Old-growth forest carbon sinks overestimated | Hacker News Reader

Old-growth forest carbon sinks overestimated

nature.com

87 pointstokai68 comments

Some basic conservation of matter arguments apply. As long as a plant is growing, it will sink carbon. When it dies and rots, it will release that carbon again. So, a growing forest will sink carbon, but a steady state one will not. This is why I have always been sceptical of tree planting as a carbon offest . . . it is short term, at best. Of course, there are a myriad of other reasons why tree planting might be desirable.

The caveat to the above is the extent to which dead plant matter gets permanently buried (turned into peat or whatever) rather than rotting. Under some conditions, forests really will work as a carbon sink, but it has never been as simple as tree planting == good, and I am glad to see research acknowledging that. As with everything, it is complicated.

The matter is conserved: it becomes soil. Most of the carbon an individual plant absorbed when it was growing will become soil when it dies and decays. (Should note "absorbed" here refers to the carbon that became part of the structure of the plant - not the carbon the plant breathed in. Plants breath out most of the carbon they breath in.) Yes, it releases some of it during the decay process, but a lot of it (most of it) does get sequestered even after death. That's where humus comes from. And it's pretty easy to meassure that. And for deciduous trees, they are sequestering yearly as the leaves they drop decay.

It wouldn't surprise me if the amount that is being sequestered is being overestimated in many studies, but the mechanism for it happening is pretty clear and that it is happening is pretty incontrovertible - it falls right out of the chemical math - and I would be much more likely to question any study that questioned that than to question that mechanism itself.

It's really frustating to me that what is being posted here is basically just a headline and not even an abstract unless you have a subscription to Nature.

Yes, but bacteria action continues in that soil and continues releasing CO2 for a long time to come. Some of it will feed new plants (anything taken from the soil is not being taken from the air). The question is how much of that soil carbon truly becomes permanently sequestered? Some of it may be there for a long time, but the humus layer will not become infinitely thick. Once we are talking about an old growth forest, the forest will have reached a somewhat steady state. I am not saying it cannot continue to act as a carbon sink, just that I suspect it to become less effective over time, and that it will not work in all conditions. A number of research papers have pretty much shown this lately, not just this one.

Assuming rich soil accumulates over time in a long-running forest, what happens to the lower layers? Do they release carbon more quickly as they "die" or do they store it? If they store it, then that's sequestration, right?

It is sequestration, but it can take thousands of years for an inch of rich soil to accumulate in a 'steady state' system.

Most fossil fuel carbon was originally sequestered in a world where very few things could break down dead trees. That world is millions of years gone, current natural carbon sinks are 99% temporary.

There's a certain argument to be made for timber farming as a way to extend our temporary carbon sinks -- trees that fall over rot, releasing their carbon, trees that are cut down and turned into houses may be protected from decay for tens or hundreds of years.

You could really go whole-hog into attempting to maximize the amount of timber you can stuff into a house -- start building double-stud walls with 12 inch on-center spacing just to triple the number of 2x6's used, fall in love with giant cross-laminated timber panels, etc etc.

That is incredibly labour and energy-inefficient. You're better off burying raw timber, as opposed to processing it, cutting it up, transporting it to a worksite, and paying professionals to hammer it into place.

How do you feel about baking the timber in solar ovens first, before burying the charcoal, to go full terra preta?

It seems to me that the low-hanging fruit is to sequester the sawdust and scraps that result from sawing logs into boards. I assume that much of that ordinarily finds its way into other products (paper, particle board, etc...), but that it's economically less valuable than usable boards and thus burying it the ground has lower cost.

There does seem to be increasing interest though in making large structures out of wood, so maybe that's a pretty good route for carbon storage.

Just to precise, "tree-roting" is incredibly slow, and the tree mainly turns into other living forms (thus, also sequestered carbon) rather than in carbon dioxide (one part does, but far from 100%).

It's like when you eat, you do not turn your food in pure gas, do you? And then the excreted/remaining mass will be buried etc (and will partly be turned into earthworms, nematodes, bacteria, moss, fungi, etc).

Hmmm, that's an interesting question.

A person breaths out about 2.3 lbs of CO2 each day, which amounts to 0.6 lbs of carbon, all of which must have come from your food.

Meanwhile, a person defecates about 1 lb per day, about 75% of which is water. Of the residue, about 25-50% of it is bacteria, and the remainder the undigested portion of your food. Bacteria is typically accounted 53% carbon; the undigested fats, carbohydrates and proteins will vary a lot in percentage, but is 40% carbon for carbohydrates, 50% for proteins, and "more" for fats.

Which leaves a lot of variance, but something like 0.15 lbs of carbon exiting at as a solid seems like a reasonable estimate.

So the answer would be: you turn about 80% of the carbon in your food into CO2 gas.

That seems a lot, 80%.

But after reading several times your calculation, it seems legit. Thanks.

Then, as a conclusion, when you eat you delay 20% of your carbon footprint, and a fraction of these 20% may well be stored for a long duration (in sediments, or back into the food chain, etc).

Then again, similarly, the fallen leaves of trees may store a fraction of their carbon for a long duration.

You can basically look at that 80% as the efficiency of the body machine. To a first approximation, once we have all the matter needed to build a body, you are just taking in carbon to oxidize it for fuel.

https://phys.org/news/2021-04-trees-world-offset-society-car...

Here:

> While this process naturally releases CO2 through the respiration (or breathing) of microbes that break down dead organisms, some fraction of plant carbon can remain underground for decades or even centuries. Together, land plants and soils hold about 2,500 gigatonnes of carbon—about three times more than is held in the atmosphere.

This was the point I wanted to make, probably worded awkwardly.

> It is sequestration, but it can take thousands of years for an inch of rich soil to accumulate in a 'steady state' system.

And as anyone that's been to western England knows, it takes a single generation for it to all blow away and leave you with nothing but a barren heath that's good only for grazing sheep.

Some will be sequestered in this way, but how much and under what conditions? Tropical rain forests, for example, are known to have very poor soils despite the massive amount of biomass growing in them. Almost everything is recycled back into the forest very quickly. Clearly, no significant accumulation of sequestered carbon is occurring there. On the other hand, peat bogs occur because dead plant matter falls into a low oxygen environment and doesn't really decay, so there can be a build up of sequestered carbon there, but that is rather specific conditions.

Some of the biomass is also washed away and becomes marine sediments, which can store a significant amount of carbon. https://cbmjournal.biomedcentral.com/articles/10.1186/s13021...

> The matter is conserved: it becomes soil.

So for that mechanism to work, the ground level in old growth forests would slowly rise over the centuries, and more and more soil with some carbon content accumulates.

Is that what actually happens?

Some soil accumulates. Other is washed out to lower areas of elevation. My guess is it's another thing that reaches steady state.

But you can imagine how much a large tree weighs, particularly a hardwood, a large portion of tree being made of carbon sucked from the atmosphere. Then the foliage becomes soil and buried. Plant a trillion trees, you’re going to see some changes to the composition of the atmosphere over time.

If the forests of said trees are self -regenerating, it’s going to be a decent sink.

> tree planting == good

In which cases is it not? Other than short term carbon sink we get more trees, wild life gets more trees allowing it to flourish, we get wood for building material, dead trees become soil.

You are missing the context; I think suggesting tree planting as a serious method to combat climate change is absolutely harebrained. That does not mean I am against tree planting in principle. I agree with you and I am strongly in favour of rewilding for other reasons. Of course, that still means planting the right trees in the right places. Random planting of random trees is not likely to help much and may be a net negative if it gives people the feeling that they are doing something good when they aren't really.

Maybe it just asks for being combined with a reasonably accurate accounting of the forest's carbon store at different points of time.

This way, startups proposing forestry as a solution for carbon capture would be able to present accurate results and figures.

Don't you leave out the thickening of trunks, and most importantly, carbon storage from fallen leaves of deciduous trees? Imho, the humus is also a compartment of the forest that stores carbon (even if it will be slowly transformed).

Also, the decay of fallen trees is very slow, and is not 100% into carbon dioxide (the carbon will remain a very long time in the body of numerous plant and animal species), whereas the combustion for human use is complete and on a much smaller scale of time.

The problem is that fallen leaves tend to decay in fairly short order. The total carbon capture is basically the current leaves plus one or two years of fallen leaves and that's it. The rest have returned most of their carbon to the atmosphere.

And when you think about it, this makes sense. Trees aren't constantly burying themselves in their own leaves. The roots of trees stay about the same depth year after year, and they're not growing new root systems higher up the trunk.

While I would guess that you are right for most trees, the Giant Sequoia redwood trees in California actually do bury themselves in their own pine needles. They can also grow roots out at a higher level, if needed. Of course they can live for thousands of years.

It doesn't have to stay in the form of leaves. Only a fraction (maybe not tiny) really goes back to CO2, but a lot turns into other organisms, raw fiber, etc.

So you argument would be: why then do trees not bury themselves in humus?

I think that a fair part goes away with rain and water systems, and a small portion circulates in the form of mobile organisms.

Leaves may blow around some, but on most of the forest floor there isn't anywhere for them to go. Some of the leaves get eaten or turned into fungus or whatnot, but those die off too and eventually return to the air.

There was a time in the Earth's history where this wasn't the case. During the carboniferous phase plant life didn't decay and biomatter piled up on the forest floor in miles-thick layers. The CO2 levels in the atmosphere dropped so low that the planet was verging on an iceball at times. The carbon balance was only maintained via enormous forest fires in the oxygen rich atmosphere.

I think that we are missing the earthworms in this model. Carbon from leaves would be accumulated in invertebrate's bodies for a while... except by our burning soil addiction.

It's more like decades, not mere two years, for at least a fraction of the biomass in the humus.

See my previous comment: https://news.ycombinator.com/item?id=26935290

IMHO we need to do everything we can to buy us enough time to innovate our way out of the problem, so yeah, tree planting == good so long as the trees live long enough to do the job.

True enough, but the situation currently is not one of expansion or even of a steady-state: old-growth forests are declining rapidly, and a good deal of the carbon they are squestering is being released over the short term. The issue with forests is their being reservoirs that are being emptied.

Naive tree planting definitely isn't as simple a carbon capture method as 1 ton trees -> -1 ton carbon, but in current circumstances it's still a good idea. For one, thanks to deforestation most tree planting (so long as they're planted in a sustainable way) is growth, increasing total biomass. For another, if I could snap my fingers and offset some current day carbon emissions by a few decades, I definitely would. We are right against the wall at the moment, buying a bit of time could do a lot for us.

> When it dies and rots, it will release that carbon again.

What happens when a tree dies and turns into a house?

> it is short term, at best.

If the house stands for 100 years, then you've successfully sequestered said carbon for over 100 years (because only after then will the rotting process start). Even then, a lot of carbon turns into humus (which btw: we're also running out of Topsoil for our farms). So creating more carbon-rich humus as a topsoil replacement for our farms is ALSO a priority.

Since humus is the final product of the rotting process, I'd argue that the carbon-content of humus represents the "permanently sequestered" bits of the carbon. Which IIRC, is still a substantial portion of the weight of the tree.

There's also all the leaves that fell off the tree and turned into humus. Probably not as much by weight, but its quite possible that the leaves themselves allow the tree to sequester MORE than its weight in carbon.

We can go a long way in removing carbon from the atmosphere by regenerating our degraded soils by bringing them back to life. Some estimates of atmospheric carbon sources list released soil carbon around 40%. Fertile soil is filled with carbon in the form of life and decomposing material (food for life). While this video doesn't address carbon directly it does show how much soil around the world is degraded to the point where all life and carbon in the soil is gone.

https://www.youtube.com/watch?v=c4p-kQ6D8aA

Edit: Soil as carbon sink https://news.climate.columbia.edu/2018/02/21/can-soil-help-c...

Not sure about your definition of "short term" here. Where does the carbon in coal come from? The last time I read it's from the plants.

The carbon in coal was previously sequestered underground and not part of the current carbon cycle. By burning all of that coal, oil, and natural gas we un-sequestered all of that carbon. So even if we replanted all of the forests there are still many gigatons of extra carbon in the atmosphere.

It's "short term" because the plants will release the carbon back into the atmosphere in a few decades.

So to combat climate change we need ways to sequester the carbon that will keep it out of the atmosphere permanently. There are no good solutions for this currently. Maybe at some point someone will figure out a way to economically create massive pure diamonds using CO2 from the atmosphere with renewable energy so we could use them as building materials or something. Thanks to the laws of thermodynamics we'll have to invest something like all of the energy we produced using fossil fuels to fix this problem. We can cheat a bit by going to a stable form instead of making it straight up crude oil again, but it's still an almost unfathomable amount of energy we're going to need. And our politicians still arguing if it is a good idea to stop digging the hole, and fighting tooth and nail against it. There are no jobs on a dead planet.

Many species of trees live for hundreds of years, not decades.

> it is short term, at best.

Short-term, where short-term is around 70 to 100 years [1], seems just fine to me. It'll buy us time to develop better technology for carbon sequestration and clean energy.

In fact, the problem with planting trees might be the opposite -- trees don't reach their maximum rate of carbon sequestration for a couple of decades, and by then it might be too late.

[1] https://www.fpl.fs.fed.us/documnts/pdf2011/fpl_2011_lippke00...

Properly managed agroforestry is an excellent carbon sink, where properly managed is doing all of the heavy lifting here.

The napkin strategy is to grow fast woods, kiln them into charcoal, spread some of it on the forest as a soil amendment, spread the rest on agricultural land, repeat.

Elemental carbon weatherizes into CO2 very slowly, on the order of centuries, the half-life is more than a thousand years. So-called "biochar" (it's just charcoal) is an excellent soil amendment, mitigates topsoil depletion, supports good microbiomes, adsorbs fertilizers and releases them slowly, it's a big win. If we manage to do this on the necessary scale, we might eventually hit the point where adding more char to soil isn't worth it from an ecosystem health perspective, and then we can just put it into the various large holes we've dug into the earth to extract minerals. There is something very tidy about filling an old coal mine with synthetic coal!

Yes, back when I was doing archeology, we'd find pieces of charcoal 700-800 years old. And that was in the forests of Belize.

In the California Sierra Nevada mountains almost all of the land was harvested for timber in the last 150 years. These forests contained mostly trees that were many hundreds to thousands of years old. So if we plant the right trees and let these harvested forests keep growing, instead of harvesting them again, we will get a thousand years of increasing carbon capture. I'm not sure about other forest ecosystems, but "short term" could be hundreds to thousands of years. A quite useful timescale for a large carbon sink as we try to get to a more normal atmospheric carbon dioxide level in the future.

There are some debates in the comments involving process-based comparisons, e.g., when carbon release from decay outweighs carbon sinking from plant metabolism.

It may be helpful to give this top-level summary from the National Climate Assessment [1]:

"Net storage of atmospheric carbon by forests (742 teragrams, or Tg, of CO2 per year from 1990 to 2015) has offset approximately 11% of U.S. CO2 emissions. Assuming no policy intervention – and accounting for land-use change, management, disturbance, and forest aging – U.S. forests are projected to continue to store carbon but at declining rates (35% less than 2013 levels by 2037) as a result of both land use and lower CO2 uptake as forests grow older."

[1] https://nca2018.globalchange.gov/chapter/6/ -> "Forest Carbon Dynamics"

The NCA is your best one-stop-shop for the best current synthesis on such questions, unless you're literally publishing in this specific area (in this case, carbon cycle science at decadal timescales).

I remember reading about this decades ago. I guess it's still being discussed?

Short summary of the theory:

Trees are largely made of thin air ;-) : Carbon Dioxide provides the required (C)arbon, water that rains out of the air provides (H)ydrogen and (O)xygen, and (N)itrogen fixated from the air provides the basis for amino acids. This covers about 99% of the chemicals in the woody parts.

Young trees want to grow a lot, so they make a lot of lignin and cellulose. These consist of C, H, and O. Once trees have grown tall enough, they tend to slow down, so you'd expect them to not grab as much carbon anymore.

It says the original paper was published in 2008, and a reply was published last month.

> Once trees have grown tall enough, they tend to slow down, so you'd expect them to not grab as much carbon anymore.

I don't think that's accurate (or at least not very precise). Studies have shown that when evaluating the metric of annual addition of tree mass per acre in the rainforests of the Pacific Northwest, old-growth forests add as much or more when compared to "young" forests.

I don't have any specific sources, but if you want to know more the research by Robert Van Pelt should be a good starting point.

This is definitionally untrue. Forests have a maximum biomass carrying capacity, and they rarely reach that capacity, instead losing carbon to stochastic events like fire. Old growth forests "store" a lot of carbon, but their capacity to sequester is finite.

https://www.fs.fed.us/psw/topics/fire_science/ecosystems/car...

You're not listening to what I'm saying. I'm not talking about carbon sequestration, I'm refuting the argument that "once trees have grown tall enough, they tend to slow down". Here, I pulled a source for you: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.45...

> Wood production of the entire main trunk and whole crown both increased with size and age up to and including the largest and oldest trees we measured.

This is even supported by your own link:

> In the past, some researchers have suggested that converting old forests to young, fast-growing plantations, whose harvested wood products could store carbon for several decades, would create a net increase in long-term carbon stocks. This approach was based on the idea that old forests are slow growing and thus carbon sequestration slows down as forests age. More recent research generally does not support this idea, as a global survey of old forests found that many continue to sequester carbon and have stocks that far exceed young, managed forests.

Of course they have stocks that far exceed young managed forests (they store a lot). But forests do not continue packing biomass on indefinitely (the rate of sequestration slows and eventually stops). See this summary of Malmshmeimer (2011) [1]:

“Trees do die, and at a rate that eventually reaches some kind of a stasis at a landscape level,” says Fried. “In some stands, up to one hundred percent of the trees will be killed by a fire or insect outbreak; other stands con- tinue to grow, but over the entire forest you’ll eventually reach a plateau, after which the net in-forest growth and carbon accumulation rates decline—eventually to zero.” Many pro- tected forests on public lands, especially those in parks and wilderness areas, are no longer increasing carbon storage, he says.

[1] https://www.fs.fed.us/pnw/sciencef/scifi155.pdf

One correction, plants, including trees, don't get their nitrogen from the air, but from the soil.

True in basically all relevant cases but they recently found one type of mountain plant that pulls nitrogen from the air, which holds some promise for GM crops.

There are actually a bunch of plants that can pull N2 directly from air, by entering into a more direct symbiosis with nitrogen fixing bacteria in root nodules. (not typically trees though)

Depending on where you live, you might even plausibly have some in your garden.

https://en.wikipedia.org/wiki/Root_nodule

Technically correct, the best kind of correct!

Of course, the way the nitrogen gets into the soil (as ammonia) is due to nitrogen fixing bacteria that do in fact get their nitrogen from the air. So the tree gets nitrogen from the air in a roundabout way.

(Earth's atmosphere is 78% nitrogen, so as a bacterium you'd be a bit silly to ignore such a rich source)

Same is true of Hydrogen and Oxygen of course, which fall as rain and enter the soil first, before being taken up by the roots.

Here's a non-paywalled summary: https://phys.org/news/2021-03-old-growth-forest-carbon-overe...

Longer term storage: peat.

https://en.wikipedia.org/wiki/Peat

" Peat forms when plant material does not fully decay in acidic and anaerobic conditions. It is composed mainly of wetland vegetation: principally bog plants including mosses, sedges, and shrubs.... "

"Across the world, peat covers just 3% of the land’s surface, but stores one-third of the Earth’s soil carbon...."

"In natural peatlands, the 'annual rate of biomass production is greater than the rate of decomposition', but it takes 'thousands of years for peatlands to develop the deposits of 1.5 to 2.3 m [4.9 to 7.5 ft], which is the average depth of the boreal [northern] peatlands', which store around 415 gigatonnes ... of carbon...."

Now and then a well-preserved body thousands of years old has been found in a bog.

Am I missing sth? I don't even see an abstract there, yet they want $8.99 for it?

It's in the Matters Arising section of volume 591. These are short 1-3 page long contributions, shorter than research articles. And being so short, they don't have an abstract.

Table of contents: https://www.nature.com/nature/volumes/591/issues/7851

As of writing this comment, these short papers (this, and the counterargument, right after in the table of contents) don't seem to be in sci-hub. Until someone uploads them to sci-hub, they probably aren't available in the free web.

Thanks for explaining. Didn't know that format. :)

There is already a rebuttal:

https://www.nature.com/articles/s41586-021-03267-y

The rebuttal too is behind a paywall and doesn't have an abstract...

I think the link is to the rebuttal, right? The original paper is from 2008.

2008: Original article [1]

2021: Rebuttal of the 2008 article [2]

2021: same issue of Nature, next 2 pages: Rebuttal of the rebuttal by 6 (out of 8) of the authors of the 2008 article [3]

[1] https://www.nature.com/articles/nature07276

[2] https://www.nature.com/articles/s41586-021-03266-z

[3] https://www.nature.com/articles/s41586-021-03267-y

I’m curious if these competing rebuttals get to read each other during the editing process. Otherwise will there be two more rebuttals in the next issue?

Genuine question, if old-growth forest do not work as carbon sinks, how all the petroleum we are extracting comes from? Is all from ancient marine environments?

The article doesn't say old growth forests don't work as carbon sinks, just that don't work as well as they initially thought.

Petroleum is mostly marine microorganisms. Coal is mostly old forests from a time before fungi developed the ability to break down lignin. Dead trees back then just continued to pile up without ever rotting.

Always good to remember environmentalist don't care about old growth forests or eco-systems or humans.

It's just about CO2 in the air. Not even the measurable amount, most can't tell you any numbers.

They have found a way to control the population that will work for decades.

Who cares what old growth forests do, why are we even worrying about CO2 if it's not about getting old growth forests back? I guess because CO2 has become god.

Would you please stop posting flamewar comments to HN? We've asked you many times and you're not only still doing it, you've been flaming up a storm lately. This is seriously not ok and if you don't stop it we are going to have to ban you. I don't want to ban you, so please fix this.

https://news.ycombinator.com/newsguidelines.html

'Flamewar comments' is not really accurate since only you really replied, but I get the sentiment, perhaps you mean this one

"Be kind. Don't be snarky. Have curious conversation; don't cross-examine. Please don't fulminate. Please don't sneer, including at the rest of the community."

Or maybe you think I'm really bad at flame wars, but... "strongest plausible interpretation"

Perhaps a shadow ban and I'll aim for vouch if that's a middle ground.

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