How Much Carbon Do Our Forests Capture?

17:08 minutes

large tree with lots of dead branches backlit by setting sun
Credit: nairnbairn/flickr/CC BY-SA 2.0

It’s one of the first things you learn in elementary school science class: Trees take in carbon dioxide and breathe out oxygen. That may have satisfied our childhood questions about how trees work, but as adults, we understand the picture to be a lot more complex.

Carbon dioxide is a potent greenhouse gas, which means our forests are some of the biggest natural weapons we have to fight rising global temperatures. But how much carbon do our forests truly capture?

Christopher Woodall, project leader with the USDA Forest Service joins guest host John Dankosky to crunch the numbers on carbon sequestration. And Christa Anderson, research fellow at the World Wildlife Fund, talks about how forests may be our best weapon for fighting carbon emissions.

a graph with rising lines showing an increase in carbon split up between China, India, United States, European Union, and the rest of the world
Annual CO2 emissions from fossil fuels by major country and rest of world from 1959-2017, in gigatons CO2 per year (GtCO2). Credit: Carbon Brief

Further Reading

Read the original paper in Nature.

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Segment Guests

Christa Anderson

Christa Andersen is a research fellow at the World Wildlife Fund in San Francisco, California.

Christopher Woodall

Christopher Woodall is a research project leader with the USDA Forest Service in Durham, New Hampshire.

Segment Transcript

JOHN DANKOSKY: This is Science Friday. I’m John Dankosky. It’s one of the first things you learn in elementary school science class– trees take in carbon dioxide and breathe out oxygen. They may have satisfied our childhood questions about how trees work, but as adults, we understand this picture could be a lot more complex. 

Carbon dioxide is a potent greenhouse gas, which means our forests are some of our biggest natural weapons to fight rising global temperatures. But how much carbon do our forests truly capture? That’s a question for my next guest, Christopher Woodall is a Research Project Leader with the USDA Forest Service. Welcome to Science Friday. Thanks for being here. 

CHRISTOPHER WOODALL: Hi, there. Good afternoon. 

JOHN DANKOSKY: If you want to join our conversation, 844-724-8255, or 844-SCI-TALK. Christopher, you said that the future of our carbon trajectory hinges on the future of forests. So forests play that big a role in this equation, huh? 

CHRISTOPHER WOODALL: Yeah. Yes, indeed. Our forests annually sequester about 15% of what our fossil fuel emissions give off. So when we look at the mass balance as far as CO2 in the atmosphere, we’re looking at emissions as far as burning fossil fuels, concrete production. But also, we’re looking at what goes into our force– force taking in carbon dioxide, sequestering it in wood and dead wood and soils, et cetera. And so annually, that’s about 15% reduction. 

JOHN DANKOSKY: So they play this huge role, but it’s important to remember the trees also emit carbon when they die. Tell us what happens there. 

CHRISTOPHER WOODALL: Yeah, it’s like a checkbook balance. You have some emissions from burning of trees, decomposition. But largely in the US, our growth and expansion of forests still outweigh those losses to the atmosphere. So over time of the millennia, as far as forests sequestering carbon, that’s really tallied up to quite a store of carbon in US forests. That is equivalent to almost 70 years of emissions of fossil fuel emissions at our annual rate. So not only is our annual sequestration of carbon being taken out of the atmosphere by US forests, but there’s tremendous amount of carbon currently stored in forests across the United States. 

JOHN DANKOSKY: So there’s all this carbon being stored in the forests. Just so we understand, though, how much does a tree release when it decays, and how much does it put into the soil and keep there? 

CHRISTOPHER WOODALL: Well, a large portion of it– probably a majority– is going to be admitted through heterotrophic respiration, or what you call decomposition. Areas out west, where it might be even drier, you might have immediate combustion through wildfires and immediate release. But always a portion of it is going to be fragmented and decomposed into the soil for long-term storage. Like I said at higher latitudes where it’s colder, that can be millennia of carbon stored deep into the soils. 

JOHN DANKOSKY: This is, in some ways, difficult to measure, because we’re not exactly sure how much carbon is in any given tree. Reporter Patrick Skahill from Member Station Connecticut Public Radio talked with forest pathologist Bob Marra about this. 

BOB MARRA: What’s going on inside of these trees? It’s kind of hidden to us, for the most part. Trees that otherwise look to be perfectly fine, and you would have no reason to think otherwise, can have internal decay taking place. If we’re going to look to forests as a way to sequester carbon, we should develop much more accurate estimates of how much carbon is actually sequestered. 

JOHN DANKOSKY: So Christopher, how do we measure how much carbon is being sequestered right now in any given tree? 

CHRISTOPHER WOODALL: Yeah, so I’ll talk about the population of trees we’re typically talking about The US Forest Service, for about 20 years, has conducted an annual inventory of forests across the United States. Hundreds of foresters a year going out there, measuring millions of trees, going back and re-measuring them. Not only measuring live trees, but standing dead trees, dead wood, soils. And from that, we apply models to estimate carbon at time 1, time 2. Hopefully that produces a fairly certain estimate of changes in a forest’s carbon. 

Now, it is correct a lot of the models we apply to these measurements– like measuring the diameter of a tree– are still just models, and there’s current work ongoing– and has been for decades– to refine our modeling approaches. And emerging science, like laser scanning, tomography– which was just reported in that story– can really augment our ability to refine our estimates of carbon and force and trees. So 

JOHN DANKOSKY: The different techniques that are used go from just observation of a tree to actually scanning through the middle of it. Do you feel fairly certain that we know how much carbon any given tree stores? Are there new technological advances coming that might be able to give us even more information than we have currently? 

CHRISTOPHER WOODALL: Yeah. Yep. So at the large scale, I’d say we’re fairly certain. In preparation of our annual submissions to the United Nations as far as US forest carbon, it’s been a sink as far as storing net carbon since 1990. We’re very certain on that. When you scale down to an individual tree, indeed, you need to start using a lot more science to bear on that. 

So the emerging technologies I mentioned, such as laser scanning from satellites, other information like Landsat changed detection and just refined modeling. A lot of the machine learning algorithms, et cetera– there’s a lot more statistical advances we can use these days with our big data sets to refine our estimation of forced carbon. 

JOHN DANKOSKY: And are there different types of forests, or different types of trees that are better at storing carbon than others? 

CHRISTOPHER WOODALL: Well, certainly, we can see in our greenhouse gas reports that the Western US, just due to droughts and struggles they’ve had out west, is probably not sequestering as much carbon as we are in the east right now. It’s been a little bit wetter the past decades in the Eastern US, coupled with land use change where agriculture has reverted to forest in many areas. 

So we’re gaining forest. Forests are growing rapidly in parts of the US. So certain forest types, indeed, are currently able to sequester more carbon than other forest types, but all forests are valuable and part of the equation. We can’t lose forest, because if we do, that’s carbon emissions. So in the viewpoint of increasing carbon sequestration, it’s about having more forests and healthier growing forests. 

JOHN DANKOSKY: Denser forests, old growth forests– are they different than the newer forests, or places where there are just some trees planted amongst other, say, agricultural fields? 

CHRISTOPHER WOODALL: You know, I like to talk about a portfolio approach to looking at carbon sequestration. Certainly, across wilderness areas, we have old growth where old trees are putting on tremendous increments, still storing a lot of carbon. But also younger forests, coupled with forest industry producing long-lived wood products, building condos out of wood instead of concrete, et cetera– really present a lot of opportunities to have a lot of carbon-friendly approaches of increasing forest across our diversity of forest conditions, whether old growth, young forest plantations, et cetera, across the United States. 

JOHN DANKOSKY: I want to bring in another guest, now. Christa Anderson is a Research Fellow at the World Wildlife Fund. She studied California’s Forest Carbon Offset program. Christa, welcome to Science Friday. Thanks for joining us. 


JOHN DANKOSKY: These carbon measurements are being used in a forest carbon offset program in California, so maybe you can explain how exactly that works. Once we figure out how much carbon is in a forest, how exactly is it used in this program, in this market? 

CHRISTA ANDERSON: Oh, sure. Well, I’ve been researching the California Forest Offset program for several years, and most of the work that I’ve done occurred prior to joining the Wildlife Fund, so the science and views I present are my own. And what I’ve looked at in this research is really this premier example, as you said, in California of how the state is working to store carbon in forests. 

The main idea and how it works is that a forest owner can opt to join [? in ?] [? the ?] offset program, and thereby, they would change their forest management practices so that they’re going to store additional carbon in their forest. They measure that carbon using some of the techniques that Chris already mentioned, and then once they’ve measured it and it’s been verified, they get what’s called a carbon credit. And that’s for each additional ton of carbon that they can measure that they’ve stored in their forest land. 

And those credits they can sell to greenhouse gas emitters here in California like power plants, refineries. In California, we have a cap and trade program. So those greenhouse gas emitters are required to reduce their emissions, and one of the ways they can do that is by buying these forests offset credits. 

JOHN DANKOSKY: So they buy these credits, and the credits have to come from forests that are actually doing something in their management plan to create more carbon storage. Can you explain that a little bit more? That it’s not just any piece of property with some trees on it that you can get into this California market. 

CHRISTA ANDERSON: You know, they have a number of options for ways that they can change the management of their forest, but I think the easiest one to think about is that if you’re a forest owner who’s harvesting wood from your forest at a regular rotation cycle, in order to, on average, store more carbon, you might say, hey, instead of doing a harvest every, say, 20 years, I’ll stretch it out and harvest every 30 years. That way, on average, I’m storing more carbon. That’s the easiest way to think about the options that they have for storing more carbon. 

JOHN DANKOSKY: And the forest owners can be anywhere, not just in California. Who exactly are they? Are they big companies? Are they conservation groups that have large parts of forest that they wanted to preserve? 

CHRISTA ANDERSON: Yeah, that’s right, actually. It’s a California program, but the forests can be located anywhere in the US. And one of the really interesting features of the program is that a lot of different landowners have been participating. There are private companies. There are nonprofit forest owners. There are individuals. There are investment firms. And there are a number of tribes who are also participating as forest owners. 

JOHN DANKOSKY: Christopher, I’m wondering if I can bring you back in and talk a bit about what you think a marketplace like this does for the ability for America to think about preserving forests to sequester carbon in this way. What do you make of this California marketplace? 

CHRISTOPHER WOODALL: Yeah, yeah. I think it’s important. Having a marketplace for anything we’re interested in, whether it’s stocks or forest carbon, really draws attention to the science and objective information for trading and for investment. So if we’re seeing investments in forest carbon, where there’s high risk, whether from wildfire, et cetera, it can really be impetus to refine our management and adopt carbon-centric management techniques to increase the carbon and/or mitigate the risk from wildfire, insects, disease– the other threats that our forests face. So I think it’s a very valuable tool to drawing attention to this and increasing the science as far as refining our estimates of forest carbon with new technologies. 

JOHN DANKOSKY: Christopher, what else can you tell us, though, about the management techniques used to increase carbon sequestration? To actually apply for these credits and get them, force have to be better than average at carbon sequestration. So how do you get to that point in a forest? 

CHRISTOPHER WOODALL: Sure. And that’s a good question, because professional foresters, for over 100 years, have been really focused on whether increasing timber production, wildlife habitat production, yield of water, now carbon is another thing. So adjusting the structure of trees, adjusting species composition over time, adjusting the density of trees that is pertinent to the forest type and the climate you’re in can really increase the growth of trees, and hence increase the carbon yield, as it were. 

JOHN DANKOSKY: And Christa, what are some of the limitations of this program? It seems like it’s something that could really work to preserve forests, to make sure we’re growing and preserving more trees, and also create a marketplace for this. But what are some of the limitations or the problems down the road, you see? 

CHRISTA ANDERSON: Yeah, one thing is that it’s really hard to figure out how do you know that the thing that’s happening in the forest to store more carbon wouldn’t have happened without the program? So how do we know that we’re really storing additional carbon? And the California Offset program has taken a few steps to safeguard and make sure that that is the case, but it’s definitely one of the ongoing questions that we’re working on, and we’ll continue to need refinement as forest offset programs grow and expand. 

And the bigger picture– a forest offset program is one of the tools and a good approach to incorporate forests and forest management into our overall picture of climate change mitigation, and forest carbon can really do good– especially when you think about the co-benefits of forests as well, and we can do more with forests than we have been doing– but it’s certainly not the whole climate solution. We need forest carbon, and we need to be decreasing all of our energy and [? industrial ?] [? emissions ?] as well. So it’s an important piece of the puzzle, but one of the pieces. 

JOHN DANKOSKY: We’re talking with Christopher Woodall, and that was just Christa Anderson. She’s a Research Fellow at the World Wildlife Fund. Christopher Woodall’s a Research Project Leader with the USDA Forest Service. And we’re talking about carbon sequestration, how to store carbon in forests. We’re going to get to some of your phone calls in just a moment. 

I’m John Dankosky. This is Science Friday from WNYC Studios. I want to go to Scott. Scott is calling from Lake Siskiyou in California. Hi, there, Scott. Go ahead. 

SCOTT: Yeah, I was just wondering about the clearcutting that’s been going on in our. Area my 90-year-old father used to be a timber faller back in the early days, and he’s noticed just an incredible amount of clearcutting, especially at Mount Shasta, which is just northwest here. On the east side of Mount Shasta, there’s virtually no trees around there, and it’s just really astounding how much clearcutting’s happened. 

JOHN DANKOSKY: I thank you for bringing us this issue, Scott. I’m wondering, Christopher, if you can talk about that– about the impacts on carbon with clearcutting. 

CHRISTOPHER WOODALL: Yeah, sure. I mean, I can’t speak to a western example, as I’m located in the east. But generally, clearcutting is used as a regeneration technique– a very valuable one. Some forest situations where you need a lot of light to come in to reduce competition and allow trees to regenerate. In other areas, it’s more used as an industrial technique as far as plantation forestry. 

The key thing is is that regeneration. Whether you’re using clearcutting or something with just patches or selection treatments where you’re just taking out a few trees in a forest, the key is regeneration. So that’s the question, as far as if you’re clearcutting, are you clearcutting it and paving it over? Land use change to agriculture or urbanization? That’s a real, real risk to forests. As far as clearcutting to regenerate another stand of trees, that can have carbon benefits over the long run, but it has to be professionally applied. 

JOHN DANKOSKY: Christa raised something before I’d really like to hear you talk about, Christopher. It’s this idea that there are questions in some people’s minds about whether or not the carbon that is stored in forests through a program like this California program that’s incentivizing people to do different forest management techniques– whether or not that carbon would have been stored anyway. How do you respond to that? 

CHRISTOPHER WOODALL: Yeah, that’s always been a trying thing with the markets, as far as trying to demonstrate the additionality that Christa talked about. So that continues to be an issue, but we’ve got to keep trying as far as having a marketplace find out what works and what doesn’t work. We’re looking at adaptation as far as managing our forests in the future. 

So if we’re changing climate so much in the future projections as far as climate change, we’ve got insects and disease that are coming into our borders. We have deer browse, et cetera. We have a lot of threats to our forests. 

So adaptive management as far as maybe changing the species composition, looking at alternative stand structures in the future. That might be a way to really get the forest management on a step into the future where we can have a productive force despite the changes that we see coming down the pike as far as climate change. 

JOHN DANKOSKY: I should ask you, quickly, if you could, about a recently-published study that suggested something unusual– land that was 50% to 60% forested could take in as much carbon as land that was 100% forest. Can you explain how that works, briefly, if you would? 

JOHN DANKOSKY: Yeah, sure, sure. I was the author of that study. So a lot of landscapes– we shouldn’t forget about where it’s not 100% forest as far as carbon sequestration. These might be areas where very productive forest was clearcut but transferred to an agricultural land use, urbanization, suburbanization. 

So a lot of those [? soils ?] are very productive, and so the [? forests ?] [? remain– ?] even though it’s only 60% forested in the landscape, they’re very productive, growing a lot of trees at a fast rate. So those shouldn’t be overlooked. In addition to our national forests and wilderness areas that are 100% forested, we need to also consider private forest lands, and areas where we have mixed agriculture in with them due to the productivity of the soils. 

JOHN DANKOSKY: Christopher Woodall, a Research Project Leader with the USDA Forest Service. Thank you so much for joining us. I appreciate it. 


JOHN DANKOSKY: Thanks, also, to Christa Anderson, a Research Fellow at the World Wildlife Fund. Thank you, Christa. 

CHRISTA ANDERSON: You betcha, thanks.

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