Wood and Its Carbon Debt, Again

Wood and Its Carbon Debt, Again

This letter came in to us the other day.

To the Editors:

By this time, you and many of your Vermont readers have most likely heard the podcast on VPR on wood heat. It was a good and thorough treatment of the subject, but there was some material on the “con” side by academic experts that I question and I think your magazine is a good forum for discussing it.  I’ve heard these concerns voiced in other places and it leaves me wondering if these academics spend much time in the woods. They don’t seem to be familiar with how the forest really responds to a harvest.

They point out that it could take 60 to 100 years for a new tree to grow to the size of the harvested tree, and they contend therefore that it will take that long for the CO2 released by burning to be reabsorbed by the forest. They claim further that this is too long to alleviate the immediate crisis with global warming.  

Now there are several things wrong with this analysis. Replacement trees don’t wait 60 years to take up all the CO2, but begin as soon as they start growing and take it up all during their lives at an ever increasing rate.  [Editor’s Note: Check out this op-ed that we ran in 2011 by William Strauss that quantifies this point and perspective.]

Another flaw in the analysis is that it only looks at the replacement trees and not at how the forest as a whole responds. Arguably the most drastic harvesting we can do is a clearcut. In this case there is a profound response in the next growing season in that a very dense stand of annual and biennials rapidly establishes itself along with the tree seedlings. This stand takes up far more CO2 than the just-sprouted replacement tree. This succession goes on, and the area eventually becoming a thick stand of pole wood, out of which the ultimate survivors will be the replacement trees. It has a very large CO2 uptake.

Of course all of these die except the replacement trees, but even dead they store a lot of carbon. Some of this is released as the dead wood rots, but some is sequestered as humus in the soil. The point here is that all of the plants in the succession grab on to CO2 during all the years until the replacement trees are harvested and the process begins again.

Any comments on the above are welcome.

Arthur Krueger
Shrewsbury, VT

This seems like an informed and well-reasoned reaction to me, Arthur. Some comments:

The 60- to 100-year figure was provided by Dartmouth professor Andrew Friedland, who has a reputation for being a provocateur on this subject – here’s a blog I wrote in 2013 in response to one of his talks. I sent Friedland an email asking him to explain the scenario where you’d get a 100-year debt; specifically, I asked how we got from a 30-year debt, which was the headline that came out of the big Manomet carbon accounting study we covered about 10 years ago, to a 100-year debt. He wrote back: “Since [the Manomet study], there has been a body of literature, including some contributed by my lab, that suggests that deep mineral soil carbon (20 cm to 60 cm below the O horizon) is lost in the 15 to 30 years after logging and can take ~75 years to recover. So I am also including that. Perhaps 100 years is on the high end, but I certainly do not think that there is justification to assume that all carbon released after clearing and burning a stand is reclaimed in 30 years.”

So there’s his logic. He would acknowledge the flush of growth you rightly point out that is sequestering carbon in a clearcut, but he’d say you’re not taking into account what’s going on in the soil. You can read one of his related papers here. The Cliff’s Notes version is that a significant amount of carbon is sequestered in the soil. And when a stand is clearcut, we don’t know for sure how that effects deep soil carbon stores. If the soil gets scarified, or compacted, or blasted with sun and rain because there’s no longer a canopy, then the soil chemistry may change, and we might lose a percentage of the carbon that was sequestered in the deep mineral soil. (In this particular study, there was no difference in C levels in the top 20 cm of soil compared to stands that hadn’t been harvested, but the uncut stands had 5 to 31 percent more carbon in the mineral soil.) The paper admits to statistically insignificant data and states: “at present, researchers do not have a full understanding of how and on what timescale soil C responds to forest harvest.” In other words, it’s good science. Friedland’s found a niche that hasn’t been well studied. He’s documented something that bears looking into. Other researchers will consider his work and scrutinize it, try to replicate it and report on what they find at their sites. Grad students will scratch their heads and try to figure out a better way to quantify things; better methods that might be used. This will unfold over decades and we’ll learn a little bit more with every incarnation.

What I agree is not good, as you allude to, is cherry picking the idea of a theoretical 60- to 100-year carbon debt and plastering it on a figurative billboard, because it’s not grounded in reality and it confuses people. Note the phrase “clearing and burning a stand” in his note to me. This scenario seems to be what the big-number carbon debt models are based on, and yet this hasn’t happened commercially in the Northeast since we were a colony sending potash back to Great Britain. So much of the firewood on the market in Vermont and the rest of the region comes from thinnings, where the overstory is not completely removed. The soil in this case is not radically disturbed, and the trees that are left flourish and sequester carbon more efficiently. And even if the land is clearcut, I can’t imagine a modern scenario where all the wood gets burned, which would affect the carbon debt calculation. In the gallery below you can see a sorting bunk in front of a slasher – this was part of a clearcut we had done last summer. As the wood was cut to length, the operator graded it. In the case of this job, 44 percent of the wood became either pellets or firewood, the rest went out as sawlogs or pulp, and the carbon in that wood was sequestered in 2x4s and cardboard boxes.

“Well what if all the wood did get burned?”

“What if the logger, with a family and mortgage and debt up to his eyeballs on a $300,000 feller buncher, decided he’d drive a load of sawlogs worth $3,000 to the pellet mill and get paid $700 for it?


What use is this intellectual exercise outside of academia? And what damage does this sow in a region where we have the resource base, and a logging culture, and the institutional knowledge to harvest and use wood more efficiently and responsibly than anywhere else in the world. And yet we spite our own face. The planet’s falling apart because of fossil fuel emissions and we dither.

I apologize, Arthur, for getting on my soapbox. (Though you did start it.) The bottom line is that the inputs in these carbon-debt models – what kind of land, what kind of harvest – make a huge difference in the number at the end. In the aforementioned Manomet study, we were told that if you switched from oil/gas to wood pellets, in 50 years you’d reduce your greenhouse gas emissions by 25 percent. The Northern Forest Center asked one of that study’s authors to run the model again using different inputs that better reflected the reality on the ground in northern New England, and lo and behold switching to pellets resulted in a 50 percent reduction in greenhouse gases right away.

The simple truth is that we don’t have a complete grasp on the carbon implications of burning wood, which is a statement meant to reflect the things we’re still learning, like how logging effects deep carbon stores in the soil, and the things that we just won’t ever know, like when the next ice storm is going to level a stand of trees that some model assumed would sequester X amount of carbon over the next 50 years. I’d be wary of anyone who pretends to know otherwise. The best we can say right now is that when we cut and burn a tree, it will take some amount of time – a timeframe ranging from negligible to decades – to offset the emissions produced.  

What we do know with absolute certainty, though, is that from a carbon emission perspective, burning wood is always better than burning coal or gas or oil, since none of that fossilized carbon should be in our atmosphere. And we do know burning wood provides the financial means to keep new forests growing, and gives our friends and neighbors jobs, and it’s cozy, and all the other good points the VPR piece makes. You might look at burning wood like you’d look at eating butter. Do you want to be aware of the health implications and moderate your intake? Yes. But there’s no reason to be scared of it. It’s natural; our bodies are built to handle it in moderation. Burning fossil fuels is like eating artificial trans-fat. It’s unnatural and bad for you in all cases.

Alright, choir (and those who want to tell us what we're missing): the floor is yours.

Photo Gallery

  1. Dan → in Cornish NH
    Feb 01, 2019

    Thanks for this added perspective. As I listened to the VPR piece, I wondered about the issue along the lines discussed here, but did not take the time (and perhaps lacked the perspective) to frame the question this articulately.

  2. Pete LaFlamme → in Maums
    Feb 01, 2019

    An old growth forest actually has less carbon-consuming capability than, for example, a recently clear cut forest. Research shows that stomata densities ( where CO2 enters the leaf to be converted to carbohydrate during photosynthesis) are much higher in a dense stand of new growth following a clearcut than in an old growth forest.

  3. Dave → in Maine
    Feb 01, 2019

    Well said, Dave. Especially the point comparing wood harvesting to fossil fuel extraction which always results in adding more carbon to the equation that could have remained locked up.

  4. Scott Nichols → in Orford, NH
    Feb 01, 2019

    Mr. Mance,

    Thank you for a thoughtful response that leaves the door open for further learning and discussion.  Wouldn’t it be nice if that happened more often?

    I wish to point out one thing: With regard to carbon dioxide, I’m not sure that burning wood is always better than burning coal, gas, and oil if that wood is being burned to produce power.  The original VPR piece was about heating with wood, so I suspect that’s what you had in mind.  The distinction between heat and power is important given that heating with wood is 3 times more efficient than making power with wood. From purely a carbon standpoint, the news about biomass power often bleeds into biomass heating to negative effect even though the two are very different uses of wood.

    Thanks for hearing me out.

  5. Carolyn → in East Wallingford, VT
    Feb 01, 2019

    The thing that breaks my heart about all this is, having lived with oil heat and wood heat, I would give an appendage to be able to go back to pushing a button and having the house warm. I hate heating with wood. It’s dirty, exhausting, body-straining, creates complex storage problems, and gives me six months of sinusitis. Yeah, the science indicates that burning wood is better than fossil fuels. I’m not going to argue with that. I just think it’s a tragedy that humankind spent so many generations trying to come up with a better way to create warmth and power, only to find it has a long-term negative effect on a global scale. What a BUMMER!

  6. Bill → in Strafford VT
    Feb 01, 2019

    Thanks Dave. So pleased we still have intelligence to challenge academia.

  7. Stephen Jones → in Princeton, Ma.
    Feb 01, 2019

    Great article! When science and the use of statistics get distorted to support any agenda then we are cheated out of finding a true answer to the problem in question.

    I am a retired owner of a forest products company and was taught in forestry school that science should be pure because opinion is not.

  8. Chuck Wooster → in White River Jct., VT
    Feb 02, 2019

    Burning wood is a great example of how the perfect can be the enemy of the good. Some carbon is released, as is true with every form of energy production. But the benefits - biological, economic, social aesthetic - are profound. Thanks, Dave, for giving us such a thoughtful assessment.

  9. Jeff → in Vermont
    Feb 02, 2019

    Thanks for your discussion. I like your title “again.” We just have to keep calling out the bad information going round and round. In their effort to be fair I think VPR/BLS gave a littler too much respect to some false equivalencies. We cover carbon, particulates, and how working forests protect against permanent loss to development in our follow-on podcast: https://www.sustainableheating.org/podcast-the-pros-and-cons-of-heating-with-wood/

  10. Mark Bowen → in Barnet
    Feb 03, 2019

    We have a small woodlot, 163 acres, and all of our firewood comes off our land, always as a side benefit of doing a selective timber cut, prescribed by our forester, or in onesies and twosies that have fallen down on their own. We’ve also done some patch cuts, 2- or 3-acre clearcuts for wildlife habitat management, leaving dead trees and large slash and trunks in place, and these have dramatically increased, among other things, the number of songbirds on the land. (This certainly benefits “the environment,” irrespective of its effect on carbon storage.)

    The selective cuts leave the forest as a whole essentially unchanged, preserving or enhancing the age diversity in the original stand, while releasing healthier stems that promise to be commercially valuable in the future. Every cut also produces several semi loads of chips for one of the regional biomass power plants. The soil is deeply affected only on the skidder trails, most of which were there already.

    We burn about six cords a year. Somehow I just cannot believe that the way we have managed our woodlot does not promote carbon storage in the forest to at least the tune of six cords per year. And I expect that a significant proportion of the firewood burned here in Vermont is produced in the same way.

    Carbon accounting in forest management is a tricky business, partly because the science is still being explored and partly because, as Mr. Mance points out, wood is used in so many different ways. So I don’t think one can really make a broad statement about whether burning firewood is renewable in the carbon storage sense or not. The devil is in the details. This was part of the message, I think, in the Brave Little State podcast—which was provocative, and that’s a good thing. But I don’t think, all-in-all, that it treated the subject of carbon storage with the sophistication it requires.

  11. Arthur Krueger → in Shrewsbury, VT
    Feb 04, 2019

    Dave -  I read the Petrenko and Friedland paper you referred me to.  I am an engineer by trade and spend much time in test pits analyzing soil.  I also spend at least as much time managing my sugar woods.  All I can say is that the paper is mighty thin gruel to base energy policy on.  It is a limited sampling on one particular not overly common soil type. 

    The authors actually state that their results are not statistically significant.  They focus solely on the carbon contained in about 2 feet of soil immediately below the organic soil layer.  They did not even include the carbon in the root fragments within this band.  They sifted them out and threw them away.  They did not look at what went on below this layer.  They certainly did not look at the carbon in the biomass growing in the forest itself.  They made a tacit assumption that the depth of the organic layer itself did not change over time.  In short, they didn’t account for most of the carbon stored in the system they are studying.

    The paper is a fine academic exercise, but it is in no way useful in establishing forest/energy policy.  It is way too limited in scope.

  12. Gregory Cox → in Hawley, Massachusetts
    Feb 05, 2019

    I too was struck by some of the statements made by Brave Little State in their program “The Pros and Cons of Heating with Wood”, particularly in their carbon calculations.  Here is some recent research to consider:

    At a Mohawk Trail Woodlands Partnership meeting in October, UMass researcher Paul Catanzaro presented results of a study he did with Anthony Amato from UVM on the Impact of Forest Management on Carbon.  By reviewing Forest Cutting permits in Massachusetts, they found that most harvesting here is partial cutting, removing about 4Mbf/acre or about 13 metric tons of carbon/acre, cutting roughly 1/3 of the trees at a time.  A typical harvest reduces the net carbon storage/acre by about 17 percent, 10 metric tons/acre from the harvested timber, and another 3 tons from disturbing the duff.  Below surface carbon appears not to change much if BMPs are followed to protect soils from erosion.  By their calculations, the carbon removed in partial harvesting is replaced by new growth primarily of existing trees in about 9 years (at a rate of 1.5 metric/tons per acre per year).  A shelterwood harvest that cuts 2/3s of the net volume would reduce total stored carbon by about 30 percent and would take about 15 years to replace the stored carbon.

    This contrasts sharply with the 60 to 100 year carbon replacement scenario cited in the broadcast.  That scenario seemed to be based on the idea that if you cut down a sizable tree, it will take 60 to 100 years for an equal sized tree to grow to replace it.  While that might be true for an isolated tree in the open, it doesn’t reflect actual forest growth.  With the partial cutting common here, the trees uncut use the increased sunlight to grow faster and replace carbon much faster.  Added growth absorbs more carbon than new trees would initially.

    Another carbon study was done recently by Mass Wildlife on clearcuts they did to create early successional habitats for wildlife.  The study reportedly that net carbon on the clearcuts was within 2 percent of the total before harvesting after just 6 years regrowth.  Whoa – how can that be?  I thought. There’s no way the trees could have regrown that fast.

    I thought about that last weekend when I skied in a family woodlot where we had a seed tree harvest to regenerate an ice damaged stand 4 years ago.  In the snow, the bare new trees seem pretty small and puny compared to the seed trees that tower above them.  There’s no way those sprouts and saplings can hold anywhere near the volume of carbon as the trees that were cut did, even if there are thousands per acre.  Seems obvious, right?  There might be a lot of carbon in the rotting tops that we haven’t cleaned up that are slowly tuning to duff, but that wouldn’t replace all the wood taken out.

    But as I whacked around, I suddenly realized that we often spend too much time looking at the trees and overlook the vegetation.  While the new trees in our woods range from 1 foot to 8 feet tall, the forest floor is densely covered by an nearly impenetrable mass of low woody vegetation: blackberries, raspberries, hobble bush, striped maple, fire cherry, which rises to 6 foot high or more in many areas, way more was there before the harvest. It all stores carbon, not permanently, of course, but for that period until the new trees grow tall enough in 5 or 6 years to close their crowns and shade it out.  As the undergrowth dies back, that carbon will then be released and taken up by the fast growing trees.

    So carbon storage in a regrowing forest may initially be more horizontal in the brush layer than vertical in the trees we normally measure it in.  Calculating the carbon just in the new trees doesn’t really show us the whole picture.

    That insight got me thinking about another part of the broadcast which talked about concerns about particulate emissions from wood burning.  We all know that wood burning, while much more environmentally sustainable than burning fossil fuels, can produce high levels of Pm25 emissions which can pose health issues.  Whenever anyone reloads their stove or furnace, they produce considerable smoke until the combustion gets going. Why, anyone can see that wood burning seems to produce more emissions that fossil fuels.

    Or does it?  Some information from an recent study about wood pellet boiler emissions by the UMass Department of Environmental Health Sciences might make you start to rethink that assumption.  Using a state grant, EHS Professor Rick Peltier and his students equipped a van with emissions monitoring equipment and took it around to gather weeks of emissions data at six locations downwind from modern pellet boiler installations.  Because wood burning emissions differ chemically from those from fossil fuels, they could measure which emissions came from pellets and which from nearby fossil fuel systems .

    The first finding from their research was that all the pellet boiler installations have very low Pm25 emissions, in most cases much lower than the EPA limits. This shouldn’t be a surprise as the pellet boilers are specifically engineered to burn cleanly with low emissions.  What was surprising, on the other hand, was their finding that the pellet Pm25 emissions were often much less than the Pm25 emissions from neighboring oil-fired systems.  In one example where a pellet boiler heated part of a town building complex and oil boilers heated the rest, the Pm25 oil emissions were so high that a simple way to improve air quality might be to just switch the other buildings over to pellet heat.

    It turns out that just because we don’t see obvious smoke when fossil fuel systems fire, that doesn’t mean that they are burning cleanly, particularly as those systems age.  Conversely, while emissions are more visible with wood heating systems,  a lot of what we see may be steam, not particulates.  In the case described in the broadcast where Goddard College installed a central wood-fired heating plant and distribution system to replace 20 oil-fired boilers in individual buildings, the particulate emissions from the modern wood system may turn out to be less than the less visible emissions from the 20 old oil-fired units.

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