Researchers say “proforestation” policies are the fastest and most effective way to draw excess CO2 out of the atmosphere.
Kate S. Petersen
Jan 19, 2021
HAMPSHIRE COUNTY, Mass.—Bob Leverett walked away from the trunk, looking up through the canopy, trying to get eyes on the crown.
He crushed the thick pine needle duff with each step, while a light drizzle tapped on the leaves above him, and birds called from a distance. Then he saw it, the top of the tree, and measured its height with a small instrument he raised to his eye. He would combine this measurement with others to calculate the mass of the tree, a monolithic white pine in western Massachusetts. Once he found the mass, he could approximate how much carbon it contained, carbon the tree had been pulling out of the atmosphere, in the form of carbon dioxide, for well over 100 years.
The drizzle stopped by the time he finished taking his measurements, so that only sporadic drops of water fell as they lost their balance in the canopy. He attended to his calculator and then his eyes moved up the furrowed grey-brown trunk, “Twenty tons, roughly, of carbon dioxide absorbed from the atmosphere. That’s the contribution of that whopping big tree,” Leverett, co-founder of the Native Tree Society, told EHN.
An engineer by training, Leverett has worked for decades to document and educate the public about remnant stands of old growth forest in Massachusetts and, more recently, to quantify differences in carbon uptake and sequestration between younger and older forest stands.
This difference is particularly relevant as researchers and lawmakers consider the potential for natural solutions—Earth’s intrinsic carbon sequestration systems—to be part of large scale climate change mitigation strategies.
The Intergovernmental Panel on Climate Change (IPCC) 1.5°C special report released in 2018 found that, in addition to dramatic emissions reductions, humans must quickly find a way to remove a tremendous amount of existing carbon dioxide from the atmosphere in order to stay below a 1.5°C rise in average global temperatures and avoid the worst climate change related harms.
Given the magnitude of this task and no proven technology to accomplish it, trees—for their ability to cleave carbon dioxide molecules and tuck the carbon into their tissue as they grow— have received widespread attention as a means to sequester significant amounts of excess carbon.
But as talks of massive tree planting ventures get under way, Leverett and other researchers are attempting to make an important distinction. They say that, while tree planting campaigns can play a role in climate change mitigation, it is the forests that are standing now that can sequester carbon most effectively in the near term.
They also warn that these invaluable assets are being squandered as forests are cleared worldwide.
A new scientific term
Bill Moomaw is a co-author on five IPCC reports and the co-director of the Global Development and Environment Institute at Tufts University. He started working on climate change solutions in 1988, researching technological and policy strategies for reducing emissions. But more recently, his focus shifted toward the natural sequestration systems that are already working to reduce the impact of human emissions. For instance, according to the U.S. Environmental Protection Agency, U.S. forests presently sequester roughly 9 percent of the nation’s greenhouse gas emissions in 2016.
“We’ve got to cut emissions, but the other side of it is we could increase removal rates,” said Moomaw. “A lot of people are talking about technologies to do that, but none of those will be in place in the next 10 years when we need it.”
In 2019, Moomaw and his co-authors published a scientific review finding that the capacity of forested lands to sequester carbon dioxide could be increased significantly. They say the fastest way to do this is through what they call “proforestation,” the natural growth and development of standing forest ecosystems.
They devised the term because, unlike forest-based interventions currently being evaluated for their climate change mitigation capacity, such as reforestation or afforestation, there was not a succinct term that scientists and policymakers could use to discuss the carbon value of naturally developing, undisturbed forests.
A proforestation management style is comparable to management currently practiced in U.S National Parks and Wilderness areas—besides necessary intervention for safety, trail maintenance, or restoration—forest ecosystems are left to do their own thing.
Moomaw advocates for an expansion of protected lands where forests are allowed to grow and develop, uninterrupted by resource extraction, as soon as possible.
“Proforestation will sequester more total carbon in the near term, when…it’s most important to do it, than anything else that is out there,” he said.
One reason for this is that newly planted forests may take “decades to a century before they sequester carbon dioxide in substantial quantities,” according to the proforestation review.
“It depends on species and forest type as to where the take off period is,” Moomaw said, but he emphasized that there is no time to waste.
The 1.5°C special report found that, to stay below 1.5°C, net emissions must be nearly halved within 10 years, and net zero accomplished by 2050. Ongoing greenhouse gas removal must continue thereafter through 2100.
Older trees are typically more efficient carbon extractors than younger trees (in comparable environmental conditions) because, as trees get larger, they add more carbon rich mass each year than the year before. For example, one study found that, on average, a 100 cm diameter tree added biomass at three times the rate of a 50 cm diameter tree of the same species.
This is not to say that smaller diameter trees or younger forests have no climate change mitigation value. For instance, if there is a hypothetical 40-year-old stand of a tree species that is known to really escalate carbon sequestration rates at 60 years, that’s still better than starting from scratch.
Recent studies have shown that the opportunity to improve carbon sequestration through proforestation is significant on the timeline laid out by the IPCC.
Researchers recently identified forested areas of the northwestern United States that would be particularly beneficial to preserve for proforestation given their high carbon sequestration potential and relatively low vulnerability to climate change related disturbance such as wildfire. They calculated that these areas could sequester 6 years’ worth of regional fossil fuel emissions by 2099.
Another study found that if currently regenerating secondary forests were allowed to grow worldwide, they could sequester 120 billion metric tons of carbon by 2100—the equivalent of 12 years of global fossil fuel emissions.
“It gives you a sense of the scale that’s possible,” said Moomaw.
Many studies have identified the potential of tree planting efforts to be a viable climate change mitigation strategy, and the proforestation review authors don’t disagree. However, they outline some potential logistical problems with tree planting as a strategy.
Tree planting efforts of any kind require resources and labor which may or may not be available, and afforestation (planting on land that was not previously forested) is particularly fraught because it may require land that is already utilized for other purposes, such as agriculture.
Moomaw said that proforestation, on the other hand, requires no additional land, limited resources, and no labor. “It [utilizes] forests that are already in place,” said Moomaw.
The review also addressed a commonly touted technological solution called bioenergy with carbon capture and storage (BECCS). In this strategy, trees would be grown in plantations to sequester atmospheric carbon. The trees would then be burned to create energy, and the carbon collected during the burning would be injected into the ground.
In addition to requiring an enormous land mass (something nearly the size of Australia by some estimates), the technology has not, to date, been demonstrated to be viable on a large scale.
Even so, excess carbon removal by BECCS is incorporated into various IPCC climate change mitigation scenarios, but Moomaw said that it’s really just a stand in for a removal system of some kind.
“It’s an abstract idea,” he said. “It should have just said ‘Option X is needed to close this gap.’ Instead, they named it…and therefore it sounds like it’s real.”
A Chatham House research paper on BECCS echoed this sentiment.
“The danger at the moment is that policymakers are ‘sleepwalking towards BECCS’ simply because most models incorporate it,” the authors wrote.
Jennifer Skene, an Environmental Law Fellow at the National Resource Defense Council, outlined a related concern pertaining to tree planting campaigns in an article on the organization’s website. She wrote that policies such as the Trillion Trees Act have the potential to become a distraction from the more imperative goals of reducing emissions and protecting standing forests.
Losing carbon banks
Any distraction from forest preservation goals is particularly consequential right now as global tree cover is lost at a rate of about 78,000 square miles per year, according to Mikaela Weisse, a project manager at Global Forest Watch. This is an area about the size of Nebraska. Old, intact forests, those that are relatively free from industrial extraction and typically have high carbon sequestration and biodiversity values, are being lost to cutting and fragmentation at a pace of about 80 square miles per day.
The consequences of these losses include both the forfeiture of future sequestration potential and also the release of ancient carbon stores back into the atmosphere. When a forest is cut, it becomes a greenhouse gas emitter instead of a sink.
According to a Dogwood Alliance report, logging in the U.S. releases as much carbon into the atmosphere as the commercial and residential sectors combined.
Researchers attempting to refine carbon accounting techniques and quantify carbon losses from Oregon forests tracked greenhouse gas emissions from wood products after they leave the forest.
“We track it from the forest, through the manufacturing process, through to product usage and recycling, and [into] landfills,” Bev Law, a co-author on the study and professor emeritus at Oregon State University, told EHN.
The carbon stored in forested lands is lost incrementally over each stage of extraction and processing. According to the researchers, 40% of harvested wood may be discarded on its way to becoming a marketable product.
Trees are limbed and cut to length after they are felled in the forest and logging detritus is often burned to clear harvest sites, releasing stored carbon, according to Tara Hudiburg, a co-author on the paper and an associate professor at the University of Idaho.
When the logs arrive at the mill, more carbon is lost as they are shaped and processed into their final marketable form.
Then there’s the wood products themselves, which the authors write “ultimately release CO2
to the atmosphere as they are manufactured, disposed of, and decompose or are burned.”
Carbon from single-use products such as paper are lost to the atmosphere fairly quickly. While often touted as long term carbon storage assets, forest products such as lumber don’t stick around as long as one would think. Buildings get torn down, replaced, and renovated. Furniture gets discarded. According to the researchers, if current trends continue, 75% of U.S. buildings will be renovated or replaced by 2035.
Of the carbon that has been removed from Oregon forests by logging over the last 100 years (which the researchers noted had taken 800 years to accumulate), Hudiburg, Law, and their team found that 65 percent has been released to the atmosphere.
Forests, on the other hand, can reliably store carbon for hundreds or thousands of years.
Soil carbon sequestration
Leverett walked off the trail, stepping over a row of 8-inch mushrooms and onto the thick, vibrant green moss mat that covered the forest floor. With each of his steps, the moss compressed and then slowly rebounded, lightly jostling the oddly erect and bristly Lycopodium, which stood a scattered army of ancient 6-inch tall spore-bearing plants.
“Look here. Now here’s something that is so characteristic of these old woods,” said Leverett.
He pointed at a long cylindrical mound in the moss, “This is a nurse log. See look here, where these little birches seed in on the log,” he said. “They land on a little bit of organic matter on top…and they sprout.”
Nurse logs, fallen, slowly decaying trees, serve multiple ecological purposes, including a special habitat for more trees to grow and a moisture repository to cool the forest and sustain it through drought. Snags are another classic old forest feature, long dead trees, still standing, providing nutrients and habitat. Unlike the bulk of extracted wood products, researchers have found these features can hold on to their carbon for hundreds of years in temperate regions.
Forest ecology influences rates of decomposition and also the ultimate destination of stored carbon. Interconnected systems of biological decomposers such as bacteria, fungi, and invertebrates facilitate the transfer of carbon from decaying material into the soil.
Carbon is still released to the atmosphere when woody material decays in a forest, but Moomaw and his team report that in old, intact forests, more than half of total carbon stores may be located in the soil, nurse logs, snags, and other woody debris.
For this reason, advocates of climate policies that include proforestation say that forest ecosystems must be emphasized in conversations about carbon sequestration, rather than just the trees—ecosystems that develop over time to entertain the food webs, species composition, canopy structures, temperature and moisture regimes, and special habitats that allow for full carbon sequestration potential, not to mention increased biodiversity and other old forest benefits.
An old, intact forest is “a much more complex place than a young forest,” said Leverett.
Just like the carbon sequestered in trees, soil carbon is often lost to the atmosphere after logging, which researchers say may be due to disturbance related changes in physical, chemical or microbial make-up of the soil.
Making sense of the curve
Recognizing forests’ role in storing carbon and the potential of that carbon to be released, is important for understanding the ramifications of changes in carbon sequestration rates on the stand level as forests mature.
While individual canopy trees usually continue to accelerate carbon sequestration well into old age, rates of carbon accumulation in a forest area as a whole may slow down because of changes in forest structure, such as smaller trees dying or being suppressed by shading from larger trees.
This fact may be misinterpreted to mean that older forests no longer have carbon sequestration value and should be replaced with younger stands.
“Lots of sources pit young forests against old ones and advocate cutting older forests,” said Leverett.
But this position ignores not only the length of time it takes new forests to ramp up sequestration rates, but also their stored carbon.
For instance, in white pine stands, Leverett found that stand level sequestration rates were lower for a 140-year-old forest than for an 80-year-old forest (but still higher than for a 0-20 year-old forest).
But if a 140-year-old forest is cut, the majority of its sequestered carbon would be released into the atmosphere.
“You’ve got so much there that you’re holding on to, the last thing you want to do is release it all,” said Leverett. “You can’t make it up for a long time.”
Also a stand level slow down does not mean that old forests stop accumulating carbon. Forests continue to sequester carbon after their peak, just at a slower rate.
There is an open question as to whether, when, and how old forests finally stop increasing carbon stores and the answer seems to be at least partially related to species composition. In Pacific Northwest Douglas fir forests, researchers found negligible net carbon addition after 400 years.
However, redwood stands of northern California persist for many thousands of years and Robert Van Pelt, forest ecologist and affiliate professor at the University of Washington, said that it would take at least 1500-2000 years for a redwood stand to reach a “steady state.” Even after this time, carbon dynamics would continue to fluctuate depending on stand density, canopy gaps, and fire history.
Moomaw points out that, from a near-term carbon sequestration perspective, these are just details. Regardless of how long ancient forests can continue adding significant carbon, only a small fraction of U.S forests are old, due to widespread logging. There is significant opportunity for proforestation to capture additional carbon and for climate policies to protect the carbon already stored in U.S forests in the critical time frame outlined by the IPCC.
In fact, with the fourth largest forested land area in the world (as of 2010), the U.S. is well positioned to be a global leader in utilizing proforestation to help meet climate goals.
The Pacific Northwest is home to some of the most carbon dense forests in the world and hosts an array of very long-lived tree species.
In the northeastern United States, significant natural forest has recovered since farmlands were abandoned roughly 100 years ago. Researchers have found that these forests have the potential to double or even quadruple their carbon stores if allowed to mature.
However, Moomaw says that standing forests in developing nations end up being centered in global conversations about forest carbon sequestration objectives, allowing wealthier nations to avoid responsibility for their share of the climate change mitigation burden.
“There are four international agreements on forests,” said Moomaw. “They all apply to tropical forests. Isn’t it interesting that Europeans and Americans don’t think that their forests should play a role?”
According to Global Forest Watch, the U.S lost the equivalent of roughly 100,000 square miles of tree cover to logging between 2000-2019.
The majority of U.S. forests are not protected from logging and the threat against the Tongass National Forest in Alaska is a prime example of the vulnerability of pristine and carbon rich forests even on American public lands.
“The U.S. is the world’s largest wood [producer],” Danna Smith, executive director of the Dogwood Alliance, a group that is fighting the expansion of the biomass industry in the southeastern United States, told EHN. “And so we have a systematic institutional policy framework that supports that.”
Smith said that increasing forest protections would not mean that there cannot be a wood products industry in the U.S, but that the economics and patterns of wood product consumption would need to change to eliminate wastefulness.
“Industry wants people to think that people who want to protect forests [are] out to stop all logging, [that] we’re against all wood products,” said Smith. “That’s not true, but we do believe that the 21st century requires us to be a lot more discerning and careful about the resources that we are consuming.”
Moomaw and his co-authors conclude their review with policy recommendations that include inventorying American forests to identify the best areas for proforestation and practicing proforestation on suitable public land. They also wrote that private landowners could potentially be incentivized to maintain carbon sequestering forests on their properties.
Finally, in addition to producing wood products with an emphasis on recycling and limiting waste, the forestry industry could broaden to encompass proforestation-based jobs such as public education and health, scientific data collection, and other positions related to the litany of forest based ecosystem services.
“What about the ones we’re already got?”
Leverett paused where an old black cherry tree split the wet, leaf strewn trail. The cherry, about a meter in diameter, emerged, mottled green and black, from a tangle of encircling roots. It leaned a little at its base, but about 50 feet overhead it jagged hard sideways, and then pushed up again through the canopy, finally reaching sunlight.
He made contact with an outstretched palm.
“We do have forests here,” said Leverett. “Not that planting trees isn’t a good idea. Of course it is. Everybody is for that. But what about the ones we’ve already got?”