If you, say, clear cut some land, or turn previously unproductive land into fertile land, you start from roughly zero carbon ‘at rest’.

The trees, when they grow, pull the vast majority of the carbon in their structure from the atmosphere. 99% or close to it.

At some point, the tree growth plateaus - it may have been slow the first year or two, then accelerated the next 10 years, then gradually tapers until it hits it’s maximum growth potential at that site (give or take). That may be in 20 years, or 100, but it will happen sooner or later.

During this time, trees die - and their carbon gets broken down and re-released in the form of methane or co2.

Roots, debris, etc. roughly equal the amount of plant matter above ground.

All of this rots when the trees die or burn (which happens on an ongoing basis), which releases the carbon back to the atmosphere in the form of co, co2, methane, etc. except in rare exceptions.

You can tell this is the end result because if you dig into/in these forests, they aren’t sitting on thick beds of carbon. Typical soil depth is 3-6ft before you hit bare mineral soil. So they’re in equilibrium over time.

According to this random link I found [https://www2.nau.edu/~gaud/bio326/class/ecosyst/USFScarb.htm] in the US it averages out to about 17.7kg/m^2 across all forests.

So if you start with bare/no carbon and add trees, you’ll on average be able to temporarily store 71,614 kg of carbon per acre. Until the trees die, or burn.

If you harvest the trees and either bury them (where they can’t rot) or use them for construction, you can reset it and sequester the carbon. That has costs however.

If that isn’t done, then the carbon store is temporary - the carbon will eventually get back into the atmosphere, and the forest will have net zero carbon uptake at some point in the meantime.

The EPA recently estimate the US releases 6.1 billion metric tons of Co2 and equivalents per year (as of 2021). If we estimate an average acre of forest plateus at ~ 72 metric tons of co2 stored, that means you'd need to plant ~ 85 million acres of forest to EVENTUALLY store that co2 - per year. Just for the US alone. And hope none of those forests completely die off, don't end up growing, or burn eventually.

There are approximately 800 million acres of existing forest in the US (a heavily forested nation). Accounting for 7.5% of all forested land in the world.

It's not clear there even IS that much land which could support a forest to last even a decade, let alone land that would support it for long enough for these forests to mature - or not just burn/die soon.

But if we look at farmland in the US (currently ~ 900 million acres) - https://www.nass.usda.gov/Publications/Highlights/2019/2017C...

We'd have converted 100% of our existing cropland to forests in about a decade, just trying to counteract each years co2 equivalent emissions.

And that is 40% of all land in the US!

Add together 800 million acres of existing forest, and 900 million acres of farmland and we're nearing almost all fertile land in the US total. Even if we could squeeze out another 10% AND keep all these forests going (while we starve as we have no farms/crops anymore?) we'd still be completely out of room in less than 2 decades, with all potentially tree bearing land covered. While we still continue to produce carbon.

And if these trees die or burn, any carbon they stored is back into the atmosphere.

Carbon wise, trees are springs/capacitors, not 'sinks'. They help buffer spikes, but they don't solve the real problem - too much carbon coming from 'out of the carbon cycle' geological stores and being injected directly into the atmosphere at massive scale.

To solve that problem, the carbon needs to get taken out of the carbon cycle again. And as long as those trees are above ground, that can't happen.