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发布日期:2020年06月30日
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Musings of an Energy Nerd
Limiting CO2 Releases When Clearing Land

A green builder pursues the idea of biochar production

Loggers in northern New England recently cleared ten acres of spruce forest to create a pasture. After the stumps were pulled, the contractors had to decide how to dispose of the woody debris in an environmentally appropriate manner. [Photo courtesy of Martin Holladay]

When a developer builds a new home in a rural area, the first task is usually land clearing. The steps are fairly routine: a logger clears some of the land, and an excavation contractor prepares for the foundation and puts in a driveway and septic system.

When a logger is hired to clear land, some of the trees may be marketable—perhaps the land includes valuable saw logs, pulp logs, or firewood logs—or the logs may have little value. If the logs have little value, they may be chipped. If chip prices are high, the chips may be trucked to a biomass plant or a wood-pellet factory; if chip prices are low, the chips may be spread in the woods or trucked away to be disposed of elsewhere. Regardless of the value of the standing timber, though, the developer or landowner usually wants some of the land to be cleared.

For most builders, the decisions around land clearing are basically financial. But if you are a green builder, there is another factor to consider: What are the carbon consequences of this type of land clearing?

If you’re attempting to evaluate the carbon consequences of land clearing, here are some starting points for your analysis:

Assuming you’re a green builder who wants to clear land in a responsible manner, it makes sense to delay any carbon releases associated with logging. There are several ways this might be done:

Biochar is a type of charcoal. It is produced by burning wood in a kiln or an earthen dome designed to limit the available oxygen—a process called pyrolysis. Biochar is considered to be a relatively stable form of carbon—a form of carbon that is not readily released into the atmosphere.

When biochar is incorporated into soil, it can…

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20 Comments

  1. jkonst | | #1

    I feel pretty passionate about reducing clearing on any building project, and replanting natural habitat where clearing has already happened. I look at real estate listings in the Hudson Valley and Catskills, and the amount of lawn for many homes seems so wildly unnecessary. Does your 10 acre forested plot really need to be turned into 8 acres of lawn with some trees on the periphery?

    I appreciate the thought that has gone into this article, and I hope some environmental benefit can be derived from the practices mentioned here. But I can’t help but feel the answer here is a bit like recycling vs. reducing - the greener approach in most cases is to reduce the harm done to the land than to try to mitigate the impact after the damage is done.

  2. Bryan Coplin | | #2

    We all would do well to remember that a willingness to pay a premium to lessen environmental impact adds far less value than simply consuming less.

    If we fail to address climate change, it won't be for lack of technology, but rather an inability to tolerate inconvenience.

  3. {block name="guanjianci"}{/block}
  4. User avater
    Stephen Sheehy | | #3

    It's so much easier, cheaper and greener to leave land as is, as much possible. Almost 30 years ago, we bought an old farmhouse with around 40 acres, about half wooded, half fields. Once a year, I brush hog the fields in the Fall to keep them from turning into woods again. I use up maybe 10-12 gallons of diesel fuel, but the wildflowers and other field plants provide habitat for birds and bugs. The woods continue to suck up CO2.

    Spending $140-150,000 to clear ten acres is crazy. In most of northern New England, you can buy a lot more than ten acres of already cleared land for much less money.

    1. Charlie Sullivan | | #8

      I appreciate your minimal mowing, as do the birds. I know a commercial mower who, even though he enjoys the actual work mowing large rural fields in VT and NH, hates destroying bird nests just for someone's aesthetic preferences, and wishes he had more customers like you.

      As far as CO2, going a step further, and letting those fields turn into woods again is a big opportunity to sequester more carbon.

  5. Antonio Oliver | | #4

    Martin, interesting article and case study. There was one sentence that I found particularly suprising in this article: "Whether [a] tree burns or rots, the amount of carbon released is the same—the only difference is the rate of release." I've seen the residual matter (ash) that remains after a log is burned, and I've seen the residual matter that remains after a tree has rotted to a 'compost' that can be crumbled between one's fingers. It appears that the burning process is far more carbon comsumptive, with the rotting process leaving far more organic solid matter. Or in other words, the terminal point of the two processes appear not to be identical. Would this not imply that less carbon has reacted with oxygen to form CO2? What am I getting wrong?

    1. User avater GBA Editor
      Martin Holladay | | #5

      Antonio,
      Carbon isn't destroyed by rotting or burning. The carbon atoms still exist. When you crumble the compost left by a rotting log, there is probably carbon in the compost. But the compost hasn't finished decomposing yet. More rot will occur, and roots of nearby plants (and soil microbes) will continue using the nutrients and converting the carbon compounds in the compost. Eventually the carbon is released into the atmosphere as carbon dioxide or is sequestered (temporarily) as new plant growth.

      Ashes also contain nutrients (including potassium) that aid plant growth. I imagine that the rate of plant growth near a rotting log or a pile of wood ashes differs due to many factors. I'm also fairly confident that composting delays atmospheric releases of carbon dioxide compared to burning.

      The process of wood rot or composting occurs relatively slowly in the Arctic and remarkably quickly in a tropical rain forest -- where leaves and twigs are consumed almost immediately when they hit the forest floor, by countless organisms including plants, insects, fungi, and microbes.

  6. Antonio Oliver | | #6

    Martin,
    First, as you made clear, this is not a simple topic. Also, I do understand that carbon atoms are not destroyed--there's that law of conservation of matter and energy. When I use the term consume, I'm referring to the carbon atoms that are initially bonded to other carbon atoms (and other elemental atoms within the wood) that react with oxygen and become CO2 in the atmosphere. (/article/explainer-how-and-why-fires-burn#:~:text=Wood%2C%20for%20example%2C%20contains%20molecules,pyrolysis%2C%20releases%20atoms%20and%20energy)
    Wood IS carbon, mostly--so there is definitely carbon in the compost. It would seem reasonable that more solid carbon remaining means less carbon converted (better word?) to CO2. Yes, I hear your point that further wood decay can occur, but the action of the microbes and wood eating insects will prevent a lot this carbon from being converted directly to CO2. Will a carbon atom from this decayed log eventually become an atom in a CO2 molecule in the air many enviro-chemical reactions/processes in the future? Maybe, probably, likely. But is it part of the original log decaying accounting at that point? Probably not. It might likely be part of another botanical or even zoological organism or organic matter in the soil.

    But, I'm a physicist, not a chemist.

    I don't want to belabor this topic; I very much agree with what I believe to be the underlying point of this article.

    1. User avater GBA Editor
      Martin Holladay | | #7

      Antonio,
      As far as I can tell, we agree. Healthy soil contains organic matter--that's carbon--as organic gardeners understand. The organic matter in soil is always rotting and therefore disappearing; much of the carbon ends up as atmospheric carbon dioxide.

      It's fairly easy for me to keep a decent level of organic matter in my Vermont garden soil by regularly adding compost, because the organic matter decays slowly. For a tropical gardener, however, it's much harder to keep a high level of organic matter in garden soil--it disappears too quickly. Where does it go? It rots, and much of it becomes atmospheric.

    2. Brian Bailey | | #11

      I also wonder about the role of methane when considering whether to let wood rot vs. burn.

      I've heard (but do not know) that some of the carbon in rotting wood is emitted as methane, which would militate against letting it rot against the backdrop of the climate change emergency. Although methane is much shorter-lived compared to carbon in the atmosphere, it's 84x more potent as a greenhouse gas during the first couple of decades after emission. I guess that is probably the relevant time scale for mounting a response to the climate emergency (or failing to do so), so allowing carbon emissions in order to prevent methane emissions may be a worthwhile trade.

      1. Charlie Sullivan | | #15

        As I understand it, methane is likely when the rot occurs without sufficient oxygen. I suspect that burying wood makes methane more likely. I suppose you could set up to collect the methane as is one in landfills, and flare it or maybe even burn it to heat the house or even generate electricity, but that's not likely to be practical at this scale. That was part of my motivation for my unpopular idea to pile the wood above ground.

  7. Charlie Sullivan | | #9

    How about simply piling the logs and stumps in an inconspicuous area of the land, on the principle that above ground it will stay dryer and rot more slowly than below ground, and will be less likely to go anaerobic and release methane. If the pile is too ugly, maybe plant an row or ring of fast-growing evergreens around it. You could even put an EPDM cover over it keep it drier and slow the rot. Maybe not a good idea in an area with high fire risk but it seems like there could be a way to do this on a "large parcel of land" in New England.

    I've been trying similar approaches on a my small parcel of land (< 1 acre), and in that case it's challenging to avoid carpenter ant colonies thriving too close to my house or my neighbors, and to avoid excessively ugly piles. But if I had dozens of acres I would expect it would become easier.

    1. User avater GBA Editor
      Martin Holladay | | #10

      As someone who is familiar with the sight of the brush piles and log piles produced by a commercial logging operation (clearing around 10 acres), I would have to say that your suggestion is impractical. These piles quickly become 20 to 30 feet high, and would be hazardous to neighborhood children. After 10 years, such a pile wouldn't be pretty.

      1. Charlie Sullivan | | #16

        Good point about the hazard to children--I was imagining a sufficiently remote area where that isn't an issue, but it's something to consider for sure. As far as aesthetics, I was assuming it could be somewhere out of sight, but that assumes that there is enough land to put it far away and hidden by hills or woods that are left intact, and a way to get there without too much roadbuilding work.

  8. Armin Z | | #12

    Hi Martin, are you familiar with Hugelkultur and any studies examining its use for carbon sequestration? My understanding of Hugelkultur is that's it's a way of using disposing of brush and logs by digging and filling trenches in layers of coarse to fine organic, logging "wastes" to create what is a combination of raised bed, swale, and windrow. Great article btw I appreciate the realistic assessment of a well-intentioned effort. I often wonder if we are in fact at "a time when the carbon impacts of all new construction projects appear to be serious enough to justify a construction moratorium"...or at least a moratorium on single family dwellings on large greenfields. And I say that as someone who predominantly builds those types of projects.

    1. User avater GBA Editor
      Martin Holladay | | #13

      Armin,
      I was not familiar with Hugelkultur before I read your comment. I found a few references online, including this research paper: "Hugelkultur Gardening Technique Does not Result in Plant Nutrient Deficiencies."

      This method doesn't sound fundamentally different from the traditional New England approach mentioned in the article -- the low bid on the job ($80,000) -- which involved "burying the stumps in a low spot or excavated hole." If you wanted to go "full Hugelkultur" with a New England logger, you'd be back to Plan B, which would involve chipping the logs and brush as well as burying the stumps, and at that point, you're probably back up in the $140,000 range.

      One final point: burying woody biomass delays, but does not prevent, carbon releases to the atmosphere. Buried wood will rot. Whether it rots more slowly or more quickly than wood that is directly exposed to rain and wind depends on many factors, including how deeply it is buried (since there is more biological activity close to the surface than several feet below the surface -- a fact that matters in septic system design). If the wood is chipped (as it was in the research project I linked to), it will rot faster than if it isn't chipped.

      1. Armin Z | | #14

        Hi Martin totally I take your point that wood will eventually rot (also there's the logistical question of what to with fill that's generated from digging trenches). I found some other research re: wood burial as carbon sequestration /articles/10.1186/1750-0680-3-1
        but this question is way above my pay grade/brain capacity. That said it would be great for state forestry services to enshrine best practices for land clearing. Especially relevant given ash blight and tree death that will happen regardless of new land clearing :(

  9. Nate Reik | | #17

    Was there any discussion about the carbon required to create the biochar? You have to heat it with something to do that.....

    I suppose, on the other hand, chipping and burying would burn fuel as well...

    1. User avater GBA Editor
      Martin Holladay | | #18

      Nate,
      Any logical analysis of the climate change implications of land clearing options would certainly need to include consideration of the number of gallons of diesel fuel required to operate the equipment needed to implement the plan. On the job in question, diesel fuel was required for pickup trucks, dump trucks, bulldozers, tractors, chippers, and perhaps feller-bunchers, as well as for several pieces of unusual equipment I don't know the name of -- not to mention gasoline and chain oil for the chainsaws.

      In general, I agree with Stephen Sheehy (Comment #3) -- the best option (usually) is to change the land as little as possible.

  10. Jon Lipstate | | #19

    I would theorize that the way to approach biochar would be to use a forestry mulcher on a skid or excavator to grind the woody materials to chips, and then fill burn chambers setup using the TLUD method (Top Lit Updraft). Water isn't required for this method, and it uses the wood's own energy to convert to charcoal, making it substantially more efficient. Materials need to be roughly the same size in order to evenly convert, hence mulching.

    designing chambers that are both large, and portable would take some effort however.

  11. Tyler Keniston | | #20

    Good piece and discussion.

    In line with many of the points already made, I wonder why the following would not be considered:

    -Send the above ground material to biomass: we still need energy, and I wonder if the displacement effect (of fossil fuels) wouldn't outweigh any possible C sequestration from composting it. And it sounds cheaper. Win-win.

    - Depending on intended use of the land (pasture... what does that mean in this case?) grind the stumps slightly below grade but don't go disturbing all that soil. This has the dual benefit of not having stumps to deal with, and not disturbing the soil structure as much.

    - "...complicated topic that is best addressed by forestry scientists, atmospheric scientists, and soil scientists." True, especially when it comes to carbon sequestration in soil, which seems to be the crux of the issue here. And well above my paygrade.

    "Whether the tree burns or rots, the amount of carbon released is the same—the only difference is the rate of release." This is strictly true, but these rates can change drastically when one considers that the 'rotting' carbon can be worked into soil structure: a place where carbon release rates can change dramatically (vs, say, a pile of woody debris sitting in the corner of a field). Indeed it depends on many factors, including climate, existing soil structure, and what sort of vegetative overstory remains.

    When it comes to biochar, there really isn't a fundamental difference to burning or rotting of woody debris other than—once again—rates.

    There is more carbon stored in forest soils than above ground (most of the time). This should give us some indication that getting carbon into soil may be a worthy goal. Biochar is perhaps one option, but considering soil structure and the above ground interface (conditions) is part of it too. This, perhaps, is most relevant to large-scale forest management as opposed to small-scale field conversions.

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