New studies raise issues that complicate the ambitious goals to use mass tree-planting to fight climate change. The researchers warn that in some cases, carbon-offset tree plantations could reduce biodiversity, doing more harm than good for the environment.
Trees take in CO2 and store it in their woody mass and the soil, making them a natural solution for reducing greenhouse gases in the atmosphere. Globally, it is estimated that forests absorb enough CO2 each year to make up for one-and-a-half times the greenhouse gases that the United States emits annually. That kind of carbon drawdown capacity motivated large-scale tree planting efforts such as the global Trillion Tree campaign, launched in 2006. Sophisticated carbon credit markets have also been developed, which allow polluting companies to pay for tree planting in order to offset their emissions.
But trees that die, burn or are cut can release that CO2 back into the atmosphere, and trees planted in large stands of only a few species do not function the way that naturally diverse forest ecosystems do.
A study of tree planting in the tropics published Tuesday, in the monthly journal Trends in Ecology and Evolution, argues that these carbon-offset plantations can come at the expense of biodiversity and other important services that forests provide. The study’s authors also warn that an emphasis on carbon reduction alone can lead to poor environmental decisions.
“It is crucial to shift from the narrow focus on carbon and adopt a more holistic perspective if we aim to effectively conserve and restore natural ecosystems and combat climate change,” the study’s lead author, Jesús Aguirre-Gutiérrez, an ecologist and senior researcher at the University of Oxford’s Environmental Change Institute, told Newsweek via email.
The scale of tree planting necessary to offset global greenhouse gas emissions would require vast areas of land, the authors write, often leading tree planters to displace other ecosystems. Afforestation, or planting trees in places they didn’t previously grow, can replace tropical grasslands that also provide carbon sequestration and other important services, such as regulating water flow and enriching soil, the authors contend. Grasslands support biodiversity as well, and many species adapted for those environments suffer when grass is replaced by trees.
For example, in Brazil’s Cerrado savannah, increasing tree cover by 40 percent reduced the diversity of plants and ants by about 30 percent.
“Planting trees is great as far as they are planted in areas where they belong,” Aguirre-Gutiérrez said.
The study finds that tree plantations using only a few species can also reduce stream flow, deplete groundwater, contribute to more intense wildfires and acidify soils. Aguirre-Gutiérrez said that biodiversity and other forest functions are not as easily quantified—and thus monetized—as carbon reduction, leading to imbalanced priorities in forestry.
Meanwhile, another study, published Monday in the monthly journal Nature Geoscience, provides the first map of yearly changes in global forest biomass over the past decade, revealing that temperate zone and northern, or boreal, forests have become the world’s main carbon sinks, drawing down more carbon than tropical forests have.
Younger, faster-growing trees typically absorb more carbon than mature ones, and northern and temperate forests have comparatively more areas of younger growth.
Lead author Jean-Pierre Wigneron, of France’s National Research Institute for Agriculture, Food and Environment, said tropical forests hold an immense amount of carbon, especially in older trees. But many of the world’s tropical forests are now so degraded by deforestation, fire and drought that they are nearly carbon neutral or, in some cases, sources of CO2 emissions.
“With tropical forests, the main takeaway is not to plant more trees, necessarily, but to protect the ones that are there,” Wigneron told Newsweek, adding that corporate money directed to tree plantations would be better spent on conservation. “If they really want to protect the planet, they should first take care of protecting forests instead of replanting.”
The new research comes as tree-planting climate campaigns have come under high-profile criticism. Microsoft founder and clean energy philanthropist Bill Gates took a jab at forest climate solutions at a September 19 Climate Week event in New York City, sponsored by the New York Times.
“I don’t plant trees,” Gates said, lumping tree-planting in with what he called less-proven approaches to reducing emissions.
When event moderator David Gelles said that some proponents claim that planting enough trees could resolve climate change, Gates called that “complete nonsense.”
Jad Daley said he finds that kind of criticism frustrating. Daley is President and CEO of American Forests, a nonprofit conservation group that has been promoting forestry for more than a century. American Forests is also a partner in the Trillion Trees campaign, which is promoted and funded by billionaire Salesforce founder Marc Benioff.
Daley told Newsweek that what should be an important and nuanced discussion about forests and climate change has instead fallen into a “false, all-or-nothing narrative” that ignores the clear benefits of planting more trees.
“No one is saying that a trillion trees or any other amount of forest could solve climate change. We never said they could do it alone,” Daley said. “With that said, trees and forests are an absolutely essential part of the climate change solution.”
Daley pointed to the 2022 report from the Intergovernmental Panel on Climate Change, which includes forestry solutions to help meet emissions reductions targets. The IPCC’s summary for policymakers concludes that most pathways to limit dangerous warming will require “some combination of reforestation, afforestation, and reducing deforestation.”
In the U.S., EPA data shows that forests in the U.S. absorb nearly 16 percent of the greenhouse gas emissions that Americans produce each year, and Daley said responsible forestry practices could boost that carbon-emission offset to more than 20 percent of net U.S. emissions.
American Forests worked with the Nature Conservancy over the past three years to map out nearly 150 million acres in the U.S. where tree cover could be appropriately increased.
Daley agreed with the emphasis that the researchers behind this week’s new studies place on balancing biodiversity with carbon-removal goals. But he said he’s concerned that without proper context, the new studies could be misinterpreted in ways that undercut forest climate solutions.
For example, Daley fears that the findings that show some tropical forests no longer remove as much CO2 from the atmosphere could erode public support for forestry work in those areas.
“It’s actually a reason to work more in those places,” he said. “The question that we want to ask is, what forestry actions could help those forests get back to being a larger net sink of carbon?”
The studies out this week provide important guidance and new information to help foresters and government leaders better manage their woods for both climate solutions and broader environmental goals.
The new remote sensing data and mapping provided by Wigneron and his colleagues at France’s National Research Institute will give many nations their first accurate accounting of the carbon cycle in their forests.
“Most of the countries don’t know if they are carbon sinks or carbon sources,” Wigneron said. “It could help them to understand better where they could put efforts.”
The work by Aguirre-Gutiérrez and his co-authors presents a framework for certification of carbon-offset projects that could ensure that other ecosystem functions are not negatively impacted.
“Current and new policy should not promote ecosystem degradation via tree plantations with a narrow view on carbon capture,” the study’s authors concluded.
In the push for natural climate solutions, Aguirre-Gutiérrez and other scientists are urging, let’s not lose sight of the forest for the trees.