[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.