In recent years, there has been increasing discussion of the potential role of afforestation and reforestation in addressing climate change by effectuating the removal of large amounts of carbon dioxide from the atmosphere. Indeed, a number of recent studies have indicated that tree planting could contribute substantially to meeting the temperature targets of the Paris Agreement. Most prominently, modeling by Bastin et al. projects that increasing forest cover by 25 percent could result in the storage of a quarter of current atmospheric carbon pools. In the United States, this had led to both legislative and executive initiatives to drive massive tree-planting programs. Four U.S. senators recently introduced the Trillion Trees and Natural Carbon Storage Act, which, inter alia, calls for support of the 1t.org initiative, an international program for reforestation of a trillion trees. In October 2020, President Donald Trump signed an executive order establishing the One Trillion Trees Interagency Council to help further the federal government’s contribution to the World Economic Forum’s One Trillion Trees initiative.
However, while tree planting assuredly may have a modest role to play in the battle against climate change, the vision of a “trillion trees” could prove to be chimerical, as well as counterproductive from an environmental and social justice perspective.
Massive Tree Planting and Climate Change: Potential and Risks
Projections of massive potential carbon sequestration from large tree-planting initiatives suffer from a number of methodological errors. First, most of these studies fail to take into account the carbon currently stored in the ecosystems that would be converted to forests, overestimating potential carbon sequestration by 85 percent or more. Moreover, to avoid land-use conflicts, the Bastin et al. study, and others, assume that as many as 25 percent of the trees would be planted in ice-covered regions, including in Alaska, Canada, Finland, and Siberia. However, this would substantially reduce surface albedo (reflectivity), which would result in the absorption of much more incoming solar radiation, potentially negating most, or all, of the cooling benefits of planting trees in these regions, or perhaps even exerting a net-warming effect. This could also be the case for tree planting in mid-latitude regions.
Tree-planting programs also often occur on agricultural lands. This can result in agricultural production being displaced to native forest areas, resulting in deforestation, and the release of substantial amounts of carbon. One recent study projects that a mass tree-planting initiative could displace 2.5 billion hectares of agricultural land.
Finally, mass forest planting programs in countries such as Chile have resulted in expansion of plantations into native forest regions, displacing native species and substantially reducing sequestration of carbon.
There are also serious questions about the permanence of carbon sequestration through reforestation and afforestation. A recent study in the journal Science concluded that nearly half of the world’s forests are susceptible to “stand-replacing disturbances”––that is, factors such as wildfires, drought, and disease that can imperil their long-term existence. Forests, especially plantation stands of pine and eucalyptus, are highly vulnerable to fires, and will become far more susceptible to loss as temperatures increase due to climate change. Massive forest fires in places such as California, Oregon, and Australia in recent years, for example, have released huge amounts of carbon, including in forest stands designated for carbon sequestration. In 201 to 2016, California experienced a severe drought that converted California’s forests from a net sink to a net source, releasing more than 600 million tons of carbon dioxide into the atmosphere. Bark beetles in Canada, whose populations have been fortified by climate change, have killed billions of trees over the past few decades. Finally, as we have seen in Brazil recently, forest gains may be dramatically reversed by new governments with different priorities.
In areas where forests do not naturally thrive, such as savannas prone to drought and fire risk, afforestation may reduce resilience to climate change, and could thus compromise long-term climate storage. Moreover, poor project planning, and a lack of seedling maintenance in tree planting programs often results in massive tree losses within a few years.
Ill-designed mass tree planting programs may also have serious environmental implications. Tree-planting rushes, such as the Bonn Challenge, which seeks to plant over 350 million hectares of forests by 2030, are targeting many areas incorrectly identified as “degraded” or “unproductive.” This includes vast areas of open ecosystems, such as savannas, grasslands, and shrubs, which are predominant in developing countries. Overall, as much as 50 percent of planned international sequestration contemplates planting trees in savannas and grassland areas. However, these areas are far from “unproductive,” often harboring high levels of biodiversity, particularly in developing countries. Large-scale afforestation programs could substantially denude species-diverse biotic communities in such regions. Moreover, planting forests in these ecosystems could deplete groundwater, which could severely undermine the dry season water supply for many local communities in the South.
There are also potential social justice and economic implications associated with tree-planting programs. Many fear that seeking to afforest/reforest an area potentially equivalent to half the area of Africa or more would result in massive land grabs from vulnerable populations that rely on this land for their livelihoods. Such land grabs, often involving violence, have occurred in the past in the quest to sell credits on carbon markets.
Overall, many forest policy researchers believe that the upper potential of sustainable forest carbon storage globally is no more than one to two billion tons of carbon dioxide per year. One study has concluded that a “realistic” assessment of the potential of tree planting programs yields a mere 0.25-.0.45°C reduction in temperatures relative to projections for 2100. The time has come to acknowledge, as one commentator concluded, what while “forests can certainly be a useful part of a climate change solution . . . we’re not to going to plant our way out of the climate crisis.”
Rather than focus on dubious methods for achieving optimal levels of carbon sequestration from tree planting, the following policy recommendations avoid the risks associated with indiscriminate reforestation/afforestation initiatives.
Tree-planting initiatives should not target savannas and grassland ecosystems for conversion to forests. Many of the studies that advocate planting a trillion trees rely heavily on savannas and grasslands for planting, on the specious grounds that these areas are “degraded.” However, these areas often support extremely high levels of biodiversity. Moreover, ecological restoration of savannas and grasslands rarely involves planting trees. To the contrary, effective programs often require tree-cutting and prescribed fire to promote biodiversity and ecosystem services. A focus on these areas, largely concentrated developing countries, could be yet another plan to exploit tropical countries. At the end of the day, greater fire potential in such ecosystems, as well the presence of tree-killing mega-fauna such as elephants suggests that tree planting will provide less long-term carbon sequestration.
Tree-planting programs should focus on the planting of mixed native species to protect biodiversity and preserve ecosystem services. By contrast, many mass tree-planting initiatives simplistically emphasize “tree cover” without taking into account the implications of planting large stands of non-native monocultural species in terms of factors such as species richness and preservation of water resources.
Tree-planting programs should seek to foster further development of the agroforestry sector. Agroforestry is the intentional incorporation of trees and shrubs into crop and animal farming systems. While it is already widely practiced throughout the world, the Intergovernmental Panel on Climate Change has estimated that 20 percent of unproductive agricultural land could be converted to agroforestry, potentially sequestering up to 3.4 billion tons of carbon dioxide annually.
Robust funding for research into other carbon dioxide removal options is critical. Many scientists believe that achieving the Paris Agreement’s temperature targets may require both aggressive reductions of greenhouse gas emissions and sequestration of ten to twenty billion tons of carbon dioxide annually by the middle of the century. This argues in favor of more federal support for research, development, and deployment of other so-called carbon dioxide removal options that might help us to supplement tree-planting initiatives. Direct Air Capture is a technology that can draw in ambient air and extract the carbon dioxide, while returning the rest of the air to the environment. The CO2 can then be stored underground, or used, such as for green fuel production. Bioenergy with Carbon Capture and Storage (BECCS) is a technological process that uses biomass for energy production and captures the carbon dioxide emissions for storage or use. Biochar is produced by the burning of organic materials in a low or no oxygen environment. This form of charcoal can store carbon dioxide for hundreds, or thousands of years. While in the early stages of research, there are also several potentially promised ocean-based approaches that could store huge amounts of carbon, and some of which could help us to address the threat posed by ocean acidification.
In conclusion, support for tree-planting initiatives could help to foster a bipartisan effort to address climate change in the United States. However, it is important to ensure that we proceed in a manner that follows the science and ensures sustainability.