Ideal tree size (at harvest) for carbon farming

I was wondering if there was any resources out there on the best size of tree to cut for the purposes of carbon farming? Obviously this will depend on the individual species of tree.

Years ago a friend of mine said one of the best things we could do to sequester carbon would be to cut down trees and not burn them (as long as other trees were planted/allowed to grow to replace them). I was skeptical because I figured the existing tree was already pulling carbon out of the atmosphere.

My friend's response was that younger trees are better because they are growing so much faster. I can see his point, but while a very young tree puts on dramatic growth that is easy to see, does a very large old tree put on more mass that is just harder to perceive with the naked eye? How many saplings doubling their mass does it take to equal a 100' tall tree putting on 1% additional mass? (That's not a question that can actually be answered without knowing a lot more details, I'm just using the question to show my reasoning.)

I remember reading that the growth rate of black locust really slows down after they get about 30' tall, and that many foresters choose to harvest them at that point to maximize their profits. Kind of like butchering an animal not because it's full size, but because the feed to weight ratio is unfavorable after a certain point. (I don't know if the black locust thing is actually accurate, it's just something I read.)

Is it common for trees to slow down once they reach a certain size? If so, are there any resources online that give the sizes for various species?

I imagine coppiced trees might be worth cutting at a smaller size. The fully developed root system would probably help them put on even more mass when they are younger.

I also expect that from a practical standpoint trees will be more often harvested based on what they are going to be used for (fence posts, cabin logs, etc.) rather than their ideal size for carbon sequestration. Still, it would be interesting to know what that ideal size is.

I suspect that it would vary greatly on species and on your region's ability for rapid regeneration. Example: Poplar reproduces volume of biomass much quicker than Douglas fir, but a mature Doug Fir forest has much more biomass than a mature poplar forest. However, if you cut a tenth of the poplar, the regeneration of the poplar grove (through clonal shoots) will be much more rapid and complete than harvesting 10% of a Doug Fir forest. Also the quantity or mass of fungi that associate with the longer term forest probably has a role in carbon sequestering that should be at least considered.

You are right to consider the desired use of the tree. Fence posts? Rocket fuel? Coppice? Pollards? Hedgerows? Dead Hedges?

Are you cutting it and burning it? Are you cutting it and burying it in hugulkultur? Are you cutting it and leaving it on the ground as chop and drop? Some species are much better to use for a given purpose than others.

Roberto pokachinni wrote:I suspect that it would vary greatly on species and on your region's ability for rapid regeneration. Example: Poplar reproduces volume of biomass much quicker than Douglas fir, but a mature Doug Fir forest has much more biomass than a mature poplar forest. However, if you cut a tenth of the poplar, the regeneration of the poplar grove (through clonal shoots) will be much more rapid and complete than harvesting 10% of a Doug Fir forest. Also the quantity or mass of fungi that associate with the longer term forest probably has a role in carbon sequestering that should be at least considered.

That's a good point. In my area balsam poplar (we call it cottonwood, but IIRC it's balsam poplar) is both the largest and fastest growing tree there is. Our spruce trees can eventually get about as tall, but even then are not usually as big around. On the other hand mature spruce trees grow more densely here than mature. The forester at the university I went to said that for biomass (for the purposes of making pellets) nothing came close to poplar/cottonwood in this area.

I believe Paul mentioned poplar (or was it cottonwood?) in his podcast on poop beasts. Supposedly they can take very high amounts of nitrogen and not only don't die, but thrive. Hopefully that applies to the poplar growing here as well. The thread about humanure in the carbon farming forum makes me think that combining it with poplar coppicing might be the fastest way to accumulate biomass. Poplar/cottonwood is considered a trash tree around here though. It's not that it's useless, it's just that everything it does other trees do much better (other than growing big and fast). If I put together enough money for a sawmill it has some use for making furniture. It also made decent siding for my grandmother, she just had to make it really, really oversized to accommodate all the shrinking.

Hugelkultur would probably be the best use for it. I would like to implement some large-scale hugelkultur mounds, but my family seems pretty skeptical. I'm going to try to get one built (by hand, unfortunately) this summer, and hopefully it will work out well and sell them on the concept. I like the idea of poop making cottonwoods, cottonwoods making hugelkutur, hugelkultur making food and food making poop.

There are two species. Black Poplar or Cottonwood (Trichnocarpa), which tend to grow a lot larger and have deeply furrowed bark when mature, and particular have larger branches in the upper canopy, and Trembling Aspen or Poplar (tremuloides) which are smoother, thinner in profile and with less robust tops. They are closely related, but separate species.

I like the idea of poop making cottonwoods, cottonwoods making hugelkutur, hugelkultur making food and food making poop.

Just be sure the cottonwood is good and dead before putting it in your hugulkultur or you will be growing more cottonwood than food in your mound.

Hi Tyler good question. I do hear people talk about this. At a certain point when a tree slows growth its carbon sequestration also slows. If you want to really maximize carbon sequestration in your agroecosystem you could cut trees at that point and use them for building materials. If they then re-sprout or you plant another tree to replace them you can keep your system sequestering at a faster rate because it doesn't "saturate" and start to slow down. However there are certainly some loss when a tree is cut no matter what, and this can vary from almost negligible to quite serious depending on a number of management factors like how much the soil is disturbed during the harvest process. Bamboo is a classic example, managed bamboo forests actually sequester more carbon than wild bamboo forests both because of the stimulation of faster regrowth and the use of treated bamboo as a long – lived building material.

Great, I also thought that it depended on the speed of growth.
I had a similar question, so I do not open a separate post:

trimming is thus also good, because the tree has to regrow what we cut?

And there is sth I do not understand here : "Bamboo is a classic example, managed bamboo forests actually sequester more carbon than wild bamboo forests both because of the stimulation of faster regrowth and the use of treated bamboo as a long – lived building material. "

I understand only the 1st point.

I think what Eric is saying Xisca, is that there are two carbon sequestering advantages to managing bamboo grove over what happens in a wild unmanaged one.

1.) With bamboo, there is rapid regeneration of volume shortly after harvesting takes place, thus maximizing the carbon sequestering potential in the rapid growth of earlier stages.

2.) The use of the bamboo as a building material; this acts to sequester carbon much longer as it is kept away from potential decomposition and thus the potential for atmospheric carbon release that would happen in the wild un-managed forest.

Old growth forests capture and sequester more carbon than young forests.

http://www.natureworldnews.com/articles/5658/20140116/older-trees-grow-faster-take-up-more-carbon.htm

Think about it: trees add mass every growing season by adding a thin layer of tissue under the cambium. These are the "rings" we see on a stump. Not just the trunk, but the branches and roots all add a layer of wood year after year. Big branches get thicker every year. Big roots continue to grow thicker. The greater the surface area, the more carbon is added. A young tree may be growing quickly and getting taller every year, but the overall surface area of the tree is a fraction of that of a huge old tree. If both trees add a millimeter of thickness to their trunk and to their branches, the big tree would add exponentially more carbon than a host of smaller trees occupying the same square footage of forest floor.

Further, 60% of a plants energy is directed below the ground, given to root growth and root exudates. The larger the plant, the greater the volume of root exudates they are pumping into the soil food web.

Roberto pokachinni wrote:

I like the idea of poop making cottonwoods, cottonwoods making hugelkutur, hugelkultur making food and food making poop.

Just be sure the cottonwood is good and dead before putting it in your hugulkultur or you will be growing more cottonwood than food in your mound.

Ha, that's definitely good advice. Quite a few times I've been digging out a birch or cottonwood tree and discovered that it got its start as the branch of a tree that had been either cut down or pushed down by a dozer.

I'm going to double girdle a few cottonwoods for future harvest. Right now I have lots of already rotten birch and cottonwood available. Of course I'll leave some of that for the wildlife.

I was trying to figure out the best size to harvest cottonwoods for biomass. I found a source on growing hybrid poplars that gives 10" as the best time to start harvesting. I'm not sure if that will directly apply to cottonwoods or not. I'll probably just have to observe them, but judging how much mass a tree is really putting on over time seems like it would involve a lot of measuring, photographing and record keeping, none of which I'm very good at. I'm not too worried about it, I'll probably harvest them based on ease of access more than anything.

Marco Banks wrote:Further, 60% of a plants energy is directed below the ground, given to root growth and root exudates. The larger the plant, the greater the volume of root exudates they are pumping into the soil food web.

I never thought about that aspect of it, thanks for the link!

I have worked with a bunch of foresters and their advice varies. Some prefer to take mature trees and have a "young healthy forest", but they are forgetting that the conversion from smaller trees to sawlogs is where the landowners forest starts to become very valuable. Other foresters grow trees with an eye for the future where every tree has the potential to be a sawlog. The type of forester that is kept and retained is entirely up to the landowner.

My current forester is more of the later type, and my forest is very mature in age. It is like that, not so much because it is by design, but because we have have pretty much always used small equipment (a low impact farm tractor) and it could not pull the big wood out. But also by my Grandfather who taught me, "if you always cull the junk wood, you always have good wood."

All this is beyond the scope of the original question, but here foresters only get paid by what is harvested or in the making of a forestry plan. I will say however that in my forestry plan, that at a minimum I am to keep 3-4 wildlife trees per acre. These a big trees ( 3-4 feet in diameter), well rotted, with holes and such for porcupines, bears and the like. I also plan to keep the larger trees on top of the ridges for raptor perches. Whatever you decide, to cut big trees or small ones, you should leave a few of the biggest ones for wildlife.

There ought to be some well-established mass/age curves out there somewhere for common species. This is a big deal in the pulpwood and chipwood industry. I don't know where you would look, though - maybe university forestry extensions and USDA Forest Service research stations in your area would have the information, or know where to look.

I did find some information on white spruce from my local extension office a while back. If I remember correctly they recommended letting them grow for 90-110 years, but I'll have to see if I can dig up that paper.

If you search "tree mass at harvest age" or similar you'll find some documents like this one: http://extension.psu.edu/natural-resources/forests/finance/forest-tax-info/publications/forest-finance-8-to-cut-or-not-cut-tree-value-and-deciding-when-to-harvest-timber

My woodlot has a forest management plan and part of it contains soil data in each individual stand...a stand being defined as an area with a certain land characteristic, dominant tree species or soil type. Anyway, on this forest management plan they have one column that lists common trees that are present and then what specific trees should be managed for; all based on soils. It is interesting, and is both useful and useless at the same time.

Here is why:

It is useful because I can go into a particular stand and know right off the bat which tree species are present, but not ideally suited for the soil in which they grow. Those I can cull without a lot of guilt. However, this information is rather useless too because it is based on a lot of factors. Here is a case in point; Eastern Hemlock. I love this tree because it has a lot of uses, and honestly, because I have a lot of it. About 1 in 4 trees, yet nowhere does my forest management plan even suggest managing for it. That got me to investigating, because if 25% of my forest is covered in hemlock, obviously my soil is ideal for it! Nature never lies. So I found out that its not a tree species that is liked by foresters because it is shade loving and slow growing. Slow growing being relative as White, Black and Red Spruce are all tree species managed for, however they have more commercial value even they grow much slower.

It is kind of like having sheep; the USDA considers sheep a minor livestock and therefore us sheep farmers are not given priority, however as homesteading livestock they are ideal because they provide meat, wool, milk, and clear browse. The Eastern Hemlock is similar. For a conifer it makes excellent firewood, can be used for tanning, makes strong lumber, is semi-rot resistant, can produce paper, and grows reasonable fast (if given sunlight). The problem lies in that it is very heavy and modern carpenters prefer Spruce, fir and pine framing instead. So for its low economic value to the building industry, it is not considered a tree to manage. That is where I disagree with the USDA and on my farm, go out of my way to stay clear of hemlock saplings (hemlock has a high germination rate for a variety of factors).

I say this for educational reasons, and it is a long way of saying be wary of what the professionals may tell in what to harvest. I am in no way smarter than they are, I just also realize why they are prone to say what they do.

My suggestion for carbon removal would be birch. The value of it has not gone up in over 40 years, and it breaks down really fast in the soil, yet is a relatively fast growing hardwood if given sunlight. It would be excellent for hugels I know that. I just put in a main logging trail to a new section of forest where natural hugels abounded, and where yellow and white birch were heavily present, and even my wife who grew up in town saw the soil and was amazed by how "rich it was".