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The potentials and limitations of tree plantings as a climate solution


In 2020, the World Economic Forum launched an initiative to plant one trillion trees to tackle climate change. Similar initiatives can be found across the world, yet the international conversation on tree plantings as a solution to reducing CO2 emissions in the atmosphere and mitigating the rise in global surface temperature is mixed, with judgments ranging from helpful to harmful. For example, an article in The Guardian says tree planting “has mind-blowing potential” to tackle climate change, whereas an article in Slate says it won’t stop climate change and may “do more harm than good.”

To better understand the role of tree plantings as a climate solution, Climate Feedback reached out to scientific experts in the field. In this Insight article, we examine the benefits and limitations of tree planting as a climate solution based on comments from reviewers and current scientific evidence. We highlight the complex role tree planting plays in addressing climate change and how focusing too heavily on planting trees can distract from other important climate solutions.

SUMMARY


Overall, the ten scientists who provided comments emphasized that the most important climate solution is reducing greenhouse gas emissions[1]. They also highlighted that avoiding deforestation is a cheaper and more effective climate solution than planting new forests (see figure below)[2,3,4]. “First and foremost, reducing greenhouse gas emissions from fossil fuel combustion and deforestation are far and away the best climate solutions,” said William Anderegg, Assistant Professor at the University of Utah.

Figure—An assessment of cost-effective natural climate solution strategies in Latin America, Africa, and Asia. While all pathways demonstrated positive benefits, protecting forests, peatlands, and mangroves provided twice the climate mitigation potential as reforestation. From Griscom et al. (2020)[3].

Although tree planting is a potentially powerful tool in addressing climate change, it is not a panacea. “Tackling climate change is like building a house, we need every tool in the toolbox,” said Susan Cook-Patton, Forest Restoration Scientist at The Nature Conservancy. “Tree planting is like a screwdriver. Useful tool? Absolutely! Can you build a house with a screwdriver alone? Absolutely not.”

The benefits and limitations of tree planting as a climate solution vary considerably depending on geographic region, local ecology, social context, planting approach, and a multitude of other factors[4,5]. Although scientists are beginning to unravel the trade-offs of planting trees as a climate solution, the net climate effects are not fully understood yet.

Despite this complexity, scientists who provided comments agreed that the focus on tree planting can be a distraction from other important climate solutions and natural carbon sinks. “There’s no possible way that we can plant our way out of climate change,” said Anderegg.

The benefits of planting trees

Planting trees in areas that were previously forested (reforestation) and in areas with no prior tree cover (afforestation) are two relatively cheap and readily available solutions to help mitigate the effects of climate change caused by greenhouse gas emissions[6]. Trees actively remove carbon from the atmosphere as they grow and can store this carbon for long periods of time[7]. As a result, forests generally function as a carbon sink and are currently estimated to absorb 4.0 billion tons of CO2 per year[8].

Along with carbon sequestration, trees support biodiversity, stabilize soil, prevent flooding, enhance habitat connectivity, and increase resilience to climate impacts[2,4,5]. In urban areas, trees can provide additional benefits. “Trees have exceptional powers against climate threats, not so much through the absorption of CO2, which is extremely limited in comparison to the emissions of a city, but by cooling the air during heat waves, absorbing part of the pollutants and fine dust, and reducing the outflow of surface waters,” said Giorgio Vacchiano, Assistant Professor at the Università di Milano.

For example, trees have been shown to reduce average land surface temperature in temperate, arid, and tropical regions (see figure below)[9,10].

Figure—Changes in land and air surface temperature as a result of changes in forest cover from 2003 to 2012 in four different regions around the world (A). In temperate, arid, and tropical regions, afforestation reduced mean air and land surface temperatures (blue shaded region in B and dark red lines in C and D). From Alkama et al. (2016)[10]. 

Reforestation and afforestation efforts also have the potential to serve as a sustainable source of food and income for local communities if these efforts are implemented appropriately[4,8,11,12]. “Biodiversity, water infiltration, drought resistance and other factors should be considered in tree planting efforts, as well as its influence on the local community,” said Sara Vicca, Postdoctoral Research Fellow at the University of Antwerp.

The limitations of planting trees

While there are many benefits to planting trees as a climate solution, there are several limitations as well. Tree planting can only offset a fraction of the carbon emitted by humans each year. “Global fossil fuel emissions totaled about 9.5 Gt C per year from 2009-2018, while Earth’s land ecosystems (i.e., forests, grasslands) sequestered about 3.2 Gt C per year from the atmosphere[13],” said Logan Berner, Assistant Research Professor at Northern Arizona University. “Thus, burning fossil fuels emitted about three times as much carbon each year as was taken up by all of Earth’s land ecosystems!”

In addition, the process of carbon sequestration is slow and not a guarantee over the long term. “Trees only capture carbon as long as they are alive,” said Valerie Trouet, Associate Professor at the University of Arizona. “As soon as a tree dies, all the tree planting and carbon capture efforts have been for naught, unless the wood (in which the carbon has been captured) is used for other long-term storage uses, such as construction.”

Carbon can also be released from trees that are lost due to fires, droughts, floods, pest outbreaks, and land conversion[2,8,14]. “As temperatures rise and precipitation patterns change, large areas are becoming unsuitable habitat for trees due to direct water stress and indirectly through fires and insect outbreaks,” said Karen Holl, Professor at the University of California, Santa Cruz.

Forests also saturate in their ability to sequester carbon[7,15]. “The biggest limitation is that carbon uptake by trees cannot be sustained [forever]—forests can only take up so much carbon before they saturate and provide no further uptake,” said Erle Ellis, Professor at the University of Maryland, Baltimore County.

Finally, reforestation and afforestation efforts often favor single-species plantings over restoring native forests, which can lead to the global spread of monocultures and invasive tree species[12,16,17]. As described in Seddon et al. (2019), “pledges tend to focus more on the extent rather than the quality of forest to be protected, afforested, or reforested. The approach encourages the establishment of monoculture plantations of fast growing species, including exotics.”[16]

As a result, some tree planting efforts can replace native ecosystems and reduce biodiversity. “A large body of literature shows that even the best planned restoration projects rarely fully recover the biodiversity of intact forest,” as described in Holl et al. (2020)[4].

Reforestation and afforestation can also negatively impact local communities if they remove land available for food production, reduce water supply, increase social inequity, or displace people from the land[4,8,16,18].

The climate effects of tree plantings depend on context

“It all comes down to planting the right trees in the right place in the right way,” said Cook-Patton.

Estimates of carbon sequestration from tree plantings can vary by a factor of 10 depending on which tree species are planted, tree age, the amount of land reforested, how long the trees remain on the land, and where they are planted[2,4,6,18].

Trees also have an array of biophysical effects on the environment, including lowering albedo, emitting volatile compounds and methane, altering the roughness of the landscape, and increasing evapotranspiration[19-22]. “Tree planting in temperate and tropical ecosystems will likely lead to overall cooling since the effects of carbon sequestration outweigh the effects of lower surface albedo; however, this is not the case in boreal forests,” said Berner. Specifically, in boreal regions, planting trees can increase warming because forests reflect less sunlight than the snow-covered landscapes they replace (i.e. lower albedo)[22].

Trees also alter the roughness of the landscape, which affects the amount of energy and water transferred to the atmosphere[20,22]. As described in Anderson et al. (2010), “Tropical forests have high rates of transpiration that contribute to cloud formation, considerably reducing both surface temperatures and the amount of sunlight reaching the Earth’s surface.”[22] The biophysical effects of trees vary depending on location and forest type, shaping the overall climate impact of tree plantings(see figure below)[21,22].

Figure—An assessment of the biogeochemical (carbon storage and carbon exchange) and biophysical (net radiation – Rnet and latent heat flux – LE) climate services provided by 18 natural and agricultural ecoregions across the Americas. Biogeochemical (first column) and biophysical (second column) effects were estimated by comparing changes in surface energy relative to a bare-ground baseline if vegetation was cleared over 50 years. The third column combines the biogeochemical and biophysical changes to estimate overall climate regulation values (CRVs). Natural forests in tropical and temperate regions have the greatest net positive effect on climate regulation. From Anderson-Teixeira et al. (2012)[21].

Tree planting and restoration efforts must also be designed, implemented, and monitored effectively over time. “Whether a forest is managed or not is more important than whether a forest is planted or naturally regenerated,” said Sebastiaan Luyssaert, Associate Professor at Vrije Universiteit Amsterdam.

Long-term commitments to protecting, monitoring, and managing reforested lands are important determinants of how effective tree plantings are as a climate solution[4,8,20]. “People often focus on the biophysical issues of tree planting but social issues are equally important. If tree planting efforts are going to have the desired impacts over the long-term then stakeholders need to be engaged in the efforts or the trees will be re-cleared,” said Holl.

Tree planting can distract from other climate solutions

Scientists agreed that the focus on tree planting as a climate solution has the potential to distract from other important approaches, especially the reduction of fossil fuels and avoiding deforestation. “Solving climate change is not about how to reengineer the biosphere with more forests to take up industrial carbon pollution. The problem is how to end industrial carbon emissions, which are the primary cause of climate change,” said Ellis.

“Tree planting can be a distraction, particularly when people view it as a simple solution. We have to make rapid and dramatic changes to our behavior and infrastructure to reduce greenhouse gas emissions to slow climate change. Planting trees can be a small piece of the effort but it is not a silver bullet. People perceive it as a cheap and easy way to address climate change and it is much more complicated than it seems,” said Holl.

Scientists also emphasized that tree planting is only one aspect of reforestation. Although planting trees may be essential in degraded sites, it is not always necessary. “In places where there are seed sources, nearby forest patches, good growing conditions, a forest can grow back perfectly well on its own without tree planting and at a fraction of the cost and effort,” said Cook-Patton.

Conserving other natural carbon storage ecosystems, such as wetlands and peatlands, is another climate solution that can be overshadowed by discussions of tree plantings. For example, peatlands hold 25% of the world’s carbon but only cover 2-3% of its land area[16]. “It would make no sense to drain peatland to plant forests—this would cause a massive loss of carbon. Instead re-wetting of peatland and flooding old wetlands can have huge positive carbon and biodiversity effects,” said Mark Maslin, Professor at the University College London.

While tree planting can assist in climate mitigation if done carefully, reducing greenhouse gas emissions and deforestation are the most important climate solutions. The complex benefits and limitations of tree plantings depend on geographic region, local ecology, social context, planting approach, and a variety of other factors. Because of this, scientists are still working to fully understand the net climate effects of tree plantings.

Q&A WITH SCIENTISTS


1. Are the claims that tree planting is among the best climate solutions scientifically accurate? 

William Anderegg, Associate Professor, University of Utah:
Planting trees (often termed reforestation) can be one important climate solution and it has a lot of co-benefits for humanity, such as habitat for biodiversity, water and air purification, and tourism, among others. However, the science is still fairly shaky on whether it is “among the best climate solutions.” First and foremost, reducing greenhouse gas emissions from fossil fuel combustion and deforestation are far and away the best climate solutions. Reforestation is part of a set of “carbon removal” technologies, but it’s not clear that it’s one of the best. It is likely one of the cheaper technologies, but there are a number of really crucial elements that have to be assessed for reforestation to work well as a climate solution. Most of the studies to date that claim reforestation is among the best climate solutions have, by and large, ignored these crucial elements. Three central ones – trees are only climate-cooling in some regions of the world (i.e. planting trees in very Northern latitudes will actually warm the climate because they are darker than the snow below them), tree planting has inherent constraints in that it would have lots of problems if it conflicted with land use for growing food, and finally, tree planting as a carbon removal technology is inherently very slow (trees take a long time to grow) and risky – climate change could very easily cause lots of carbon in these forests to go back into the atmosphere as fires, droughts, and pests hit forests.

Erle Ellis, Professor, University of Maryland, Baltimore County:
In a word, No. Tree growth can only slow down carbon accumulation in the atmosphere, not stop it. Ultimately, the only solution to climate change is to stop burning fossil fuels, which in turn requires a transition to inexpensive clean energy.

Karen Holl, Professor, University of California, Santa Cruz:
As Dr. Brancalion and I discuss in our brief article and a subsequent article that is currently in review, increasing tree cover is part of the solution to climate change but it is just one piece of the puzzle[4]. The most important steps are first reducing greenhouse gas emissions and second protecting existing forests and other high carbon storage ecosystems (e.g. wetlands). A few key points here:

First, planting trees is not going to solve the problem of climate change if we don’t reduce greenhouse gas emissions. As temperatures rise and precipitation patterns change, large areas are becoming unsuitable habitat for trees due to direct water stress and indirectly through fires and insect outbreaks. Moreover, some existing forests are slowing in their capacity to uptake carbon.

Second, it is important to protect existing forests first. Existing forests, rain forests in particular, store large amounts of carbon both above- and below-ground. As a restoration ecologist who has spent over 25 years studying how to restore forests, I know that it is nearly impossible to restore exactly what was there before, and it makes much more ecological and economic sense to protect existing forests so that should be the priority. Yet, large scale forest clearing continues worldwide. For example, in the U.S. the Trump administration is proposing to allow logging in the Tongass National Forest, at the same time that they have expressed support for the Trillion Trees Act. Temperate rain forests store huge amounts of carbon. Likewise, in Brazil there are extensive efforts to restore the Atlantic forest ecosystem in southeastern Brazil at the same time that deforestation and forest fires have increased substantially in the Amazon in the past couple of years. Efforts should focus on reducing the drivers of deforestation and enforcing existing legislation that protects forest.

Third, it is important to distinguish between tree planting and increasing tree cover or forest cover. Many forests are able to recover on their own if degrading factors are removed, so often it is not necessary to plant trees[17]. Allowing for natural recovery should be the first option for restoring forests, as it is more ecologically and economically sound. If it is necessary to plant trees either due to slow recovery or for socioeconomic reasons then the focus should be on restoring tree cover over the long-term. Many of these large-scale planting schemes focus on the number of trees put in the ground without considering who is going to care for and monitor the trees and whether the social conditions are in place for the trees to not be cleared. This has led to many massive tree planting failures. It also means that the cost of planting trees is substantially underestimated. The cost needs to include not only growing the seedling, digging the hole and planting the tree, but management such as clearing competitive vegetation and monitoring the outcomes of tree planting over the longer term.

Sebastiaan Luyssaert, Associate Professor, Vrije Universiteit Amsterdam:
As early as the late 1970s afforestation was proposed as a means to buy time. The initial proposition was to sequester part of the emission through afforestation while developing (technical) solutions to reduce the emissions. Since then no new evidence became available to question the approach in which forests are useful to buy time but are not a solution in themselves. Contrary to this some of the evidence that became available suggests that the forest carbon is transient.

Afforestation, forest management, forest conservation all come with considerable benefits (could be cheap, community-based, enhance welfare, enhance wealth) and it is beyond doubt that under the right conditions afforestation, forest management, forest conservation can all sequester carbon (under the current environmental conditions). Nevertheless, it remains unclear whether carbon sequestration in forests is sufficient for mitigating climate change.

Mark Maslin, Professor, University College London:
Many of these articles are well meaning but they are misleading. The magic number of 1 trillion trees is always mentioned which is about 900m hectares of new forest which is about the size of the whole of the USA. One study in Science suggested this could store 205 billion tonnes but this has been shown to be incorrect as they left out key parts of the calculation. The most recent reliable estimate from the IPCC suggests that new forests could store on average an extra 57 billion tonnes of carbon by the end of the century. This is still a huge number – but in 2019 we emitted over 11 billion tonnes of carbon – so planting an area the size of the USA with trees would take out just 6 years of pollution. So the best climate solution is to stop burning fossil fuels completely. So tree planting is useful and essential but not the best climate solution by a long way.

Valerie Trouet, Associate Professor, University of Arizona:
In short: it’s not about tree ‘planting’, it’s about creating and maintaining new forests so that they continue to capture and sequester carbon over decades. Moreover, even more important than ‘planting’ new forests, is to stop deforestation of existing forests. When done correctly and maintained over a long period of time (decades, centuries) afforestation can be a powerful way to mitigate anthropogenic climate change, but it only comes second (third) after (1) reducing anthropogenic greenhouse gas emissions and (2) decreasing deforestation.

Susan Cook-Patton, Forest Restoration Scientist, The Nature Conservancy:
The very most important thing is to reduce fossil fuel emissions! After that, there is no single “best” climate solution. Climate change is a complicated and global problem and to address that problem, we need a diversity of solutions. I would also argue that we need to think about protecting forests first. It’s a lot cheaper and more effective to keep the carbon in the forest than it is to try to regrow that carbon later. Improving management of working lands is also a promising option.

However, restoring trees to where they historically occurred is a powerful natural climate solution, because trees are great at absorbing lots of carbon from the atmosphere and storing it for long periods of time. Note that I said “restoring trees” rather than planting trees, because there are lots of ways to restore trees to the landscape. In places where there are seed sources, nearby forest patches, good growing conditions, a forest can grow back perfectly well on its own without tree planting and at a fraction of the cost and effort. Tree planting is going to be more expensive and more time-consuming than letting forests naturally regrow, but may be absolutely essential in degraded sites that are far from seed sources. Other ways to restore tree cover include setting up agroforestry systems (e.g., silvopasture) or establishing plantations, and these approaches can help optimize human livelihoods and carbon capture. The best approach is going to depend on local context. First, what does the local community want from the landscape and second, what are the best and lowest cost mechanisms for restoring trees while meeting the needs of the local community?

My current preferred analogy is that tackling climate change is like building a house. We need every tool in the toolbox. Tree planting is like a screwdriver. Useful tool? Absolutely! Can you build a house with a screwdriver alone? Absolutely not.

Giorgio Vacchiano, Assistant Professor, Università di Milano:
According to the best available science, “nature-based solutions”, which include planting trees, managing existing forests in a “climatically intelligent” way, stopping tropical deforestation, conserving wetlands and peatlands, and practicing Conservative agriculture could help us achieve 30% of the climatic mitigation required by 2030 to contain heating at the end of the century within 2°C compared to the pre-industrial era[2]. The mere expansion of forests in all available areas (excluding agricultural, urban and highly biodiverse ones such as savannas) could guarantee, through additional photosynthesis, an additional sequestration of over 10 billion tons of CO2 per year[5,11]. A contribution which is not sufficient on its own, but which cannot be dispensed with if the goal of the Paris agreements is to be achieved.

However, this contribution is not obvious. First of all, to realize this potential it is necessary to plant “the right trees in the right place”, and to ensure the new forests care and protection from drought and fires, especially in the early years. Science today knows well the best techniques for creating resistant, resilient and functional forests, which are capable of self-sustaining ecologically once they reach maturity.

Secondly, there are some situations in which planting trees could trigger opposite consequences to those desired. They are the so-called “biophysical effects”. Trees and forests have other ways, besides photosynthesis, to change the local climate. Trees can change albedo, or a color change of the earth’s surface, from white (snowy areas without trees) or dark (tropical deforested soils) to green (new areas with trees). In the first case, corresponding to the arctic and boreal areas, a darker surface color would result in heating, for the same phenomenon that we experience when we enter a dark car on a hot August day. The greater evaporation of water by new trees can favor the formation of clouds (cooling effect), but also increase the amount of water vapor in the atmosphere, a natural greenhouse gas (heating effect). The increase in surface roughness favors the dispersion of heat by turbulence (cooling effect). The production of methane, a greenhouse gas, by forests in tropical wetlands such as mangroves (heating effect), and the emission of volatile organic compounds can favor the formation of clouds (cooling effect).

As can be seen, some of these biophysical effects have contrasting effects. To obtain a precise estimate, scientists use bio-physical models of the Earth’s climate, with which they try to reproduce the effects of an increase or decrease in forests and their consequences on the climate. However, the little research conducted so far has yielded conflicting results on their overall effect – an extremely active debate. This uncertainty is crucial, because according to the IPCC, there is a high probability that biophysical effects on a local scale are more important than those caused by photosynthesis.

In reality, the net climate balance depends on the geographical area where the new forests are planted, the species used, and the soil moisture. According to the models considered by the IPCC, an increase in forests in the tropics would cause both global and local cooling (2.5 °C less in the Sahel, 1.2 less in China, and up to 8 degrees less in western Sahara).

Contrary to what emerged from earlier studies[23], according to the special report of the IPCC, in Europe and North America a cooling effect is possible as a result of forest expansion, except in the most arid areas, where there is not enough water to evaporate and the “darkening” effect would be prevalent[5,11]. According to some researchers, the heat waves of 2003 and 2010 in Europe would have been much weaker in the event of large-scale afforestation. The only area where the two effects compensate each other seems to be the boreal-arctic one where, due to the albedo effect, the planting of new forests would have a climate effect three times less effective than in tropical areas[9].

According to the most recent studies, considering both the biophysical effects and photosynthesis, the afforestation of 800 million hectares worldwide would result in a decrease in temperature of 1 degree in temperate regions and 2.5 in the boreal ones[24].

But there is a third point, the most important. The cities of the world are suffering the most severe consequences of climate change: summer heat waves, intense and sudden rainfall, the deterioration of air quality. And trees have exceptional powers against these threats. Not so much through the absorption of CO2, which is extremely limited in comparison to the emissions of a city, but by facilitating the adaptation of citizens. Cooling the air during heat waves, absorbing part of the pollutants and fine dust, reducing the outflow of surface waters. In summary, improving the health and well-being of citizens in a “highly likely” way – as the IPCC report on Climate Change and Land also concludes[5,11].

Sara Vicca, Postdoctoral research fellow, University of Antwerp:
The best climate solutions are those that lead to quick reductions of greenhouse gas emissions. This requires in the first place a fast phasing out of fossil fuel emissions (which are responsible for some 85% of CO2 emissions or 70-75% of greenhouse gas emissions). That really needs to be priority number 1 for climate action. Without reducing emissions, we cannot tackle climate change.

Another important source of emissions is deforestation, which should also be halted asap, not only for climate reasons, but also, for example, for biodiversity reasons.

But, conventional mitigation (emission reductions) alone will not suffice and we will most likely also need to actively remove CO2 from the atmosphere if we are to limit warming to 1.5 or 2 degrees Celsius. Tree planting has been put forward as one way to do this; reforestation and afforestation is one of those so-called negative emission technologies that has been suggested for active CO2 removal from the atmosphere. Trees take up carbon as they grow, and can store some of that carbon for a long time (in wood and in soil). It is one of the easier techniques with estimated C sequestration potential up to ~4 Gt CO2 y-1, at least in the short term[8]. In the long term, this rate cannot be maintained because the land that can be planted is not endless.

An important caveat of tree planting for C sequestration is that the permanence of this C storage cannot be guaranteed. For example, a new land owner might decide to cut the forest and thereby undo the C sequestration, and more importantly, climate change poses an increasing threat to these C reservoirs: more frequent and extreme heatwaves and droughts for example reduce forest C uptake and increase the chance for forest fires.

Besides afforestation and reforestation, some other technologies have also been proposed for active removal of CO2 from the atmosphere. These include improved management of agricultural soils, the use of biochar, enhanced weathering, bio-energy with carbon capture and storage, and direct air capture. Most of these techniques are not yet available at scale and they all have their own pros and cons, including potentially high costs, competition for land and biodiversity impacts.

As recently stated by Holl et al. (2020), tree planting is not a simple solution. Many factors need to be taken into account![4] “Well-planned tree-planting projects are an important component of global efforts to improve ecological and human well-being. But tree planting becomes problematic when it is promoted as a simple, silver bullet solution and overshadows other actions that have greater potential for addressing the drivers of specific environmental problems, such as taking bold and rapid steps to reduce deforestation and greenhouse gas emissions.”[4]

and

“Reforestation projects can be an important component of ensuring the well-being of the planet in coming decades, but only if they are tailored to the local socioecological context and consider potential trade-offs. To achieve the desired outcomes, tree-planting efforts must be integrated as one piece of a multifaceted approach to address complex environmental problems; be carefully planned to consider where and how to most effectively realize specific project goals; and include a long-term commitment to land protection, management, and funding.”[4]

The figure below gives an overview of the negative emission technologies that are currently being considered/developed and indicates their current readiness level as well as various pros and cons.

From Minx et al. (2018)[7].

2. Various articles express concerns about planting trees to address climate change, including limited effectiveness of trees as carbon sinks, planting non-native tree species, or planting monocultures. Other articles describe how trees can trap heat (low albedo) and release volatile chemicals that stick to aerosols. Do these concerns influence the role of tree plantings as a climate solution? 

William Anderegg, Associate Professor, University of Utah:
Yes, these are fundamental concerns to tree planting as a climate solution, which have by and large not been thoroughly considered in much of the reforestation studies to date. We have the ability to use science to inform when, where, and how to do tree planting in a climate-positive way, but that science is still being done right now.

Logan Berner, Assistant Research Professor, Northern Arizona University:
The effects of tree planting on albedo are an important consideration. Tree planting in temperate and tropical ecosystems will likely lead to overall cooling since the effects of carbon sequestration outweigh the effects of lower surface albedo; however, this is not the case in boreal forests. This great figure from Anderson-Teixeira et al. (2012) highlights how the biogeochemical (i.e., carbon) effects stack up against biophysical effects (e.g., albedo, evaporation) across ecosystems in the Americas (see figure above)[21].

From Canadell and Raupach (2008) in Science: “Although sequestering carbon in forests is good for the climate, forests also affect biophysical properties of the land surface such as sunlight reflectivity (albedo) and evaporation, with further implications for radiative forcing of climate. Climate models suggest that large reforestation programs in boreal regions would have limited climate benefits because of the substitution of bright snow-dominated regions for dark forest canopies. Conversely, the climate benefits of reforestation in the tropics are enhanced by positive biophysical changes such as cloud formation, which further reflects sunlight. These patterns of full radiative forcing reinforce the large potential of tropical regions in climate mitigation, discourage major land use changes in boreal regions, and suggest avoiding large albedo changes in temperate regions to maximize the climate benefits of carbon sequestration.”[14]

Susan Cook-Patton, Forest Restoration Scientist, The Nature Conservancy:

It all comes down to planting the right trees in the right place in the right way. Trees in places with high snow cover may have net negative (bad) climate impacts because of albedo, but there are lots of places in the world where that’s not an issue! Native trees and diverse tree communities are going to support higher levels of biodiversity. The concerns you raise are real, but those can be solved with better project planning.

Erle Ellis, Professor, University of Maryland, Baltimore County:
All of those limitations about tree planting are relevant. There are so many others, like massive loss of carbon from forests after climate change goes beyond 2 degrees (which it almost certainly will this century. But the biggest limitations are that carbon uptake by trees cannot be sustained – forests can only take up so much carbon before they saturate and provide no further uptake.

Karen Holl, Professor, University of California, Santa Cruz:
Absolutely. The point of our recent paper was to highlight that tree planting is complex. There are social and ecological tradeoffs. There are many potential benefits of tree planting such as carbon sequestration, biodiversity conservation, and providing income to landowners. But, there are also numerous potential negative consequences such as the points you note – namely, releasing volatile chemicals, as well as methane in flooded forests, and increasing albedo in northerly latitudes. As we describe in our article, other potential concerns include reducing water supply in arid areas, destroying historic grasslands, increasing social inequity if projects are top down, and displacing agricultural activities to other areas which increases deforestation[4].

This is why we emphasize that it is important how and where tree planting is done, as those impacts depend on the location, the species planted, and who is involved in the project. Our more recent paper discusses these issues in more detail. People plant trees for many different reasons, of which carbon is only one. It is critical that tree planting projects clearly state their goals, choose appropriate methods and locations to achieve those goals, and then monitor whether they have achieved those goals.

Sebastiaan Luyssaert, Associate Professor, Vrije Universiteit Amsterdam:
Substantial land-based carbon sequestration through afforestation, forest management, forest conservation requires vast amounts of land and interferes with other land uses as well as with other processes than solely C-sequestration. Given the state of knowledge, it is scientifically dishonest to study and report, for example, the impact of large-scale afforestation in semi-arid regions without considering its impact on groundwater and river discharge. The same applies to studies that leave out albedo, roughness, transpiration, volatile chemicals. There is a strong theoretical basis and an increasing observational basis that these properties/processes should be accounted for if the forest sector wants to make climate-related claims. Another way of looking at this issue is that most of the research this far should have concluded that “afforestation, forest management, forest conservation can offset CO2 emissions but their net climate effects are yet to be determined”.

Mark Maslin, Professor, University College London:
Tree planting or reforestation is used as a general catch all term – what people really mean is rewilding – restoring the most appropriate ecosystem to each area. For example it would make no sense to drain peatland to plant forests – this would cause a massive loss of carbon. Instead re-wetting of peatland and flooding old wetlands can have huge positive carbon and biodiversity effects. But there are large areas of the world which had trees and we can replant them and they will thrive because they should naturally be there. Humans have cut down half the trees on the planet, three trillion of them, since the beginning of agriculture – so we know the Earth can support a lot more trees. Forests also have very positive effects from stabilising soils, stabilising the local rainfall, preventing floods, ameliorating heatwaves and preserving biodiversity. But again it has to be the right tree species in the right place.

Valerie Trouet, Associate Professor, University of Arizona:
Yes, there are many concerns regarding the role of afforestation in anthropogenic climate change mitigation. To your list, I’d like to add that trees only capture carbon as long as they are alive. As soon as a tree dies, all the tree planting and carbon capture efforts have been for naught, unless the wood (in which the carbon has been captured) is used for other long-term storage uses (e.g., construction). This is intuitive, but often overlooked. The afforestation discussion should include:

– Plant fast-growing, but short-lived trees or long-lived, but slow-growing trees?

– Plant trees that are adapted to the current climate or to a future climate (if they are long-lived)?

– What to do to protect the forests from forest fires, insect outbreaks, etc., which will all nullify the carbon sequestration history of a forest.

– What to do with the wood after a tree has lived its life? Long-term wood use (e.g., in construction) can keep the carbon sequestered for a long time after the trees die.

– In that context: focus tree planting on trees that are useful for construction (often slow-growing)?

3. How much carbon is sequestered by planted trees? How does this compare to carbon sequestered by existing forests?

William Anderegg, Associate Professor, University of Utah:
The carbon sequestered in existing forests is far, far larger than planted trees, which further emphasizes that stopping deforestation should probably be a much higher priority than planting new forests. In addition, it will take decades for planted trees to store substantial amounts of carbon, which means that tree planting solutions are not a fast climate solution. By contrast, any effort that reduces fossil fuel or deforestation emissions are immediate climate solutions that benefit the atmosphere today and in the future.

Logan Berner, Assistant Research Professor, Northern Arizona University:
I’m not sure at a global scale, but a recent study focused on Oregon provides some insight into this question. Law et al. (2018) found that forests in Oregon currently store ~3,036 Tg C and that a combination of afforestation and reforestation could increase carbon storage by ~409 Tg C (+13%) by 2100[25]. “Reforestation, afforestation, lengthened harvest cycles on private lands, and restricting harvest on public lands increase NECB [net ecosystem carbon balance] 56% by 2100, with the latter two actions contributing the most.”[25]

Susan Cook-Patton, Forest Restoration Scientist, The Nature Conservancy:
Carbon sequestered by planted trees varies by where you are in the world, what has been planted, and how old those trees are. As I mentioned above, it’s usually cheaper and more effective to keep carbon in existing forests than it is to recapture that carbon.

Erle Ellis, Professor, University of Maryland, Baltimore County:
There are so many estimates – and uncertainties. Keeping existing forests is clearly a good idea for many reasons. So is restoring forests. But solving climate change is not about how to reengineer the biosphere with more forests to take up industrial carbon pollution. The problem is how to end industrial carbon emissions, which are the primary cause of climate change. Trees/forests are an insufficient and unsustainable way to solve the problem of climate change, even if we planted fast growing forests across the planet (ruining native habitats). Climate change must be stopped at the source, by ending demand for fossil fuels through a transition to cheap clean energy.

Karen Holl, Professor, University of California, Santa Cruz:
I am not an expert on the carbon numbers. For our paper we spent some time looking up the numbers from other papers and we found that the numbers for carbon sequestration from tree planting varied by at least a factor of 10 depending on what land one considers appropriate for tree planting, assumptions about growth rates, and several other assumptions[2,4,6,18]. To me one of the most important considerations about carbon in existing forests is that it is sequestered now, along with the native species in the forest. Planted trees take time to grow and people may or may not plant native trees and a diversity of trees. For many reasons, it makes much more sense to protect existing forests.

Sebastiaan Luyssaert, Associate Professor, Vrije Universiteit Amsterdam:
There is a large difference between the fluxes and stocks. Whether a forest is managed or not is more important than whether a forest is planted or naturally regenerated. A key element is of course how the wood is used and whether this usage is luxury consumption or really substitutes fossil fuel consumption.

Mark Maslin, Professor, University College London:
Depends on what species of tree and where you plant it. Simple rules 1) do not cut down existing forest as they are still pulling carbon out of the atmosphere – and losing them makes the reforestation targets even larger and 2) forests take a long time to mature and to take their full potential of carbon out of the atmosphere so this is a long-term solution and those forests must be protected.

Valerie Trouet, Associate Professor, University of Arizona:
There is a very large body of literature on this and it is a very active field of research, but I don’t have an answer for you without diving into the literature. As stated in Holl et al (2020) though, it’s more about (long-term) maintaining newly planted forests rather than just counting how many new trees have been planted[4]. In that sense, just comparing how much a new forest vs. an existing forest sequesters in a given year is not enough, it is also a matter of how many years (decades) in a row they can do this. In other words: it’s (long-term) carbon storage that matters more than (short-term) carbon capture alone.

Sara Vicca, Postdoctoral research fellow, University of Antwerp:
I don’t have numbers for this at hand, but C sequestration in trees/forests depends strongly on the conditions (climate, soil, species). And it is also important to take into account the destination of the soil in the absence of tree planting. For example, replacing a biodiverse grassland or wetland, which can have very high soil carbon stocks, would be very problematic and could release large amounts of CO2.

Carbon sequestration is an important ecosystem service of trees and forests, but the amount of C that a tree can store is perhaps not the best reason to plant it. Biodiversity, water infiltration, drought resistance and other factors should be considered, as well as its influence on the local community (e.g. competition for land with food production is an important risk).

4. What is the importance of tree planting relative to other climate solutions? Is tree planting a distraction? 

William Anderegg, Associate Professor, University of Utah:
I would argue that this is perhaps one of the most important risks with a focus on tree planting. The relative importance of any given approach to tackling climate change is a normative challenge – science can inform us what the costs, benefits, risks, etc. are, but it doesn’t dictate what the “best” technology is. In general, tree planting can, at best, help tackle 10-15% of the climate problem (e.g. for a reasonable calculation see Pacala and Socolow (2004)[26]). However, it must be done right and informed by the best science, which hasn’t been the case so far. Given that tree planting has some massive issues to be worked out, I would say that most scientists I know working on these areas generally feel it should be far below other approaches of reducing deforestation and reducing fossil fuel emissions. We absolutely cannot let it distract from these other approaches. There’s no possible way that we can plant our way out of climate change.

Logan Berner, Assistant Research Professor, Northern Arizona University:
Tree planting can help remove carbon from the atmosphere and provide other benefits to society; however, the single most important action to mitigate climate change is reducing fossil fuel emissions. Global fossil fuel emissions totaled about 9.5 Gt C per year from 2009-2018, while Earth’s land ecosystems (i.e., forests , grasslands) sequestered about 3.2 Gt C per year from the atmosphere[13]. Thus, burning fossil fuels emitted about three times as much carbon each year as was taken up by all of Earth’s land ecosystems! No amount of tree planting will be able to fully offset the carbon emitted by burning fossil fuels.

Susan Cook-Patton, Forest Restoration Scientist, The Nature Conservancy:
Reducing greenhouse gas emissions is the most important (from fossil fuels, from the land sector). But as I said above, restoring tree cover represents a powerful tool for tackling the climate crisis rather than a panacea. Tree planting will be an essential tool in some places, letting forest grow back is another valuable one, as well as all the other natural climate solutions.

Erle Ellis, Professor, University of Maryland, Baltimore County:
Planting trees is a distraction from solving climate change, which can only happen through a transition to cheap clean energy.

Karen Holl, Professor, University of California, Santa Cruz:
Tree planting can be a distraction, particularly when people view it as a simple solution. One of the most concerning videos I have seen was the one put out by Youtuber Mr. Beast about tree planting. It so oversimplifies tree planting and there’s no clear reason why the trees are being planted. People donate to causes like that and think that “now I have done my part to reduce climate change.” We have to make rapid and dramatic changes to our behavior and infrastructure to reduce greenhouse gas emissions to slow climate change. Planting trees can be a small piece of the effort but it is not a silver bullet. People perceive it as a cheap and easy way to address climate change and it is much more complicated than it seems.

Sebastiaan Luyssaert, Associate Professor, Vrije Universiteit Amsterdam:
Both media and academia would benefit from adding the nuance back into the debate. Afforestation and avoiding deforestation are worth pursuing even if they would come without any climate benefit. Even if it would be better for the climate to forest the whole boreal zone, this would be a very poor argument to do so. Forests have many functions and services outside the climate realm.

If the focus of the debate is climate change, tree planting is a distraction from solving the climate problem. Since the 1970s, our greenhouse gas emissions have increased so much that it is questionable if tree planting will even buy us any time. This does not imply that the forestry sector has no responsibilities to take in mitigating climate change. The most important responsibility is without doubt to stop deforestation (not only in Brazil and Indonesia). Other countries could take their responsibility through forest management or when the environmental conditions are right through afforestation.

If the focus of the debate are socio-economic and/or environmental problems in general, tree planting could be a nice solution for several often related problems, e.g., water quality, erosion control, missing school because children need to walk half a day to gather firewood, etc.).

Mark Maslin, Professor, University College London:
If we are to keep climate change to just 1.5˚C then we need to cut carbon emissions as fast as possible and hit net zero by 2050. The only way to do this is to stop burning fossil fuels. But even that will not be enough and from 2050 to 2100 we will need to have negative global carbon emissions. The most viable way of producing negative carbon emissions which we know works and has massive benefits is reforestation and rewilding. So reforestation is essential for stopping biodiversity loss and stabilising local environments. It is also essential to produce negative carbon emissions for the second half of the century – and given that many trees grow slowly then if we start planting now they will be producing the greatest effects from 2050 onwards.

Valerie Trouet, Associate Professor, University of Arizona:
Yes, I think it is often used as an excuse to not reduce greenhouse gas emissions. No one is against tree planting, so governments often use it as an easy excuse to claim that they are ‘addressing anthropogenic climate change’ without taking any of the less popular measures.

Sara Vicca, Postdoctoral research fellow, University of Antwerp:
Yes, this is a great risk. Human activities currently emit ~ 40 Gt CO2 per year. Tree planting could sequester around 4 GtCO2 per year and high estimates go up to 7Gt CO2 per year. Obviously, tree planting can never compensate for current CO2 emissions. None of the negative emission technologies that are currently at the table (under development) could. Even if we combine the potential C sequestration of all these negative emission technologies (many of which are still under development, several are (very) costly, and some are mutually exclusive), we would not reach the high level of current emissions.

5. Are there any issues with how this topic has typically been presented in news stories?

William Anderegg, Associate Professor, University of Utah:
I worry that a lot of news stories haven’t included or grappled with the huge challenges and issues that tree planting/reforestation needs to address before it is a useful and major climate solution. We have a paper in press at Science that highlights the major climate risks that reforestation projects face.

Logan Berner, Assistant Research Professor, Northern Arizona University:
Tree planting can reduce surface temperatures, especially in arid, temperature, and tropical environments (see figure above)[10]. In urban areas this can reduce cooling costs and potential impacts of heat waves on vulnerable populations. Two examples: In the UK, “Computer modelling showed tree planting could reduce maximum surface temperature by between 0.5 °C and 2.3 °C. The highest reductions occurred in housing types with the lowest tree cover, but in no housing type could new tree planting keep temperatures to current levels even to the 2020s.”[27] In the US, “Our simulations suggest that a single 25-ft tall tree can reduce annual heating and cooling costs of a typical residence by 8 to 12 percent ($10-25). Assuming annual savings of $10 per household, a nationwide residential tree planting program could eventually save about $1 billion each year.”[28] Another important co-benefit is the potential for tree planting to increase habitat connectivity.

Karen Holl, Professor, University of California, Santa Cruz:
People often focus on the biophysical issues of tree planting but social issues are equally important. If tree planting efforts are going to have the desired impacts over the long-term then stakeholders need to be engaged in the efforts or the trees will be re-cleared.

Mark Maslin, Professor, University College London:
In many of these stories they talk about reforestation that has never been done before, but there are excellent examples of it being done in the recent past. In the late 1990s environmental deterioration in western China became critical, with vast areas resembling the dust bowl of the American Mid-west in the 1930s. Six large forest programmes were introduced during the late 1990s and early 2000s, targeting over 100m hectares of land for reforestation. Grain for Green is the largest and best known of these programmes. It reduced soil erosion and desertification and stabilised local rainfall patterns. The ongoing programme also helped to alleviate poverty, as payments are made directly to farmers who set aside their land for reforestation. It has also reduced China’s grain surplus, which was depressing prices, and helped to rebalance the inequality between the eastern and western provinces. The Grain for Green programme shows that widespread reforestation can have a very positive effect on the economy as well as the environment.

Valerie Trouet, Associate Professor, University of Arizona:
I think the importance of long-term carbon storage – what to do with the wood, how to prevent trees from dying – is often overlooked (see figure below)[4]. As soon as a tree dies, and it will naturally sooner or later, it will stop sequestering carbon and unless the wood is preserved either in the forest or elsewhere (e.g., in construction), all the carbon it has sequestered over its lifetime goes back into the atmosphere and the whole tree-planting effort will have been a zero-sum-game.

From Holl et al. (2020)[4].

 

REFERENCES

UPDATES

  • 12 June 2020: This post was updated with minor edits to improve clarity.

 

Published on: 11 Jun 2020 | Editor:

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