[kanjus]

The theory that young trees capture more CO2 than older trees has been debunked as far as I know. The best carbon sinks are older large trees: they grow at lower rates height-wise, but faster mass-wise, sequestering much more carbon. A single large tree might add the same amount of carbon to a forest within a year as is contained in an entire mid-sized tree, I’ll add some sources later

[205guy]

I read once that the highest density of living entities on the planet, measured in weight per surface area, are the Sequoia groves in California. That is, the mature ones that escaped logging. Now, I'm not sure we can consider the entire tree to be alive, but relevant to this discussion, they are almost entirely composed of atmospheric carbon, tons and tons of it quite literally.

In all practicality though, I think sequoia and redwoods are more difficult to grow and have limited climate tolerances (an issue when the the climate is changing). I wonder if go fast-growing tropical trees such as albezias or others would be better. But we should also be looking for 3-4 ideal trees for each climate zone or better yet, trees adapted to several zones.