazing and browsing animals (13). Because Bastin et al. failed to account for fire, their model had low predictive power across many of the open-canopy biomes they analyzed, as shown by their own uncertainty analysis. Although we commend their intent to respect the “natural ecosystem type” by training their machine-learning algorithm on protected areas, they map many of these same areas—particularly those with grassland-forest mosaics (e.g., Yellowstone National Park, USA)—as opportunities for tree planting. Of additional concern, their method of interpolation between protected areas misrepresents some enormous savanna regions (e.g., western Los Llanos in Colombia is targeted for 75 to 100% tree cover), presumably because the protected areas are located in adjacent tropical forests, not savannas.

Bastin et al.’s model suggesting grasslands and savannas as potential sites for restoration using trees is inaccurate and misguided. Earth’s savannas and grasslands predate humans by millions of years; their formation is a result of complex ecological and evolutionary interactions among herbaceous plants (grasses and forbs with extensive roots and underground storage organs), environmental change (climatic cooling, drying, changes in atmospheric CO 2 ), fires (first ignited by lightning, then by people), and large herbivores (5, 6). These ecosystems and their iconic species are already gravely threatened by fire exclusion and afforestation, processes that replace species-diverse biotic communities with lower-diversity forests (14). Carbon-focused tree planting will exacerbate these threats, to the detriment of people who depend on grasslands to provide livestock forage, game habitat, and groundwater and surface-water recharge (11). Moreover, trees planted in grasslands will be prone to carbon loss from fires. Because these detrimental effects should preclude tree planting in grasslands, savannas, and shrublands, we excluded these biomes from Bastin et al.’s estimate in Table 1.

In combination, our corrections for SOC and corrections to avoid the unintended consequences of misguided tree planting (i.e., warming and biodiversity loss with afforestation) would reduce Bastin et al.’s estimate of potential carbon sequestration by a factor of 5, to the still-substantial amount of ~42 GtC (Table 1). Although ecological restoration, if carefully implemented, can have a role in mitigating climate change, it is no substitute for the fact that most fossil fuel emissions will need to stop to meet the targets of the Paris Agreement (15). Such action should be accompanied by policies that prioritize the conservation of intact, biodiverse ecosystems, irrespective of whether they contain a lot of trees.

References ↵ J. F. Bastin , Y. Finegold , C. Garcia , D. Mollicone , M. Rezende , D. Routh , C. M. Zohner , T. W. Crowther , The global tree restoration potential . Science 365 , 76 – 79 ( ). doi: 10.1126/science.aax0848 pmid: 31273120 OpenUrl ↵ C. Le Quéré , R. M. Andrew , P. Friedlingstein , S. Sitch , J. Hauck , J. Pongratz , P. A. Pickers , J. I. Korsbakken , G. P. Peters , J. G. Canadell , A. Arneth , V. K. Arora , L. Barbero , A. Bastos , L. Bopp , F. Chevallier , L. P. Chini , P. Ciais , S. C. Doney , T. Gkritzalis , D. S. Goll , I. Harris , V. Haverd , F. M. Hoffman , M. Hoppema , R. A. Houghton , G. Hurtt , T. Ilyina , A. K. Jain , T. Johannessen , C. D. Jones , E. Kato , R. F. Keeling , K. K. Goldewijk , P. Landschützer , N. Lefèvre , S. Lienert , Z. Liu , D. Lombardozzi , N. Metzl , D. R. Munro , J. E. M. S. Nabel , S. Nakaoka , C. Neill , A. Olsen , T. Ono , P. Patra , A. Peregon , W. Peters , P. Peylin , B. Pfeil , D. Pierrot , B. Poulter , G. Rehder , L. Resplandy , E. Robertson , M. Rocher , C. Rödenbeck , U. Schuster , J. Schwinger , R. Séférian , I. Skjelvan , T. Steinhoff , A. Sutton , P. P. Tans , H. Tian , B. Tilbrook , F. N. Tubiello , I. T. van der Laan-Luijkx , G. R. van der Werf , N. Viovy , A. P. Walker , A. J. Wiltshire , R. Wright , S. Zaehle , B. Zheng , Global carbon budget 2018 . Earth Syst. Sci. Data 10 , 2141 – 2194 ( ). doi: 10.5194/essd-10-2141-2018 OpenUrl CrossRef ↵ Y. Li , M. Zhao , S. Motesharrei , Q. Mu , E. Kalnay , S. Li , Local cooling and warming effects of forests based on satellite observations . Nat. Commun. 6 , 6603 ( ). doi: 10.1038/ncomms7603 pmid: 25824529 OpenUrl CrossRef PubMed ↵ P. M. Mykleby , P. K. Snyder , T. E. Twine , Quantifying the trade‐off between carbon sequestration and albedo in midlatitude and high‐latitude North American forests . Geophys. Res. Lett. 44 , 2493 – 2501 ( ). doi: 10.1002/2016GL071459 OpenUrl CrossRef ↵ C. A. E. Strömberg , Evolution of grasses and grassland ecosystems . Annu. Rev. Earth Planet. Sci. 39 , 517 – 544 ( ). doi: 10.1146/annurev-earth-040809-152402 OpenUrl CrossRef Web of Science ↵ J. W. Veldman , E. Buisson , G. Durigan , G. W. Fernandes , S. Le Stradic , G. Mahy , D. Negreiros , G. E. Overbeck , R. G. Veldman , N. P. Zaloumis , F. E. Putz , W. J. Bond , Toward an old‐growth concept for grasslands, savannas, and woodlands . Front. Ecol. Environ. 13 , 154 – 162 ( ). doi: 10.1890/140270 OpenUrl CrossRef ↵ J. Grace , J. S. José , P. Meir , H. S. Miranda , R. A. Montes , Productivity and carbon fluxes of tropical savannas . J. Biogeogr. 33 , 387 – 400 ( ). doi: 10.1111/j.1365-2699.2005.01448.x OpenUrl CrossRef Web of Science ↵ Y. Pan , R. A. Birdsey , J. Fang , R. Houghton , P. E. Kauppi , W. A. Kurz , O. L. Phillips , A. Shvidenko , S. L. Lewis , J. G. Canadell , P. Ciais , R. B. Jackson , S. W. Pacala , A. D. McGuire , S. Piao , A. Rautiainen , S. Sitch , D. Hayes , A large and persistent carbon sink in the world’s forests . Science 333 , 988 – 993 ( ). doi: 10.1126/science.1201609 pmid: 21764754 OpenUrl ↵ R. B. Jackson , J. L. Banner , E. G. Jobbágy , W. T. Pockman , D. H. Wall , Ecosystem carbon loss with woody plant invasion of grasslands . Nature 418 , 623 – 626 ( ). doi: 10.1038/nature00910 pmid: 12167857 OpenUrl CrossRef PubMed Web of Science ↵ E. Rotenberg , D. Yakir , Contribution of semi-arid forests to the climate system . Science 327 , 451 – 454 ( ). doi: 10.1126/science.1179998 pmid: 20093470 OpenUrl ↵ J. W. Veldman , G. E. Overbeck , D. Negreiros , G. Mahy , S. Le Stradic , G. W. Fernandes , G. Durigan , E. Buisson , F. E. Putz , W. J. Bond , Where tree planting and forest expansion are bad for biodiversity and ecosystem services . Bioscience 65 , 1011 – 1018 ( ). doi: 10.1093/biosci/biv118 OpenUrl CrossRef ↵ E. Buisson , S. Le Stradic , F. A. O. Silveira , G. Durigan , G. E. Overbeck , A. Fidelis , G. W. Fernandes , W. J. Bond , J.-M. Hermann , G. Mahy , S. T. Alvarado , N. P. Zaloumis , J. W. Veldman , Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands . Biol. Rev. Camb. Philos. Soc. 94 , 590 – 609 ( ). doi: 10.1111/brv.12470 pmid: 30251329 OpenUrl CrossRef PubMed ↵ A. C. Staver , S. Archibald , S. A. Levin , The global extent and determinants of savanna and forest as alternative biome states . Science 334 , 230 – 232 ( ). doi: 10.1126/science.1210465 pmid: 21998389 OpenUrl ↵ R. C. R. Abreu , W. A. Hoffmann , H. L. Vasconcelos , N. A. Pilon , D. R. Rossatto , G. Durigan , The biodiversity cost of carbon sequestration in tropical savanna . Sci. Adv. 3 , e1701284 ( ). doi: 10.1126/sciadv.1701284 pmid: 28875172 OpenUrl ↵ IPCC, Global Warming of 1.5°C: An IPCC Special Report on the Impacts of Global Warming of 1.5°C Above Pre-Industrial Levels and Related Global Greenhouse Gas Emission Pathways, in the Context of Strengthening the Global Response to the Threat of Climate Change, Sustainable Development, and Efforts to Eradicate Poverty, V. Masson-Delmotte et al., Eds. (World Meteorological Organization, Geneva, 2018). ↵ O. V. Menyailo , B. A. Hungate , W. Zech , Tree species mediated soil chemical changes in a Siberian artificial afforestation experiment . Plant Soil 242 , 171 – 182 ( ). doi: 10.1023/A:1016290802518 OpenUrl CrossRef A. Don , J. Schumacher , A. Freibauer , Impact of tropical land‐use change on soil organic carbon stocks–a meta‐analysis . Glob. Change Biol. 17 , 1658 – 1670 ( ). doi: 10.1111/j.1365-2486.2010.02336.x OpenUrl CrossRef C. Poeplau , A. Don , L. Vesterdal , J. Leifeld , B. Van Wesemael , J. Schumacher , A. Gensior , Temporal dynamics of soil organic carbon after land‐use change in the temperate zone–carbon response functions as a model approach . Glob. Change Biol. 17 , 2415 – 2427 ( ). doi: 10.1111/j.1365-2486.2011.02408.x OpenUrl CrossRef Web of Science K. Makoto , S. V. Bryanin , V. V. Lisovsky , K. Kushida , N. Wada , Dwarf pine invasion in an alpine tundra of discontinuous permafrost area: Effects on fine root and soil carbon dynamics . Trees 30 , 431 – 439 ( ). doi: 10.1007/s00468-015-1192-5 OpenUrl CrossRef J. P. Zhang , C. D. Shen , H. Ren , J. Wang , W. D. Han , Estimating change in sedimentary organic carbon content during mangrove restoration in southern China using carbon isotopic measurements . Pedosphere 22 , 58 – 66 ( ). doi: 10.1016/S1002-0160(11)60191-4 OpenUrl CrossRef ↵ M. Hoogmoed , S. C. Cunningham , J. R. Thomson , P. J. Baker , J. Beringer , T. R. Cavagnaro , Does afforestation of pastures increase sequestration of soil carbon in Mediterranean climates? Agric. Ecosyst. Environ. 159 , 176 – 183 ( ). doi: 10.1016/j.agee.2012.07.011 OpenUrl CrossRef