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Comment by Kim_Bruning | Hacker News Reader

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.

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> 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.

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.

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

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

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