Excerpts from Mongabay, 2021. https://news.mongabay.com/2021/05/is-planting-trees-as-good-for-the-earth-as-everyone-says/
In 2019, a paper titled “The global tree restoration potential,” published in the journal Science, created a furor. The authors of the study estimated, using remote sensing and machine learning, that the Earth had available land for about 900 million hectares (2.2 billion acres) of forest restoration. Foresting this tree-less land would help store 205 gigatons of carbon, they wrote, making it “our most effective climate change solution to date.”
The study was, however, based on various flawed assumptions and data, several independent groups of researchers countered. Among the many problems, one group noted, was that the study had relied heavily on foresting grasslands and savannas.
The World Resources Institute had published the Atlas of Forest Landscape Restoration Opportunities in collaboration with the IUCN. This influential map identified more than 2 billion hectares (5 billion acres) of land as presenting an opportunity for forest restoration. But subsequent analysis by independent researchers including Durigan showed that the Atlas had classified 900 million hectares of grassy biomes as “deforested” or “degraded.”
“They mapped the major game parks of Africa as degraded and deforested, defining degradation as anything that damages trees,” Bond says.
Authors of both maps countered by saying that their maps simply point out areas that can be potentially forested. Each area, however, needed to be assessed individually.
But critics say that by marking broad areas as potential sites for restoration — a term usually conflated with planting forests — the maps prompted a flurry of massive tree-planting campaigns and projects around the world. AFR100, for instance, is aiming to plant trees across 100 million hectares of mostly savanna in Africa by 2030, write Bond and his colleagues in a 2019 paper published in Trends in Ecology & Evolution.
“These maps have been extremely damaging,” Bond says. “It was really superficial, bad science, but then international policies are feeding into this. The vast areas then became the targets for reforestation, supported by the World Bank, the IUCN, the German government and so on.”
At the heart of many of the disagreements lie muddled-up ideas. When is an area a “forest”? What is a “degraded” forest? How is it different from a grassland or savanna with trees? How far back in time do you go to see what the original habitat of the area was like? Does a forest always trump a non-forested area?
Degraded or naturally unforested?
The WRI Atlas considered all areas with more than 10% tree cover as a form of forest; this is the broad definition also used by the Food and Agriculture Organization of the United Nations (FAO). Only land with less tree cover was considered to be either naturally non-forested or converted to some other land use.
Now, deforestation and degradation of forests can create open areas with few trees. But non-forested areas like grasslands and savannas, too, naturally have trees. Sometimes the trees are scattered, sometimes they occur in dense lots. This means that when viewed from above, many of these areas will have more than 10% tree cover and look like degraded forests.
“You’ve got to be careful of the word forest and what it means — the definition of forest is critical,” Bond says. “The definition provided in global terms by the FAO is more than 10% tree cover, which includes nearly all the world’s savannas, which are not forests at all.”
If you’re thinking just in terms of tree cover in an area, it can be hard to distinguish between a “degraded forest” and a naturally non-forested area. But there are better ways to do so. Let’s consider tropical savannas. In a paper titled “When is a ‘forest’ a savanna, and why does it matter?” published in Global Ecology and Biogeography in 2011, Jayashree Ratnam, an ecologist at National Centre for Biological Sciences, Bengaluru, India, and her colleagues recommend looking carefully at the kinds of plants growing on the land, and the kinds of evolutionary adaptations they show.
Tropical savannas, such as the Serengeti in Tanzania, the Cerrado in Brazil, or the grasslands of central India, they write, are dominated by species of grasses that use a form of photosynthesis called C4. These grasses don’t like shade, which means that the trees that grow in these landscapes are typically short, and have smaller leaf areas and open crowns that let sunlight filter to the ground. By contrast, a tropical forest tends to have grasses that use more shade-tolerant C3 photosynthesis because trees there grow tall and wide and have denser canopies.
The C4 grasses in savannas are highly flammable. The wet season prods the grasses to grow long and thick, while the prolonged dry seasons turn them into potent fuel for fire. Savanna fires, however, tend to be low on the ground, burning the grasses and young saplings, but not big or hot enough to scorch adult trees. Once the fires ebb, the grasses regenerate quickly. It’s perhaps counterintuitive, but many savannas need fires to remain savannas. Even the trees that grow in these areas have adaptations like thick bark to live with fire.
Fires in forests, on the other hand, tend to be very hot, burning not just the understory but the crowns of tall, adult trees as well. They spread rapidly to other trees, and can turn catastrophic. In fact, Ratnam and colleagues note that many areas in South Asia, currently classified as tropical dry forests, such as Bandipur Tiger Reserve in southern India, have such C4-dominated grasses with interspersed fire-resistant tree species. These areas are more like savannas than forests. “Having worked for a while in African savannas and being very familiar with the idea that mixed tree-grass ecosystems were distinctive from forests, when we returned to India and started visiting various field sites, we were struck by the similarities of these sites with African savannas,” Ratnam told Mongabay-India in 2019.
Apart from needing fire and light, savannas also have a long association with animals that graze, studies have found. They’ve evolved to support both large, wild herbivores like wildebeest, rhinoceros, zebras, and antelopes, as well as nomadic pastoralists whose livestock feed on the grasses and small plants and keep the savanna ecosystem an open one.
In a country where land is an incredibly valuable resource, marginal communities often end up losing their lands for these “compensatory forests,” usually without their knowledge or any form of consultation on what the communities might want.
Grassland mixed with forest in Bandipur Tiger Reserve, India. Image by Jaseem Hamza via Wikimedia Commons (CC BY-SA 3.0).
Even if the local communities are consulted, whether they will support those trees’ growth for years to come and care for them depends on whether they see more value in the trees remaining standing, or in cutting them down or not tending to them. Land tenure, where the farmers have an ownership in the trees and land, can provide that value, examples from Africa have shown. “Everybody now knows that land tenure is a big problem,” Chomba says. “People also know the solution, but they don’t want to get into that because it means you have to engage with the local authorities for a long period of time in trying to change the laws. It’s not as simple as planting a tree and saying I planted a million trees. So we need to be able to understand the policy bottlenecks and be prepared to do the hard work to change them.”
Overall, agroforestry, if done well and keeping in mind the local context, can achieve lots: it can increase the productivity of soil, improve microclimate as well as water and food security, and build resilience to climate change. But whether these benefits actually materialize needs to be monitored systematically, Chomba adds.
Durigan says planting trees in farmlands is a good way to restore degraded lands. But she doesn’t consider agroforestry to be true reforestation or afforestation.
“I do like productive systems with trees spaced, especially in degraded land, no matter if it was not a forest before. It is better than monocultures,” she says. “But agroforestry does not result in a true forest. It is not afforestation nor reforestation, since both are expected to create a continuous canopy and a forest structure. Agroforestry is a productive system where trees and crops share the space, aiming at improving degraded soils or to have an ecologically ‘healthier’ land use.”
Not everyone loves a forest
Forests are culturally important for many people around the world. Dense, mysterious forests have been a part of stories, nursery rhymes, poems and movies. But those who live in areas that have naturally been non-forested — grasslands in India, rolling meadows in Scotland, Cerrado in Brazil — don’t necessarily want them.
Bastin, J., Finegold, Y., Garcia, C., Mollicone, D., Rezende, M., Routh, D., … Crowther, T. W. (2019). The global tree restoration potential. Science, 365(6448), 76-79. doi:10.1126/science.aax0848
Veldman, J. W., Aleman, J. C., Alvarado, S. T., Anderson, T. M., Archibald, S., Bond, W. J., … Zaloumis, N. P. (2019). Comment on “The global tree restoration potential”. Science, 366(6463), eaay7976. doi:10.1126/science.aay7976
Veldman, J. W., Overbeck, G. E., Negreiros, D., Mahy, G., Le Stradic, S., Fernandes, G. W., … Bond, W. J. (2015). Where tree planting and forest expansion are bad for biodiversity and ecosystem services. BioScience, 65(10), 1011-1018. doi:10.1093/biosci/biv118
Bond, W. J., Stevens, N., Midgley, G. F., & Lehmann, C. E. R. (2019). The trouble with trees: Afforestation plans for Africa. Trends in Ecology & Evolution, 34(11), 963-965. doi:10.1016/j.tree.2019.08.003
Ratnam, J., Bond, W. J., Fensham, R. J., Hoffmann, W. A., Archibald, S., Lehmann, C. E., … Sankaran, M. (2011). When is a ‘forest’ a savanna, and why does it matter? Global Ecology and Biogeography, 20(5), 653-660. doi:10.1111/j.1466-8238.2010.00634.x
Veldman, J. W. (2016). Clarifying the confusion: Old-growth savannahs and tropical ecosystem degradation. Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1703), 20150306. doi:10.1098/rstb.2015.0306
Bond, W. J., & Zaloumis, N. P. (2016). The deforestation story: Testing for anthropogenic origins of Africa’s flammable grassy biomes. Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1696), 20150170. doi:10.1098/rstb.2015.0170
Kumar, D., Pfeiffer, M., Gaillard, C., Langan, L., Martens, C., & Scheiter, S. (2020). Misinterpretation of Asian savannas as degraded forest can mislead management and conservation policy under climate change. Biological Conservation, 241, 108293. doi:10.1016/j.biocon.2019.108293
Skowno, A. L., Jewitt, D., & Slingsby, J. A. (2021). Rates and patterns of habitat loss across authors: South Africa’s vegetation biomes. South African Journal of Science, 117(1/2). doi:10.17159/sajs.2021/8182
Albaugh, J. M., Dye, P. J., & King, J. S. (2013). Eucalyptus and water use in South Africa. International Journal of Forestry Research, 2013. doi:10.1155/2013/852540
Chomba, S., Sinclair, F., Savadogo, P., Bourne, M., & Lohbeck, M. (2020). Opportunities and constraints for using farmer managed natural regeneration for land restoration in sub-Saharan Africa. Frontiers in Forests and Global Change, 3. doi:10.3389/ffgc.2020.571679
Banner image of India’s shola grasslands by Divinwrct via Wikimedia Commons (CC BY-SA 4.0).
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Is planting trees as good for the Earth as everyone says?
by Mike Gaworecki on 13 May 2021
- As the world searches for solutions to global climate change, tree planting has become increasingly popular, with ambitious campaigns aiming to plant billions or trillions of trees.
- These projects often have other environmental goals, too, like regulating water cycles, halting soil erosion and restoring wildlife habitat. They also often have socioeconomic goals, like alleviating poverty.
- But how effective is planting trees at accomplishing all this, and how strong is the evidence for this effectiveness? To find out, Mongabay engaged a team of researchers who conducted a non-exhaustive review of relevant scientific literature.
- We detail the results below, as part of Mongabay’s special “Conservation Effectiveness” series. Research by Zuzana Burivalova, Rodrigo Mendes and Sharif Mukul.
People have been planting trees to restock forests for a very long time.
In the 16th century, for instance, wealthy landowners in Britain and Europe established tree plantations to supply timber for shipbuilding. In the 13th century, Portugal’s King Afonso III had a pine forest planted, known as the Pinhal do Rei, to hold back encroaching sand dunes (and provide timber for the royal navy). In the 5th century, monks on the Adriatic coast reportedly planted a pine forest to supply themselves with fuelwood and food.
Even before these forests took root, Roman senator and historian Cato the Elder, who died in 149 B.C., recorded the planting of conifers to provide timber for ships. Earlier still, the Zhou Empire, which ruled China from 1100 B.C. to 256 B.C., created a forest service specifically dedicated to preserving natural forests and replanting cut forests. And there’s evidence that Indigenous peoples’ planting of domesticated species prior to the arrival of European settlers influenced the composition of the Amazon Rainforest we know today far more than researchers previously realized.
These are, essentially, early examples of tree planting aimed at supplying some service or product. Tree planting as a means of replacing functional native forests is a much more recent endeavor. “Reforestation in terms of ecological restoration, that is, reestablishing forests that will be similar to their previous ecological state, is a relatively recent ambition for Western society,” Kate Hardwick, a conservation scientist at the Royal Botanical Gardens, Kew, told Mongabay, “although it has been practiced by Indigenous communities for thousands of years.”
As recently as 20 years ago, we knew little about how to cultivate most tree species on a large scale, according to Robin Chazdon, a research professor of tropical forest restoration at Australia’s University of the Sunshine Coast. Some countries, like Brazil, have “made really great strides” learning how to collect millions of native seeds, store them, and grow seedlings with sufficient root development to survive once they are out-planted, while others still lag, she said.
Today, as the world searches for solutions to global climate change, tree planting has become more popular than ever. It’s a simple and appealing response to an overwhelming, existential crisis, and it makes for easy messaging: anyone can go out and plant a tree to help restore balance to Earth’s climate. But for many large-scale tree-planting initiatives, the focus is on the number of new trees that end up in the ground, not on planting the right trees in the right places or caring for them after planting to ensure they survive.
While mitigating climate change is the chief driver of many tree-planting initiatives, these projects often have other environmental goals, too, like regulating water cycles, halting soil erosion and desertification, and restoring wildlife habitat. They also often have socioeconomic goals, like alleviating poverty and enhancing local communities’ health and livelihoods.
But how effective is planting trees at accomplishing all this, and how strong is the evidence for this effectiveness? To find out, Mongabay engaged a team of researchers who conducted a non-exhaustive review of relevant scientific literature. We detail the results below, and you can also explore the database of findings we collected via this interactive infographic.
Before looking at our results, though, it is necessary to explain what we mean by reforestation, because the term is widely used to describe different kinds of projects. Some reforestation projects aim to restore a previously existing or degraded forest ecosystem and they usually focus on planting native species or facilitating natural regrowth of the forest — activities that fall under the umbrella term of forest restoration.
But reforestation does not necessarily lead to forest restoration. For instance, planting monocultures of non-native species is a common form of reforestation, but its end point is very different from the original forest (think of a Christmas tree plantation). Sometimes, reforestation projects plant a mix of native and non-native species to both promote the restoration of the ecosystem and provide timber or other forest products that can boost local economies and reduce logging pressure in native forests. So, reforestation and forest restoration are closely related terms, but their precise definitions are still evolving and shift depending on who uses them and in what context.
For our purposes here, we define reforestation as planting trees to restock depleted or clear-cut forests, regardless of whether the resulting landscape is a monoculture plantation or a biodiverse forest ecosystem. By contrast, we define forest restoration as actively attempting to return an area to its previous naturally forested state; the priority is the recovery of a forest ecosystem, not just tree cover. In our database of findings, we included only research evaluating the effectiveness of projects that included tree planting, though they may also have employed other forest restoration measures, like clearing invasive species or remediating degraded soil.
Reforestation is the hot new trend in our warming world
Reforestation is increasingly popular amid growing recognition of the many convergent environmental crises we’re facing, from climate change to biodiversity loss and water scarcity. In fact, the urgency of restoring degraded forests and other ecosystems is such that the United Nations declared the years from 2021 to 2030 the U.N. Decade on Ecosystem Restoration.
One of the earliest examples of modern reforestation is the Green Belt Movement, founded by 2004 Nobel Peace Prize winner Wangari Maathai in Kenya in 1977. The movement has now planted more than 50 million trees to help restore the environment and alleviate poverty. Government-led projects like the Great Green Wall across 20 countries in the African Sahel and the Three-North Shelter Forest Program in northern China (also sometimes known as the Great Green Wall of China) have planted hundreds of millions of trees over the past few decades as a bulwark against desertification.
Meanwhile, big, high-profile NGO-led campaigns like the Trillion Tree Campaign, 1t.org, Trees for the Future, One Tree Planted, and Plant a Billion Trees have proliferated, most of them targeted at carbon sequestration to mitigate global warming and other environmental benefits. And companies like tech giant Microsoft and Italian oil company Eni are planting trees to offset their greenhouse gas emissions.
Click here to learn about Mongabay’s tool for evaluating the transparency of tree planting projects.
Institutions around the world are committing to restore forests and other natural ecosystems under the auspices of various international agreements as well. The Bonn Challenge, for instance, launched in 2011 by the German government and the International Union for Conservation of Nature (IUCN), has recorded pledges by governments and NGOs in 60 countries to restore 210 million hectares (nearly 520 million acres) of land, with the ultimate goal of restoring 350 million hectares (865 million acres) by 2030. Reforestation and forest restoration also factor into many countries’ plans for meeting the emissions reduction targets they committed to in ratifying the Paris climate agreement.https://www.youtube.com/embed/0OG8MddO8OM
The tree-planting boom has been bolstered by research at times, such as a 2019 study that estimated there is room to plant 900 million hectares (2.2 billion acres) with trees in areas across the globe that are naturally suited to supporting forests and woodlands. Planting these new forests could store as much as 205 billion metric tons of carbon, about 25% of the carbon human activities have released into the atmosphere to date, the study found. “This highlights global tree restoration as one of the most effective carbon drawdown solutions to date,” it says.
While planting trees would seem to have clear, unmitigated benefits, the reality is not so simple. In fact, that 2019 study was heavily criticized for, among other things, ignoring local conditions in many of the areas it identified as suitable for restoration. There are a number of complex factors that have to be considered to determine where, when, and what kind of tree planting is warranted.
“Today reforestation projects are basically only concerned with the number of trees planted. It’s like it’s the end goal,” Pedro Brancalion, a professor at the University of São Paulo who is both a forest restoration researcher and practitioner, told Mongabay. “But the number of trees you planted is just the start of a long-term process.”
Reforestation projects can fail to achieve their intended results if not planned properly. A 2017 study of mangrove forest restoration initiatives in Sri Lanka found that, at nine out of 23 project sites, not a single lanted tree survived. Just three sites had more than half the trees survive and, in the end, only about one-fifth of the more than 1,000 hectares (almost 2,500 acres) planted successfully recovered as healthy mangrove ecosystems.
There are also documented cases of tree planting having unintended and even negative environmental consequences. Starting in the 19th century, South Africa began planting non-native Australian acacias to stabilize dunes and produce timber. But the acacias quickly spread across wide swaths of South Africa’s native grasslands and heathlands, lowering the water table and reducing water availability. The country now spends millions of dollars every year to remove the troublesome trees. “That’s a clear example of where exotic species used in monoculture plantations got out of hand, became invasive, and caused problems,” Kate Hardwick said.
Reforestation programs can negatively impact people’s livelihoods, too. Pakistan’s “10 Billion Trees Tsunami” program reportedly led to tenants being evicted from their land by landowners looking to establish tree plantations, and also lost more than $3 million due to corruption.
We could learn a lot from problems like these, but scientific analyses of them are rare. We ultimately have no way of even knowing how many reforestation projects have failed outright. “I think the main knowledge gap is that we don’t know the level of success of restoration projects, because usually people only report results and have data on successful projects,” Brancalion said. Resources that go into planting the wrong trees in the wrong place are wasted and would have been better spent on a different intervention, he added.
Lessons from a mega-reforestation project
China’s Grain for Green program is instructive in a number of ways.
In response to a series of devastating floods in the late 1990s that killed more than 4,000 people, the Chinese government embarked on the most extensive tree-planting effort the world had ever seen. The Grain for Green program (GFGP) launched in 1999 with the primary goals of mitigating flooding, reducing soil erosion, and boosting the livelihoods of the rural poor in western China. Employing a payments-for-ecosystem-services scheme, the government provided households with technical support, cash and food in exchange for planting trees in areas of degraded farmland, especially those most prone to landslides and erosion.
Based on its primary goals of reducing erosion and runoff, the GFGP has been a success. As of 2019, the project cost roughly $73 billion and participants had planted trees on 32 million hectares (79 million acres) of cropland and barren scrubland. Today, more than 23% of China’s landmass is covered in trees, up from 19% in 2000. A 2012 study found that “runoff decreased and the soil erosion significantly decreased because of an increase in the area of farmland-converted forestlands.” Many of the trees provide timber, fruit and other forest products, enhancing the livelihoods of local communities. And while mitigation of global climate change was not a principal target of the program, research showed the GFGP “largely increased” soil organic carbon stocks.
But overall, the results of this ambitious reforestation effort have been decidedly mixed, even prompting assertions that China’s new “forests” aren’t really forests at all. A 2018 study led by Beijing University’s Fangyuan Hua while she was at the U.K.’s University of Cambridge determined that, as of 2015, gross tree cover had grown by nearly a third, with 1,935 square kilometers (747 square miles) newly treed. However, that increase was due almost entirely to degraded farmland being converted into monoculture tree plantations of one single species like bamboo, eucalyptus, or Japanese cedar.
Despite all of the tree planting, Hua and her team found that native forests had declined by 6.6%, or some 138 km2 (53 mi2), at the same time. “Thus, instead of truly recovering forested landscapes and generating concomitant environmental benefits, the region’s apparent forest recovery has effectively displaced native forests, including those that could have naturally regenerated on land freed up from agriculture,” Hua and co-authors write in their study.
Hua told Mongabay that the native-forest loss resulted at least in part from people exploiting a loophole in China’s forestry regulations to make way for plantations. She added that the Chinese government rectified the situation after Greenpeace East Asia pointed out that people had illegally cleared 1,295 hectares (3,200 acres) of natural forest inside key giant panda sanctuaries.
Tree plantations, of course, cannot compare to native forests in terms of their capacity to support wildlife like pandasand provide other ecological services. A few GFGP forests are what researchers call “compositionally simple mixed forests” containing two to five tree species, and these did experience modest gains in the number of bird and bee species compared to the cropland they replaced, according to a 2016 study that Hua also led. Monocultures, on the other hand, which comprise the overwhelming majority of GFGP forests, harbor fewer bird and bee species than croplands. Both monoculture and mixed-species reforested areas had lower bee diversity than croplands, the researchers found, probably due to the lack of flowers. And neither type of GFGP forest had anywhere near the biodiversity of native forests.
“So far the argument has been that plantations are effective at curbing soil erosion, and to a large extent I think studies show that,” Hua told Mongabay. “But these studies tend not to have evaluated the soil erosion performance as compared to a proper reference benchmark, like native forest, which we have been thinking about as biodiversity specialists.”
Lack of biodiversity is not the only environmental shortcoming of China’s GFGP. Researchers found that many of the non-native tree species that were planted require more water than native vegetation, meaning GFGP forests are sucking up more rainfall and reducing the amount of runoff to rivers. This was, of course, a desired outcome of the program. But as global warming leads to an increasingly drier climate, water shortages could, ironically, become a problem in a region that suffered devastating floods as recently as 1998. Another study even explicitly warned against continuing the GFGP for this very reason.
The involvement of multiple stakeholders, especially local community members, was one of the bright spots in the program’s design. There is evidence that the program increased incomes, though those benefits don’t always appear to have been evenly distributed, and in some regions richer households were found to have had greater access to the program than poorer households. As simple a change as promoting mixed forests instead of monocultures could have improved biodiversity outcomes, researchers say, without undermining the achievements.
How to plant a forest: Context is key
Experts have generally come to agree that the first step in restoring any forest is to ensure that the underlying causes of forest destruction in the area, and ideally surrounding forest as well, have stopped. This isn’t necessarily easy to accomplish. Robin Chazdon and colleagues point out in a 2020 study that many replanted or naturally regrowing forests have no legal protection. But in addition to protecting the time and money invested in a project, there’s a strong ecological incentive to doing so: “All young forest will need the assistance, the biological assistance, of forest remnants, and we have to be sure that these remnants will be in a good situation to contribute to the restoration,” Brancalion said.
The next steps involve working with all stakeholders, and most crucially the local people who will have to live with the restored forest, to determine the project’s desired outcomes and the best approach for achieving those goals. One of the more important decisions to be made is whether or not active measures like tree planting are even necessary, as under certain conditions just protecting the land and letting forests regrow on their own, called natural forest regeneration, can be the cheapest and most effective solution. Other decisions include who will care for the restored forest and who will have access to the benefits it furnishes.
If tree planting is in order, Brancalion said that to get the benefits you want from a project, you have to be sure you’re planting the right trees in the right place. Trees that aren’t suited to the local climate, for instance, will have short lives, and trees that require too much water can deplete water tables, as in South Africa. “If you plant invasive trees, then you can sequester carbon, but you may do harm to biodiversity,” Brancalion said. “If you plant the trees in the wrong place, you may displace agricultural production, you may create economic problems for local landowners, you may destroy native biodiversity when you plant the trees in non-forest ecosystems.”
Because of the complex decision-making that must attend any successful reforestation project, Brancalion said he thinks negotiation is perhaps the most overlooked aspect. “Before planting any tree or making any field intervention, you have to negotiate, to engage people, to understand what may or may not work on that specific location,” he said.
Reforestation is a “context-dependent activity,” with no single approach suited to all areas, Brancalion said: “So in one particular condition, planting some exotic species with some native species to support firewood production for a village can be a super successful project, even though you may not have super high biodiversity. It depends on your end goals.”
As an example of successful context-specific reforestation, Brancalion cited a project he contributes to, the Black Lion Tamarin Conservation program, led by researchers at the Institute for Ecological Research (known by its Portuguese acronym IPÊ). The black lion tamarin (Leontopithecus chrysopygus) was listed as critically endangered on the IUCN Red List of Threatened Species in the 1970s. The primate’s population had dwindled to just 100 individuals due to fragmentation of its only known habitat in Brazil’s Atlantic Forest by heavy agricultural activity. After the conservation program restored a 1,000-hectare (2,500-acre) forest corridor between two important remnants of Atlantic Forest, however, the black lion tamarin population rebounded to about 1,800 individuals, and the IUCN upgraded the species’ conservation status to “endangered” in 2008.
According to IPÊ, the program also engaged thousands of rural community members and landless families who had recently settled in the area. Researchers have found that, not only was the engagement of locals crucial to the program’s success, but people benefited from the program just as much as wildlife. A 2020 study showed that by growing and planting trees and participating in other conservation-related activities, “Farmers gained knowledge, income and food security, and developed a sense of ownership and shared responsibility for protecting wildlife, conserving forest fragments and restoring forests.”
The fact that community engagement and support for local livelihoods was baked right into the plan for the Black Lion Tamarin Conservation program is key to its success. Get the planning process wrong, though, “and then it doesn’t matter how many trees you plant, because most of them will die, and in some cases if they survive they can do more harm than good,” Brancalion said.
What the science says about how well tree planting works
Mongabay launched the Conservation Effectiveness series in 2017, collating the scientific evidence for how well a variety of common strategies actually work, including terrestrial and marine protected areas, forest certification, community-based forest management, and payments for ecosystem services. (You can find the current databases of research for all the interventions at ConservationEffectiveness.org and Mongabay’s stories about them here.)
In our Reforestation and Forest Restoration database, we collected scientific findings about the effectiveness of projects that include at least some tree planting. We excluded studies that focus on silvicultural measures, such as tree thinning and liana cutting, even if they may lead to faster forest regeneration. Likewise, we excluded studies focused on forest rehabilitation or agroforestry (the planting of trees alongside food crops), and those focused on natural regeneration, because many studies of tree-planting projects use natural regeneration as a control.
We are using a crowd-sourced approach to gather published, peer-reviewed scientific findings to include. To seed the database, several scientists added findings according to their own snowballing techniques, in which they would read a relevant publication and follow the references in it to find more relevant publications. We also had an entire class of undergraduate students at the University of Wisconsin–Madison add one publication each as an assignment for their “Forests of the World” course. An editor checks whether the individual researchers or students added the findings according to Conservation Effectiveness standards. This approach is not exhaustive, but it yielded 66 studies so far, of variable scale and rigor. Our research team examined these to determine how effectively reforestation projects deliver on a variety of environmental and socioeconomic outcomes.
We have now opened the reforestation and forest restoration database, as well as those of the other strategies on the platform, so anyone can contribute additional research, subject to the same editorial process.
There are some important caveats to note about the body of available reforestation research. One is that failed projects are rarely reported or publicized; another is that even evidence of success may not be reported if the project itself didn’t include robust monitoring and reporting standards; and finally, many government-, NGO- and corporation-led projects have no obligation to report their results to anyone but the person or entity funding them. The upshot is that many reforestation efforts aren’t examined in peer-reviewed studies and hence our literature review most likely misses the lessons to be learned from them altogether.
Another important caveat is that even a successfully restored forest can take decades or more to confer anything like the benefits of natural, old-growth forest. That means the benefits of more recent reforestation initiatives may have yet to fully materialize, so they would not be reflected in current studies. (What’s more, we may never find out if they do materialize, or even if the new forest survives more than a few years, because funding for reforestation projects rarely pays for long-term care or monitoring.)
Finally, the changes the studies attribute to the tree planting mean different things depending on whether a natural regeneration site, a pasture, or a mature forest serves as a control for comparison.
Read more about our methodology here; you can access all 66 of the studies we reviewed here, as well as any others that have been added since we published this story.
The scientific evidence on reforestation and forest restoration
Use the drop down menus to select data from your country of choice or to view data for a particular type of evidence. Click on the name of an outcome (such as forest cover or jobs) to display data specific to it. Click on a square to see what evidence the outcome is based on. You can find this and other infographics in our Conservation Effectiveness series at ConservationEffectiveness.org. Data visualization by GreenInfo Network.
The verdict for environmental outcomes: Mostly positive change
Overall, most of the studies in our database show that reforestation and forest restoration does indeed increase forest cover. They have other positive environmental outcomes, too, like sequestering carbon, improving water quality and preventing erosion. However, projects generally take quite a long time, at least several decades, to provide old-growth forest levels of benefits. In some cases, all of them plantations of exotic species, reforestation actually proved worse than leaving the land in its current state, which underscores the idea that tree planters need to carefully consider the detailed context of the area they plan to reforest.
With regard to biodiversity, the findings are pretty consistent: most of the time, reforestation does increase plant and animal diversity relative to the land before reforestation, and sometimes even relative to natural regeneration (two common controls in the more rigorous studies). At the same time, biodiversity is almost never as high as in old-growth forests, although there are a few exciting exceptions, and, anecdotally, it seems that areas near mature forests recover faster. Some studies present sobering projections of the time it would take to recover to old-growth levels of biodiversity, on the order of 100 to 150 years longer than their study period.
One important, albeit unsurprising, finding is that monoculture plantations tend to decrease biodiversity, even compared to croplands, as Hua concluded in a 2016 study of China’s Grain for Green program.
We reached similar conclusions about biomass and carbon sequestration: reforestation almost always has a positive effect, in that it speeds up biomass accumulation and hence carbon sequestration compared to land before reforestation and naturally regenerating forests. However, here, too, replanted forests rarely match old-growth forests. For example, a 2016 study estimates that it would take another 96 years to reach old-growth levels of biomass accumulation in a replanted forest. Soil carbon seems to be especially slow to recover. Climate change mitigation strategies need to count on this: it can take more than a century to reach the amount of carbon stored in the soils of old-growth forest.https://www.youtube.com/embed/qrHxi_ny5nk
The survival rate of planted trees, whether they’re planted as seeds or seedlings that were germinated in nurseries, varies widely, from 0% to almost 100% success. In the studies we examined, we found a nearly even split between projects that report a greater than 50% survival rate (which we rank as a positive outcome in our database), and those that saw a less than 50% survival rate (we rank those as neutral).
It’s important to note that most studies of seedling survival in our database are highly controlled, rigorously designed, small-scale experiments. This enables a careful statistical analysis of the cause and effect of seedling survival, but does not necessarily reflect real-life conditions. High levels of seedling survival under tightly controlled conditions might not translate to successful reforestation projects at larger scales out in nature, where many additional factors may decrease seedling survival.
One 2020 study in the Philippines found that involving local people and organizations in a pilot reforestation project resulted in higher seedling survival and growth success (78% after three years) than did government-managed restoration programs. A key takeaway is that community-led restoration can work well so long as project organizers provide sufficient support, incentives, and decision-making opportunities to local communities.
Many reforestation projects are designed to decrease erosion and improve hydrology, and most of the studies we examined document improvements in these areas. However, only one study compared erosion and runoff in a reforestation project to that in a natural regeneration site, and natural regeneration came out ahead. In this category, there are cautionary tales of tree species planted where they don’t belong, causing water tables to dry up, like South Africa’s acacias (although we don’t actually have those studies in the database, for now).
It’s also worth remembering that all forest restoration interventions have different objectives and varying site quality. Many projects aim for biomass gain, for instance, so their biodiversity outcomes should not be compared to those of a project focused on biodiversity. The scale of restoration interventions (plot vs. landscape, etc.) is also important when interpreting results.
The verdict for social and economic outcomes: Inconclusive
We didn’t find enough data to allow us to make general conclusions about how effective reforestation and forest restoration are at achieving socioeconomic outcomes, such as increasing community well-being and financial security, supporting local livelihoods, and clarifying land ownership.
The studies that address the social aspect mainly cover employment, and how well the benefits of reforestation projects are shared among stakeholders. These tended to find that while reforestation projects did bring jobs to local communities, they didn’t necessarily reduce the gap between the rich and poor. Amid the general dearth of research on these topics, there is a need for more studies investigating how governments and other institutions can best work with local communities, particularly Indigenous peoples.
The few studies that cover the economic aspects of restoration tend to focus on the cost-effectiveness of tree planting versus natural regeneration and to consider the income that communities get from tree planting. They generally found that tree planting mostly did bring economic benefits to the local community. The studies that reported economic benefits often examined reforestation projects that included a payments-for-ecosystem-services scheme, such as China’s Grain for Green program. Opportunity costs of reforestation are particularly understudied, so we can’t say if enrolling one’s land in a reforestation project tends to confer more economic benefits than continuing to farm the land, for instance. It’s clear we need more studies like that, especially ones that factor in current carbon prices.
A popular narrative seems to have emerged in recent years that holds tree planting as the answer to the global climate crisis and a host of other environmental ills. Some researchers and practitioners have bolstered this perception by pushing tree planting as the number one nature-based climate solution, and large-scale campaigns that aim to plant billions or even trillions of trees have, too. From the research we examined, however, it’s evident that, just like every other strategy we’ve covered in the Conservation Effectiveness series, tree planting is no silver bullet. Tree planters need to ensure plantings makes sense in the local context, and even then there are numerous trade-offs and competing objectives to negotiate. And they need to understand that it may take a century before forests reach their full carbon-storage potential — yet we have just a decade to drastically curtail greenhouse gas emissions if we are to avert the worst impacts of runaway global climate change, according to the Intergovernmental Panel on Climate Change.
Since 1990, the world has lost as much as 420 million hectares (1.03 billion acres) of forest, much of it felled to make room for agriculture. Though the pace of forest destruction has decreased somewhat since 2015, the U.N.’s Food and Agriculture Organization says that “Deforestation and forest degradation continue to take place at alarming rates.” It might not ultimately make sense to replant all of those forests, but it’s clear that, in tandem with halting deforestation, planting trees can be a valuable part of efforts to combat global warming and restore other vital ecological functions that all life on this planet relies on.
So it’s at least partly true that “Trees are the answer,” as the slogan goes. We just have to make sure to do it right.
Banner image: A man carries trees during a reforestation project in Tanzania in 2018. Image by David Wilfred, Sky Studios, Arusha for One Tree Planted. Copyright Matt Hill.
Writer: Mike Gaworecki
Researchers: Zuzana Burivalova, Rodrigo Mendes, Sharif Mukul
Editors: Rebecca Kessler, Willie Shubert
Copyeditor: Hayat Indriyatno
Infographic: Zuzana Burivalova, GreenInfo Network
Additional contributions: students from the Forests of the World course, University of Wisconsin-Madison
Related listening from Mongabay’s podcast: “Reforestation vs deforestation: Forest losses and gains this past year,” listen here:https://html5-player.libsyn.com/embed/episode/id/19177016/height/90/theme/custom/thumbnail/yes/direction/backward/render-playlist/no/custom-color/3e5014/
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