How does planting trees help climate change


Vital Signs of the Planet

By Alan Buis,
NASA's Jet Propulsion Laboratory

It’s an intriguing premise: what if we could reduce the severity of global climate change by planting hundreds of billions of trees to remove excess carbon from our atmosphere? A recent study published in the journal Science sought to provide answers by estimating the global potential of restoring forested lands as a possible strategy for mitigating climate change.

The international research team, led by Jean-Francois Bastin of ETH-Zurich in Switzerland, used direct measurements of forest cover around the world to create a model for estimating Earth’s forest restoration potential. They found Earth’s ecosystems could support another 900 million hectares (2.2 billion acres) of forests, 25 percent more forested area than we have now. By planting more than a half trillion trees, the authors say, we could capture about 205 gigatons of carbon (a gigaton is 1 billion metric tons), reducing atmospheric carbon by about 25 percent. That’s enough to negate about 20 years of human-produced carbon emissions at the current rate, or about half of all carbon emitted by humans since 1960. The study attracted worldwide attention, as well as some criticism within the science community.

Is the concept of planting trees to help combat climate change really going out on a limb, so to speak, or might it take root? Sassan Saatchi, a senior scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, believes it has some merit. But while he says there’s potential for using reforestation as a climate mitigation tool, he cautions there are many factors to consider and that planting trees will never be a substitute for decreasing fossil fuel emissions.

“I feel there’s a strong possibility a significant portion of these lands can be reforested to their original forest cover,” said Saatchi, an expert in global forest carbon stocks and dynamics. “It’s definitely not a solution by itself to addressing current climate change. To do that, we need to reduce human emissions of greenhouse gases. But it could still have some partial impact on our ability to reduce climate change.”

A multi-country-led effort called the African Forest Landscape Restoration Initiative (AFR100) is working to reforest 100 million hectares of land in Africa by 2030. Credit: Andrea Borgarello for TerrAfrica/World Bank

Saatchi says the study establishes a reasonable estimate of global forest restoration potential and addresses the issue more directly than previous work. The researchers used new satellite-based land cover and land use maps, along with other climate and soil data and advanced techniques to arrive at their results. He says their conclusions on tree restoration aren’t that different from the recommendations made by the Intergovernmental Panel on Climate Change in 2018, which suggested that 950 million hectares (2.3 billion acres) of new forests could help limit the increase in global average temperature to 1.5-degrees Celsius (2. 7 degrees Fahrenheit) above pre-industrial levels by 2050. However, he says, “the devil is in the details.”

Many Unanswered Questions

Before a global forest restoration effort is undertaken, Saatchi says, numerous questions must first be addressed to assess the concept’s feasibility, scientific soundness, cost-efficiency, risks and other considerations. “We need to understand not only whether it’s possible to do such a thing, but whether we should do it,” he says.

“The paper has sparked a healthy debate in the science community, which has now come forward to begin to address issues that the paper did not,” he said. “The science community has been looking at these questions to some extent for a long time, but there’s more urgency to address them now, since we no longer have the same climate conditions we had 50 or 100 years ago, when humans began massive deforestation for agriculture and human settlements. Since then, Earth’s population and land use have increased drastically. In some parts of the Northern Hemisphere, countries have been able to save more forests, but other areas, such as the tropics, have seen massive deforestations because of the need to feed larger populations.”

Areas of degraded rainforest in the Democratic Republic of Congo. Credit: NASA/JPL-Caltech/Sassan Saatchi

Saatchi outlined a few of the many questions scientists and others will want to investigate. For example, how realistic are the study’s estimates of how much carbon can be sequestered through reforestation? How long will this approach take to make a dent in atmospheric carbon concentrations? Can grasslands and savanna ecosystems sustain increased tree cover? How might converting non-forest land to forests compete with food production? How much time, money and resources will it take to implement a global forest restoration of this magnitude? How do the costs of adopting such a climate mitigation strategy stack up against its potential benefits? How much carbon would be released to the atmosphere by restoring forests? How will global climate models respond to a massive forest restoration? Will an Earth with a billion hectares of new forests actually be cooler?

Fire suppression tactics have allowed this forest at the edge of a savanna in Gabon, Central Africa, to regenerate naturally. Credit: NASA/JPL-Caltech/Sassan Saatchi

“Planting a billion hectares of trees won’t be easy,” he said. “It would require a massive undertaking. If we follow the paper’s recommendations, reforesting an area the size of the United States and Canada combined (1 to 2 billion hectares) could take between one and two thousand years, assuming we plant a million hectares a year and that each hectare contains at least 50 to 100 trees to create an appropriate treetop canopy cover.”

Even once the trees are planted, says Saatchi, it will take them about a century to reach maturity. Most forests in the United States are less than 100 years old because they are recycled constantly. Trees in tropical regions take a little bit longer to reach maturity, but sequester carbon much faster. We know it will take time for new forests to absorb atmospheric carbon.”

Saatchi says scientists will want to do a comprehensive evaluation of all potential effects a mass reforestation may have on Earth’s climate and the global carbon cycle.

Currently, Earth’s forests and soil absorb about 30 percent of atmospheric carbon emissions, partially through forest productivity and restoration. While deforestation has occurred throughout human history, the practice has increased dramatically in the past 50 years. According to the United Nations’ Food and Agriculture Organization, about 7.3 million hectares (18 million acres) of forest are lost every year, and roughly half of Earth’s tropical forests have already been cleared. In the continental United States, an estimate from the University of Michigan found that 90 percent of indigenous forests have been removed since 1600.

Over time, the ocean and land have continued to absorb about half of all carbon dioxide emissions, even as those emissions have risen dramatically in recent decades. It remains unclear if carbon absorption will continue at this rate. Credit: NASA/JPL-Caltech Degraded landscapes in Colombia’s Choco region. Credit: NASA/JPL-Caltech/Sassan Saatchi

As deforestation has ramped up, Earth’s climate has changed significantly. Warmer, more adverse climate conditions are creating more difficult growing conditions for forest ecosystems.

Key questions scientists will need to address are how global reforestation might affect Earth’s surface albedo (reflectivity) and evapotranspiration. In the near term and locally, says Saatchi, forest restoration may actually have a warming effect. As the trees mature, the new forest canopy cover would presumably make Earth’s surface albedo darker, particularly in the Northern Hemisphere during periods of snow cover, causing it to absorb more heat. Increasing forest cover, particularly in the tropics, will increase evapotranspiration, causing a cooling effect. With Earth already warming significantly due to greenhouse gas emissions, will forest reforestation on a global scale have a net warming or cooling effect on our planet, and will the benefits of reforested areas absorbing more carbon outweigh their increased heat absorption? These effects may vary geographically from tropical to boreal regions and may depend largely on water and light availability. In addition, how might these changes impact climate change patterns?

“Recent Landsat satellite-based analyses show that close to 400 million hectares (988 million acres) of forests have been disturbed in this century alone (2000-2017), either by human activities or through droughts and fires – that’s almost 50 percent of the area recommended for reforestation by the authors of the new study,” he said. Some of these areas have gone back to being forests, but a large amount of these degraded forests located in tropical and subtropical regions are suitable targets for restoration.

Map of global tree loss/tree gain since the early 1980s derived from NASA Landsat and NOAA AVHRR optical imagery, revised by Sassan Saatchi from Song et al., 2016. Credit: NASA/JPL-Caltech/Sassan Saatchi

Another science question concerns biodiversity. Will ecosystems in reforested areas revert to their previous conditions and maintain their ability to sequester carbon? While ecosystems that existed before areas were deforested may have been highly diverse, reforesting them with only a single type of species (known as monocultures), might result in ecosystems that won’t function as efficiently as they did before – in other words, they may not grow the same or stay as healthy over time. Saatchi says each region of the world will need to address this question for itself. But restoring a region’s original biodiversity or its natural forests may not be easy. For example, the region’s soil health may have changed.

Yet another concern is something Saatchi calls climate connectivity. When ecosystems become too fragmented, they begin losing their natural functions. “In Earth’s tropical regions, a combination of deforestation and climate conditions may have actually changed the system so much that climate connectivity is reduced,” he says. “Once this connectivity is lost, it becomes much more difficult for a reforested area to have its species range and diversity, and the same efficiency to absorb atmospheric carbon.”

Saatchi says scientists are already studying some of these questions. He believes that by the end of the next decade, better results from satellite observations and modeling will likely enable us to determine whether a global forest reforestation will produce the carbon and climate benefits suggested by the new study, and whether it should be undertaken. In the meantime, stopping further deforestation and restoring these areas to their original forest cover of 50 years ago may be the most effective mitigation strategy.

Looking to Space for Answers

Saatchi says a number of current and planned satellite missions from NASA and other space agencies can make valuable contributions to these research efforts:

  • Instruments on NASA satellites, such as the Clouds and the Earth’s Radiant Energy System (CERES) instrument on NASA’s Terra satellite, continuously monitor the energy balance of Earth’s land surfaces, measuring their albedo, a key climate parameter that would be impacted by reforestation.
    Map created from data from the CERES instrument on NASA’s Terra satellite, showing how the reflectivity of Earth—the amount of sunlight reflected back into space—changed between March 1, 2000, and December 31, 2011. This global picture of reflectivity (also called albedo) appears to be a muddle, with different areas reflecting more or less sunlight over the 12-year record. Shades of blue mark areas that reflected more sunlight over time (increasing albedo), and orange areas denote less reflection (lower albedo). Credit: NASA's Earth Observatory
  • NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on the agency’s Aqua and Terra satellites provide a suite of measurements on global forest cover change, fire and forest carbon cycling function.
  • NASA’s ECOsystem Spaceborne Thermal Radiometer Experiment (ECOSTRESS) aboard the International Space Station, launched last year, measures evapotranspiration and stress on ecosystems, providing valuable information on how Earth’s energy, water and carbon cycles interact in ecosystems in a warming climate.
    NASA's ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) imaged the stress on Costa Rican vegetation caused by a massive regional drought that led the Central American nation's government to declare a state of emergency. The image was acquired on February 15, 2019, then processed to generate the evaporative stress image. Credit: NASA/JPL-Caltech
  • NASA scientists conduct research to map the functional traits of ecosystems. Models, combined with satellite observations, can examine whether ecosystems will absorb more carbon if we plant new trees.
  • A new NASA mission in development for launch in the next decade called Surface Biology and Geology (SBG) would give scientists a global view of the functional traits and diversity of ecosystems and their efficiency in absorbing carbon, water and energy. Other space agencies also plan to make similar measurements.
  • NASA’s recently launched Global Ecosystem Dynamics Investigation (GEDI) aboard the space station is conducting high-resolution laser ranging of Earth’s forests and topography to study how deforestation has contributed to atmospheric carbon dioxide concentrations, how much carbon forests will absorb in the future, and how degradation of habitats will affect global biodiversity.
    NASA’s Global Ecosystem Dynamics Investigation (GEDI) mission created this image of a South Carolina woodland. Darker green colors show where the leaves and branches are denser, while the lighter areas show where the canopy is less dense. Credit: Joshua Stevens / NASA Earth Observatory, Bryan Blair / NASA Goddard Space Flight Center, Michelle Hofton and Ralph Dubayah / University of Maryland
  • The NASA-ISRO Synthetic Aperture Radar (NISAR) mission, a dedicated U.S./Indian interferometric synthetic aperture radar (InSAR) mission scheduled to launch in 2022, will be able to measure the woody plants and forests that make up 80 percent of Earth’s living terrestrial biomass. NISAR’s global, detailed maps of above-ground woody biomass density are expected to cut in half current uncertainties in estimates of carbon emissions resulting from land use changes.
  • The European Space Agency’s BIOMASS mission, launching in the early 2020s, will map the global distribution of above-ground biomass in forests to reduce uncertainties in estimates of carbon stocks and fluxes in the terrestrial biosphere, such as those linked to changes in land use, forest degradation and forest regrowth.

“With these new missions, we should be able to monitor how every patch of forest around the world is absorbing carbon, and how carbon absorption is changing, on a monthly and annual basis,” said Saatchi.

Seeing the Forests for the Trees: The Big Picture

Saatchi says the study’s results can help address policy-relevant questions. In accordance with the Paris Agreement, after 2020, the global community has agreed to major emission reduction programs. Reforestation can complement these emission reduction strategies.

“With the Paris Agreement, governments around the world committed to reduce emissions by adopting low-carbon pathways in accordance with nationally determined contributions,” he said. “As a result, it’s become more urgent than ever to have realistic estimates of each country’s capacity to increase its forest cover and health. While it’s likely the burden of restoring forests will fall primarily on the shoulders of the world’s large and economically-developed countries, the developing world can also contribute by reducing land use change and deforestation. ” He adds governments will need to decide which land areas to target first and which will have the least negative economic impacts to both society and individual communities, such as indigenous populations.

A Baka woman in central Gabon makes products from non-timber forest materials. Without forest conservation and restoration, indigenous forest people will be forced to re-establish themselves outside of forest areas. Credit: NASA/JPL-Caltech/Sassan Saatchi

If it’s determined that a global reforestation effort can be successful, will the world’s governments have the will to do it? Saatchi pointed to some recent examples that show what might be possible.

Over the past 15 years or so, China has planted millions of trees and created millions of hectares of new forest cover, much of it in areas with marginal agricultural potential. “China’s land use policy increased forest cover in southern China between 10 and 20 percent, turning these areas into intense managed forests,” he said. “As a result, they created close to a carbon sink (an area that stores carbon) in their forests, almost doubling their carbon uptake. The effort has offset 20 percent of China’s annual fossil fuel emissions, and since 2012 that percentage has increased to 33 percent. So that’s a success story.”

Managed activities to increase the carbon sequestration of forests have also taken place in other parts of the Northern Hemisphere, including the United States, Canada, Europe and Russia, he says. He believes it’s possible to increase them even further and to extend the area or the capacity of these forests to sequester more carbon. In fact, he says, some foresters have been doing so for decades.

“U.S. forests have actually been a net sink for carbon for many decades,” he says. “A paper published a couple of years ago showed that reforestation could reduce U.S. annual carbon emissions from all sources by 10 to 15 percent. Imagine if we do that? It’s possible. We just need to study the cost-to-benefit ratio – is it economically feasible to plant those trees compared to how much carbon they would offset?”

The U. S. Forest Service is restoring this longleaf pine forest in Alabama. Credit: NASA/JPL-Caltech/Sassan Saatchi

Another region Saatchi says is low-hanging fruit in terms of its potential to extend global tree cover is the Amazon, where large wildfires have made headlines recently. Between the 1970’s and 2010, 20 percent of the Amazon basin was deforested for land use activities — more than 100 million hectares of trees. But prior to last year, Brazil had significantly reduced deforestation for nearly a decade. “Restoring these Amazonian forests, if possible, would certainly absorb more carbon from the atmosphere,” he said.

The Amazon rainforest near Manaus, Brazil. Fragmented landscapes in Earth’s humid tropics are suitable locations for restoration of native forests. Credit: Neil Palmer, Flickr Creative Commons / CC BY-NC-SA 4.0

Ultimately, should a global reforestation effort be deemed feasible, the biggest question may be whether it will be in time to make a difference for climate change. Saatchi is hopeful.

“We know business as usual will be disastrous,” he said. “We’ve already identified some solutions for reducing carbon emissions in parts of our society, such as in transportation and agriculture, and we’re working on ways to transform our energy consumption. So why not restore our ecosystem as well? Half of what comes out of car tailpipes stays in the atmosphere; the rest gets absorbed by the ecosystem. That’s a huge absorptive capability that must be saved.

“Maybe we’ll find we don’t need to plant a billion hectares of trees,” he continued. “Perhaps we can restore existing, degraded ecosystems to their natural state, especially in the tropics, and invest in maintaining their diversity and services. But I believe a global reforestation effort can have a gradual climate mitigation impact. What happens to Earth 100 years from now depends on the choices we make today.”

Why planting tons of trees isn't enough to solve climate change

Trees are symbols of hope, life and transformation. They’re also increasingly touted as a straightforward, relatively inexpensive, ready-for-prime-time solution to climate change.

When it comes to removing human-caused emissions of the greenhouse gas carbon dioxide from Earth’s atmosphere, trees are a big help. Through photosynthesis, trees pull the gas out of the air to help grow their leaves, branches and roots. Forest soils can also sequester vast reservoirs of carbon.

Earth holds, by one estimate, as many as 3 trillion trees. Enthusiasm is growing among governments, businesses and individuals for ambitious projects to plant billions, even a trillion more. Such massive tree-planting projects, advocates say, could do two important things: help offset current emissions and also draw out CO2 emissions that have lingered in the atmosphere for decades or longer.

Even in the politically divided United States, large-scale tree-planting projects have broad bipartisan support, according to a spring 2020 poll by the Pew Research Center. And over the last decade, a diverse garden of tree-centric proposals — from planting new seedlings to promoting natural regrowth of degraded forests to blending trees with crops and pasturelands — has sprouted across the international political landscape.

Trees “are having a bit of a moment right now,” says Joe Fargione, an ecologist with The Nature Conservancy who is based in Minneapolis. It helps that everybody likes trees. “There’s no anti-tree lobby. [Trees] have lots of benefits for people. Not only do they store carbon, they help provide clean air, prevent soil erosion, shade and shelter homes to reduce energy costs and give people a sense of well-being.”

Conservationists are understandably eager to harness this enthusiasm to combat climate change. “We’re tapping into the zeitgeist,” says Justin Adams, executive director of the Tropical Forest Alliance at the World Economic Forum, an international nongovernmental organization based in Geneva. In January 2020, the World Economic Forum launched the One Trillion Trees Initiative, a global movement to grow, restore and conserve trees around the planet. One trillion is also the target for other organizations that coordinate global forestation projects, such as Plant-for-the-Planet’s Trillion Tree Campaign and Trillion Trees, a partnership of the World Wildlife Fund, the Wildlife Conservation Society and other conservation groups.

A carbon-containing system

Forests store carbon aboveground and below. That carbon returns to the atmosphere by microbial activity in the soil, or when trees are cut down and die.

Trees removed from forests can cause carbon losses through fire, processing, soil erosion and decompositionRespiration in the soil, by microbes and other organisms, returns carbon to the atmosphereLeaf litter adds carbon to soil and retains moisture and nutrientsTrees and other vegetation take up atmospheric carbon through photosynthesisCarbon is aboveground in branches, trunk, foliageCarbon is below­ground in leaf litter, roots, soil, fungi, bacteria

SOURCE: MINNESOTA BOARD OF WATER AND SOIL RESOURCES 2019; images: T. Tibbitts

Yet, as global eagerness for adding more trees grows, some scientists are urging caution. Before moving forward, they say, such massive tree projects must address a range of scientific, political, social and economic concerns. Poorly designed projects that don’t address these issues could do more harm than good, the researchers say, wasting money as well as political and public goodwill. The concerns are myriad: There’s too much focus on numbers of seedlings planted, and too little time spent on how to keep the trees alive in the long term, or in working with local communities. And there’s not enough emphasis on how different types of forests sequester very different amounts of carbon. There’s too much talk about trees, and not enough about other carbon-storing ecosystems.

“There’s a real feeling that … forests and trees are just the idea we can use to get political support” for many, perhaps more complicated, types of landscape restoration initiatives, says Joseph Veldman, an ecologist at Texas A&M University in College Station. But that can lead to all kinds of problems, he adds. “For me, the devil is in the details.”

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The root of the problem

The pace of climate change is accelerating into the realm of emergency, scientists say. Over the last 200 years, human-caused emissions of greenhouse gases, including CO2 and methane, have raised the average temperature of the planet by about 1 degree Celsius (SN: 12/22/18 & 1/5/19, p. 18).

The litany of impacts of this heating is familiar by now. Earth’s poles are rapidly shedding ice, which raises sea levels; the oceans are heating up, threatening fish and food security. Tropical storms are becoming rainier and lingering longer, and out of control wildfires are blazing from the Arctic to Australia (SN: 12/19/20 & 1/2/21, p. 32).

The world’s oceans and land-based ecosystems, such as forests, absorb about half of the carbon emissions from fossil fuel burning and other industrial activities. The rest goes into the atmosphere. So “the majority of the solution to climate change will need to come from reducing our emissions,” Fargione says. To meet climate targets set by the 2015 Paris Agreement, much deeper and more painful cuts in emissions than nations have pledged so far will be needed in the next 10 years.

We invest a lot in tree plantings, but we are not sure what happens after that.

Lalisa Duguma

But increasingly, scientists warn that reducing emissions alone won’t be enough to bring Earth’s thermostat back down. “We really do need an all-hands-on-deck approach,” Fargione says. Specifically, researchers are investigating ways to actively remove that carbon, known as negative emissions technologies. Many of these approaches, such as removing CO2 directly from the air and converting it into fuel, are still being developed.

But trees are a ready kind of negative emissions “technology,” and many researchers see them as the first line of defense. In its January 2020 report, “CarbonShot,” the World Resources Institute, a global nonprofit research organization, suggested that large and immediate investments in reforestation within the United States will be key for the country to have any hope of reaching carbon neutrality — in which ongoing carbon emissions are balanced by carbon withdrawals — by 2050. The report called for the U.S. government to invest $4 billion a year through 2030 to support tree restoration projects across the United States. Those efforts would be a bridge to a future of, hopefully, more technologies that can pull large amounts of carbon out of the atmosphere.

The numbers game

Earth’s forests absorb, on average, 16 billion metric tons of CO2 annually, researchers reported in the March Nature Climate Change. But human activity can turn forests into sources of carbon: Thanks to land clearing, wildfires and the burning of wood products, forests also emit an estimated 8.1 billion tons of the gas back to the atmosphere.

That leaves a net amount of 7.6 billion tons of CO2 absorbed by forests per year — roughly a fifth of the 36 billion tons of CO2 emitted by humans in 2019. Deforestation and forest degradation are rapidly shifting the balance. Forests in Southeast Asia now emit more carbon than they absorb due to clearing for plantations and uncontrolled fires. The Amazon’s forests may flip from carbon sponge to carbon source by 2050, researchers say (SN Online: 1/10/20). The priority for slowing climate change, many agree, should be saving the trees we have.

Forests in flux

While global forests were a net carbon sink of about 7.6 gigatons of carbon dioxide per year from 2001 to 2019, forests in areas such as Southeast Asia and parts of the Amazon began releasing more carbon than they store.
Tap map to enlarge

Net annual average contribution of carbon dioxide from Earth’s forests, 2001–2019
SOURCE: N.L. HARRIS ET AL/NATURE CLIMATE CHANGE 2021

Just how many more trees might be mustered for the fight is unclear, however. In 2019, Thomas Crowther, an ecologist at ETH Zurich, and his team estimated in Science that around the globe, there are 900 million hectares of land — an area about the size of the United States — available for planting new forests and reviving old ones (SN: 8/17/19, p. 5). That land could hold over a trillion more trees, the team claimed, which could trap about 206 billion tons of carbon over a century.

That study, led by Jean-Francois Bastin, then a postdoc in Crowther’s lab, was sweeping, ambitious and hopeful. Its findings spread like wildfire through media, conservationist and political circles. “We were in New York during Climate Week [2019], and everybody’s talking about this paper,” Adams recalls. “It had just popped into people’s consciousness, this unbelievable technology solution called the tree.”

To channel that enthusiasm, the One Trillion Trees Initiative incorporated the study’s findings into its mission statement, and countless other tree-planting efforts have cited the report.

But critics say the study is deeply flawed, and that its accounting — of potential trees, of potential carbon uptake — is not only sloppy, but dangerous. In 2019, Science published five separate responses outlining numerous concerns. For example, the study’s criteria for “available” land for tree planting were too broad, and the carbon accounting was inaccurate because it assumes that new tree canopy cover equals new carbon storage. Savannas and natural grasslands may have relatively few trees, critics noted, but these regions already hold plenty of carbon in their soils. When that carbon is accounted for, the carbon uptake benefit from planting trees drops to perhaps a fifth of the original estimate.

Trees are having a bit of a moment right now.

Joe Fargione

There’s also the question of how forests themselves can affect the climate. Adding trees to snow-covered regions, for example, could increase the absorption of solar radiation, possibly leading to warming.

“Their numbers are just so far from anything reasonable,” Veldman says. And focusing on the number of trees planted also sets up another problem, he adds — an incentive structure that is prone to corruption. “Once you set up the incentive system, behaviors change to basically play that game.”

Adams acknowledges these concerns. But, the One Trillion Trees Initiative isn’t really focused on “the specifics of the math,” he says, whether it’s the number of trees or the exact amount of carbon sequestered. The goal is to create a powerful climate movement to “motivate a community behind a big goal and a big vision,” he says. “It could give us a fighting chance to get restoration right.”

Other nonprofit conservation groups, like the World Resources Institute and The Nature Conservancy, are trying to walk a similar line in their advocacy. But some scientists are skeptical that governments and policy makers tasked with implementing massive forest restoration programs will take note of such nuances.

“I study how government bureaucracy works,” says Forrest Fleischman, who researches forest and environmental policy at the University of Minnesota in St. Paul. Policy makers, he says, are “going to see ‘forest restoration,’ and that means planting rows of trees. That’s what they know how to do.”

Counting carbon

How much carbon a forest can draw from the atmosphere depends on how you define “forest.” There’s reforestation — restoring trees to regions where they used to be — and afforestation — planting new trees where they haven’t historically been. Reforestation can mean new planting, including crop trees; allowing forests to regrow naturally on lands previously cleared for agriculture or other purposes; or blending tree cover with croplands or grazing areas.

In the past, the carbon uptake potential of letting forests regrow naturally was underestimated by 32 percent, on average — and by as much as 53 percent in tropical forests, according to a 2020 study in Nature. Now, scientists are calling for more attention to this forestation strategy.

If it’s just a matter of what’s best for the climate, natural forest regrowth offers the biggest bang for the buck, says Simon Lewis, a forest ecologist at University College London. Single-tree commercial crop plantations, on the other hand, may meet the technical definition of a “forest” — a certain concentration of trees in a given area — but factor in land clearing to plant the crop and frequent harvesting of the trees, and such plantations can actually release more carbon than they sequester.

Comparing the carbon accounting between different restoration projects becomes particularly important in the framework of international climate targets and challenges. For example, the 2011 Bonn Challenge is a global project aimed at restoring 350 million hectares by 2030. As of 2020, 61 nations had pledged to restore a total of 210 million hectares of their lands. The potential carbon impact of the stated pledges, however, varies widely depending on the specific restoration plans.

Levels of protection

The Bonn Challenge aims to globally reforest 350 million hectares of land. Allowing all to regrow naturally would sequester 42 gigatons of carbon by 2100. Pledges of 43 tropical and subtropical nations that joined by 2019 — a mix of plantations and natural regrowth — would sequester 16 gigatons of carbon. If some of the land is later converted to biofuel plantations, sequestration is 3 gigatons. With only plantations, carbon storage is 1 gigaton.

Amount of carbon sequestered by 2100 in four Bonn Challenge scenarios

SOURCE: S.L. LEWIS ET AL/NATURE 2019; graphs: T. Tibbitts

In a 2019 study in Nature, Lewis and his colleagues estimated that if all 350 million hectares were allowed to regrow natural forest, those lands would sequester about 42 billion metric tons (gigatons in chart above) of carbon by 2100. Conversely, if the land were to be filled with single-tree commercial crop plantations, carbon storage drops to about 1 billion metric tons. And right now, plantations make up a majority of the restoration plans submitted under the Bonn Challenge.

Striking the right balance between offering incentives to landowners to participate while also placing certain restrictions remains a tricky and long-standing challenge, not just for combating the climate emergency but also for trying to preserve biodiversity (SN: 8/1/20, p. 18). Since 1974, Chile, for example, has been encouraging private landowners to plant trees through subsidies. But landowners are allowed to use these subsidies to replace native forestlands with profitable plantations. As a result, Chile’s new plantings not only didn’t increase carbon storage, they also accelerated biodiversity losses, researchers reported in the September 2020 Nature Sustainability.

The reality is that plantations are a necessary part of initiatives like the Bonn Challenge, because they make landscape restoration economically viable for many nations, Lewis says. “Plantations can play a part, and so can agroforestry as well as areas of more natural forest,” he says. “It’s important to remember that landscapes provide a whole host of services and products to people who live there.”

But he and others advocate for increasing the proportion of forestation that is naturally regenerated. “I’d like to see more attention on that,” says Robin Chazdon, a forest ecologist affiliated with the University of the Sunshine Coast in Australia as well as with the World Resources Institute. Naturally regenerated forests could be allowed to grow in buffer regions between farms, creating connecting green corridors that could also help preserve biodiversity, she says. And “it’s certainly a lot less expensive to let nature do the work,” Chazdon says.

Indeed, massive tree-planting projects may also be stymied by pipeline and workforce issues. Take seeds: In the United States, nurseries produce about 1.3 billion seedlings per year, Fargione and colleagues calculated in a study reported February 4 in Frontiers in Forests and Global Change. To support a massive tree-planting initiative, U.S. nurseries would need to at least double that number.

A tree-planting report card

From China to Turkey, countries around the world have launched enthusiastic national tree-planting efforts. And many of them have become cautionary tales.

China kicked off a campaign in 1978 to push back the encroaching Gobi Desert, which has become the fastest-growing desert on Earth due to a combination of mass deforestation and overgrazing, exacerbated by high winds that drive erosion. China’s Three-North Shelter Forest Program, nicknamed the Great Green Wall, aims to plant a band of trees stretching 4,500 kilometers across the northern part of the country. The campaign has involved millions of seeds dropped from airplanes and millions more seedlings planted by hand. But a 2011 analysis suggested that up to 85 percent of the plantings had failed because the nonnative species chosen couldn’t survive in the arid environments they were plopped into.

A woman places straw in March 2019 to fix sand in place before planting trees at the edge of the Gobi Desert in China’s Minqin County. Her work is part of a private tree-planting initiative that dovetails with the government’s decades-long effort to build a “green wall” to hold back the desert.WANG HE/GETTY IMAGES PLUS

More recently, Turkey launched its own reforestation effort. On November 11, 2019, National Forestation Day, volunteers across the country planted 11 million trees at more than 2,000 sites. In Turkey’s Çorum province, 303,150 saplings were planted in a single hour, setting a new world record.

Within three months, however, up to 90 percent of the new saplings inspected by Turkey’s agriculture and forestry trade union were dead, according to the union’s president, Şükrü Durmuş, speaking to the Guardian (Turkey’s minister of agriculture and forestry denied that this was true). The saplings, Durmuş said, died due to a combination of insufficient water and because they were planted at the wrong time of year, and not by experts.

Some smaller-scale efforts also appear to be failing, though less spectacularly. Tree planting has been ongoing for decades in the Kangra district of Himachal Pradesh in northern India, says Eric Coleman, a political scientist at Florida State University in Tallahassee, who’s been studying the outcomes. The aim is to increase the density of the local forests and provide additional forest benefits for communities nearby, such as wood for fuel and fodder for grazing animals. How much money was spent isn’t known, Coleman says, because there aren’t records of how much was paid for seeds. “But I imagine it was in the millions and millions of dollars.”

Coleman and his colleagues analyzed satellite images and interviewed members of the local communities. They found that the tree planting had very little impact one way or the other. Forest density didn’t change much, and the surveys suggested that few households were gaining benefits from the planted forests, such as gathering wood for fuel, grazing animals or collecting fodder.

But massive tree-planting efforts don’t have to fail. “It’s easy to point to examples of large-scale reforestation efforts that weren’t using the right tree stock, or adequately trained workforces, or didn’t have enough investment in … postplanting treatments and care,” Fargione says. “We … need to learn from those efforts.”

Speak for the trees

Forester Lalisa Duguma of World Agroforestry in Nairobi, Kenya, and colleagues explored some of the reasons for the very high failure rates of these projects in a working paper in 2020. “Every year there are billions of dollars invested [in tree planting], but forest cover is not increasing,” Duguma says. “Where are those resources going?”

Trees can buy time for tech testing

If done right, planting trees might give researchers time to develop some of these carbon-capture technologies.

Bioenergy with carbon capture and sequestration

Plant biomass is used to produce electricity, fuel or heat. Any CO2 released is captured and stored.

Direct air capture

Chemical processes that capture CO2 from ambient air and concentrate it, so that it can be injected into a storage reservoir.

Carbon mineralization

Through chemical reactions, CO2 from the atmosphere becomestrapped in existing rock.

Geologic sequestration

CO2 is captured and injected into deep underground formations.

images: T. Tibbitts

In 2019, Duguma raised this question at the World Congress on Agroforestry in Montpellier, France. He asked the audience of scientists and conservationists: “How many of you have ever planted a tree seedling?” To those who raised their hands, he asked, “Have they grown?”

Some respondents acknowledged that they weren’t sure. “Very good! That’s what I wanted,” he told them. “We invest a lot in tree plantings, but we are not sure what happens after that.”

It comes down to a deceptively simple but “really fundamental” point, Duguma says. “The narrative has to change — from tree planting to tree growing.

The good news is that this point has begun to percolate through the conservationist world, he says. To have any hope of success, restoration projects need to consider the best times of year to plant seeds, which seeds to plant and where, who will care for the seedlings as they grow into trees, how that growth will be monitored, and how to balance the economic and environmental needs of people in developing countries where the trees might be planted.

“That is where we need to capture the voice of the people,” Duguma says. “From the beginning.”

Even as the enthusiasm for tree planting takes root in the policy world, there’s a growing awareness among researchers and conservationists that local community engagement must be built into these plans; it’s indispensable to their success.

“It will be almost impossible to meet these targets we all care so much about unless small farmers and communities benefit more from trees,” as David Kaimowitz of the United Nations’ Food and Agriculture Organization wrote March 19 in a blog post for the London-based nonprofit International Institute for Environment and Development.

For one thing, farmers and villagers managing the land need incentives to care for the plantings and that includes having clear rights to the trees’ benefits, such as food or thatching or grazing. “People who have insecure land tenure don’t plant trees,” Fleischman says.

Fleischman and others outlined many of the potential social and economic pitfalls of large-scale tree-planting projects last November in BioScience. Those lessons boil down to this, Fleischman says: “You need to know something about the place … the political dynamics, the social dynamics.… It’s going to be very different in different parts of the world.”

The old cliché — think globally, act locally — may offer the best path forward for conservationists and researchers trying to balance so many different needs and still address climate change.

“There are a host of sociologically and biologically informed approaches to conservation and restoration that … have virtually nothing to do with tree planting,” Veldman says. “An effective global restoration agenda needs to encompass the diversity of Earth’s ecosystems and the people who use them.”

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The impact of trees and climate change

Climate change is a hotly debated topic. As more and more people are moving toward a more sustainable lifestyle to protect the planet, it's helpful to appreciate the importance of trees. In this article, we will look at the impact of trees on climate change.

How do trees affect climate change?

The importance of trees, photo: cnews

Trees and climate change are inextricably linked. There is an important relationship between CO2 levels and tree cover. In recent history, the world has lost many thousands of miles of forests when emissions have increased due to industrial and commercial activities and changes in our lifestyle. Global warming is happening at an alarming rate. The Intergovernmental Panel on Climate Change (IPCC) has issued a stern warning of rising temperatures. To preserve ecosystems and wildlife and protect against natural disasters, we need to limit warming to a maximum of 1.5°C. The temperature has already risen by 1°.

Climate change is a real threat, but how do trees affect climate change and what role can they play in lowering temperatures and saving the planet?

The most important function of trees is their ability to sequester carbon. Forests, woodlands and jungles absorb carbon by storing it in tree trunks and removing carbon dioxide from the air. According to the UK-based Woodland Trust, wood from young trees would contain more than 400 tons of carbon per hectare. It is estimated that the world's forests currently absorb about a third of our emissions. Carbon dioxide emitted by human activities, photo: ua-energy
Landing potential

A study in Switzerland suggests that a global tree planting program could remove up to two thirds of human carbon. Researchers at ETH Zurich found that there are currently 1.7 billion hectares of treeless land that can grow up to 1.2 trillion seedlings. The results of the study were published in the journal Science, and it is not surprising that they caused controversy and intrigue. According to the study, by planting more than half a trillion trees, we could capture about 205 gigatons (a gigaton is equivalent to 1 billion metric tons) of carbon.

One of the most important factors to consider when examining the results of studies such as the ETH Zurich project are the drivers of climate change. The research team aimed to point out that tree planting is not a miracle cure. We must plant trees along with a number of other measures to reduce emissions. These include switching to renewable energy sources, using less energy, reducing waste and working towards national, international and global zero carbon targets. Tree planting, photo: sad
Impact of deforestation

Deforestation is one of the most worrisome factors contributing to the acceleration of global temperatures and the increase in natural disasters. According to the Food and Agriculture Organization of the United Nations, the world is currently losing up to 7 million hectares of forests annually. Only in the USA since 1600 90% of their primary forests. In the 21st century, 400 million hectares of forests have been disturbed by natural disasters, including forest fires and floods, or by human activities such as construction, deforestation, new infrastructure or agriculture.

There are several reasons why deforestation has become fruitful. Natural disasters have destroyed some forests, but the most common cause of loss is human activity. People are now using the land to expand cities, create new settlements, or convert it into farmland. People also cut trees for commercial purposes, including cutting and selling to manufacturers and corporations for industrial or construction purposes.

Deforestation, photo: ecoportal
Are trees the answer to climate change?

Researchers around the world are working on projects and innovations that can help prevent rising temperatures, protect wildlife species and preserve the natural environment. Research shows that trees can have an incredibly positive effect on global warming, but planting isn't the only solution. Reforestation and planting new trees can help reduce carbon dioxide levels, but these initiatives are being implemented along with additional measures.

One of the problems is that in some areas trees are planted and then cut down and used as fuel, logs or building materials. The combustion process releases more carbon, defeating the purpose of using trees to absorb carbon dioxide and store carbon.

Reforestation is not the same as planting new trees (known as afforestation) and it is helpful to understand the difference between the two when considering the benefits. Reforestation refers to planting in areas that used to be forests and woodlands. In this case, seedlings replace mature trees cut down, in most cases, for human needs. Planting new trees is a means of expanding forests and identifying recent locations for young trees.

Reforestation, photo: regnum
Research shows that there is a huge amount of space on earth for growing new trees, and more plantings could have a positive impact in the future.

Secondary impact of trees on climate change

The most obvious benefit of trees is carbon absorption and carbon storage. However, there are additional benefits of increasing the number of trees on the planet.

Trees don't just help reduce the risk of rising temperatures and carbon dioxide levels in the atmosphere. They also help fight the effects of climate change. Our changing climate poses risks to the environment as well as to human health. As temperatures rise, the risk of floods, changes in weather patterns, storms and hurricanes, droughts and unpredictable conditions increases. Trees can help combat these effects in several ways.

Flooding, photo: bbc
Flooding Prevention: trees create a canopy that slows the rate of rain falling to the ground by intercepting drops. Some of the water droplets that trap the leaves can evaporate into the air, reducing the amount of rain that reaches the ground. The root system of trees also plays a significant role. Roots encourage the soil to absorb water at a deeper level, meaning less moisture reaches the surface, where it creates problems for people and businesses. Planting trees in urban areas can also reduce runoff, which is high in cities due to the abundance of paths, roads, paving, patios, and driveways.

Temperature reduction in cities and towns: trees provide shade, reflect sunlight and facilitate evaporation, providing benefits in urban areas that can be hotspots.

Pollution reduction: The world's forests absorb about one-third of emissions. Trees capture carbon dioxide, storing carbon as biomass and creating cleaner air we breathe. As emissions increase due to our lifestyle and population growth, trees play a vital role in reducing pollution. Importance of trees for urban infrastructure, photo: myc
Improving soil quality: Good quality soil is vital for agriculture, land management, healthy planting and cultivation, and food production. Trees help improve soil quality, as roots increase water uptake and leaf litter adds vital nutrients to the soil.

Useful practical advice on planting and reforestation

Tree planting can be an effective solution to reduce the risks of climate change and protect the planet, but it is important to effectively plan, manage and implement planting and reforestation programs.

Planting trees is not as easy as choosing a place, choosing a species and planting a seedling in the ground. Here are some useful tree planting tips:

Choose trees that suit the conditions, local climate and environment. Planting trees in places where they do not grow well can be counterproductive.

Selection of locations that will not negatively impact existing ecosystems or wildlife.

Mixing tree species to increase resistance to climatic conditions and pests. Planting trees in the city, photo: ecology
Ensuring that forests have time to mature and develop. It may take decades for forests to reach their full potential.

Engage and communicate with local people and communities to understand what the project includes and why it is useful. In many cases, greening programs are most successful when organizations join forces with local communities and volunteers.

Effective project management at every stage, from idea generation and site selection to natural habitat and wildlife conservation and land protection.

Summary of tree impacts and climate change

Climate change is a topic that dominates the headlines. More and more people are interested in protecting the planet and are looking for ways to do their part. Trees can have a huge positive impact on the environment, but they are not a climate change miracle cure. Trees absorb carbon dioxide and store huge amounts of carbon. The impact of trees on climate change, photo: profile
Reforestation and reforestation initiatives can make a difference, but we must also implement and advocate for additional measures. These include the use of renewable energy sources, reducing energy consumption and the use of green work, travel and the production of goods. Planting more trees can increase carbon sequestration, but it is important to choose the right sites to plant or replant and choose the right species based on environmental conditions.

Research on the impact of trees on climate change is positive and the creation of new forests could be beneficial. If we do this at the same time, while taking environmentally friendly measures and working to significantly reduce our carbon footprint, the future may look much brighter.

Planting trees will not slow down the rate of global warming

Nothing, not even the creation of huge tree plantations designed to absorb carbon dioxide, is a viable alternative to drastically reducing greenhouse gas emissions. Humanity will not be able to solve its problem by planting trees - they will not be able to absorb the ever-increasing amount of carbon dioxide emissions into the atmosphere.

According to a new study, there is simply no alternative to drastically cutting greenhouse gas emissions if governments are truly committed to limiting global average temperature rise to 2°C.

Trees can be seen as a simple mechanism for reducing carbon dioxide concentrations, and, in theory, effective management of tree plantations would absorb the carbon dioxide released by burning fossil fuels. However, scientists believe that the sheer scale of such plantations would have devastating environmental consequences.

“If we continue to burn coal and oil as much as we do today (which we will regret in the future), then the amount of greenhouse gases that we have to remove from the atmosphere to stabilize the climate will be too much for us to deal with it” , says Lena Boysen of the Potsdam Institute for Climate Impact Research, who led the study published in Earth's Future.

Trees are not a panacea

If fertile land is planted with forests, humanity will lose much-needed soil to feed 9 billion people. If trees are planted on less fertile soil, the costs required to provide them with water and nitrogen fertilizers will be simply enormous. In any case, it can cause irreparable harm to natural ecosystems.

Trees grown to absorb carbon would then have to be planted deep into the ground to prevent carbon from returning to the atmosphere, which could increase the rate of global warming and could not be contained.

“Even if we could use efficient plants such as poplar or switchgrass and retain 50% of the carbon contained in their biomass in a business-as-usual scenario of continuous and unrestricted use of fossil fuels, the size of plantations needed to limit the growth of average global temperature of 2°C will lead to devastating environmental consequences” , Boysen says.

The world's largest forests are part of the climate mechanism. In 2015, more than 195 countries signed the Paris Agreement to curb the pace of climate change, both through changing land-use technologies and moving away from fossil fuels in favor of renewable energy sources.

Thus, carbon storage through forests is only one part of a comprehensive solution to the problem. Boysen and her colleagues report that they looked at a number of scenarios to see if, even in theory, massive investment in tree planting would remove enough carbon from the atmosphere.

“In the context of the climate drama that we are seeing today on this big stage called Earth, CO2 removal will be of no critical importance if all other measures fail.”

One of the scenarios considered tree planting on 6.9 billion hectares, which would require 570 million tons of nitrogen annually, while even the smallest theoretical plantation would extend over a billion hectares and consume 96 million tons of nitrogen fertilizers annually. One billion hectares is more than 10 million square kilometers, which is larger than the territory of Canada.

Another of the scenarios considered involved filling a quarter of the world's agricultural land with infertile biomass - tree trunks that would be buried. In addition, another scenario could result in the loss of natural ecosystems equivalent to one third of the world's existing forest land.

“As scientists, we look at all future scenarios, not just the positive ones,” says Wolfgang Lucht, one of the participants in the Potsdam Institute study.

“What happens in the worst-case scenario: the collapse and complete failure of climate change mitigation policies? Will plants allow us to stabilize the climate in an “emergency” mode? The answer is "no". There is no alternative to effective climate change mitigation measures. In such a scenario, plants can play a limited but important role if managed effectively.”

Climatologists recently took inventory of the world's forests. They counted over 1 trillion trees and established the number of known species.

They determined the relative importance, since both old and young trees “sink” carbon, and even re-examined the drylands of the globe, as a result of which they were able to find vast and previously unaccounted for forest lands, an area of ​​\u200b\u200bthe size of the European Union, scattered around dry plains. However, while forest conservation is an important part of solving the climate problem, the deliberate planting of trees can be counterproductive.

The authors note that one of the biomass plantation options would require 10% to 25% of the world's agricultural land, which would be equivalent to a loss of 43% to 73% of annual global calorie production.

Fossil fuel use

Scientists considered a scenario in which the world would replace all meat and dairy production in favor of “grassland” plants, but even this “would not lead to a significant improvement in climate change” . This is due to the fact that, despite the fact that rangelands are larger than agricultural land, they are less productive. The authors once again return to the need to reduce the use of fossil fuels.

“In the context of the climate drama we're seeing today on this big stage called Earth, CO2 removal will be of no critical importance if all other measures fail.


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