Moreover, this figure likely over-estimates both the potential for forest carbon capture ( Lewis et al., 2019a) and the availability of suitable land and water for reforestation ( Veldman et al., 2019). However, it would take over 100 years to reach this C storage potential, assuming a typical C allocation rate into wood of 2 tC ha –1 year –1 ( Bonan, 2008). A recent analysis suggested that planting trees on an additional 0.9 billion hectares could capture 205 GtC ( Bastin et al., 2019), which is approximately one-third of total anthropogenic emissions thus far (∼600 GtC). Together, extant old-growth and regenerating forests absorb ~2 gigatonnes of carbon (GtC) annually, making an important contribution to the terrestrial carbon sink ( Pugh et al., 2019). How Much Carbon Can Forests Capture?Ĭurrently existing forests store ~45% of the organic carbon on land in their biomass and soils ( Bonan, 2008). However, as we discuss in detail below, realizing these co-benefits requires site-specific attention to forest management techniques, and careful consideration of the landscape context of new forests. Forests also provide important ecosystem services and generate wood products that can displace more fossil-fuel intensive materials. Reforestation and afforestation compare favorably with other negative emissions technologies in terms of carbon capture potential, although land and water requirements are often high ( Smith et al., 2016). However, the potential magnitude of carbon uptake by newly planted trees is a topic of intense debate. Total carbon capture associated with afforestation and reforestation can be enhanced by substituting long-lived harvested wood products for steel, cement, and aluminum, and by using harvest residues as bioenergy to replace fossil fuels.įorests established by reforestation (planting trees on formerly forested land) and afforestation (planting trees where they historically did not exist) can enhance the terrestrial carbon sink, thereby slowing accumulation of CO 2 in the atmosphere. Planting species mixtures frequently increases productivity, reduces disturbance impacts, and enhances biodiversity relative to monocultures. Afforestation is likely to mitigate emissions most effectively when trees are planted in formerly forested, high-productivity sites, commonly found in tropical or sub-tropical ecosystems. Newly planted forests can create “carbon debts” that take significant time to be repaid. Policy-makers must avoid generating “perverse incentives” that can compromise or even destroy existing carbon sinks in forests, savannas, grasslands and peatlands. Mature natural forests provide significant additional benefits and must be conserved, whilst regeneration of secondary natural forests is promoted. Natural forests store more carbon than plantation forests, due to complex stand structures and accumulation of carbon belowground and in the forest floor. Here, we assess the potential impact of reforestation and afforestation on the global climate system, and identify ecological, economic, and societal implications of such efforts. Because growing forests capture CO 2 in their biomass and soils, large-scale tree planting efforts have been advertised as a viable way to counteract anthropogenic emissions as part of net-zero emission strategies. The severe consequences of human disruptions to the global carbon cycle have prompted intense interest in strategies to reduce atmospheric CO 2 concentrations. 7Grantham Institute and Department of Earth Science and Engineering, Imperial College London, London, United Kingdom.6Institute of Biological and Environmental Sciences, University of Aberdeen, Aberdeen, United Kingdom.5Department of Chemistry and Biotechnology, Swinburne University, Hawthorn, VIC, Australia.4Department of Life Sciences, Silwood Park, Imperial College London, London, United Kingdom.3Institute of Silviculture, University of Natural Resources and Life Sciences, Vienna, Austria.2Grantham Institute and Department of Life Sciences, Silwood Park, Imperial College London, London, United Kingdom.1Department of Biology, Utah State University, Logan, UT, United States.Bonnie Waring 1,2* Mathias Neumann 3 Iain Colin Prentice 4 Mark Adams 5 Pete Smith 6 Martin Siegert 7
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