Then again, if my numbers are right, if humankind dedicated <1% of land used worldwide for food to growing trees and locking away the cellulose, we would cancel out worldwide CO2 emissions. Since most of that land is used as pasture, there is more than enough play to keep the world fed, and I could see up to 10% being tasked to this purpose , which seems doable in an emergency.

Have you accounted for moving the trees from where they grow to where they can be used? (E.g., milling, planing, curing.) That will take energy that likely offsets some of the benefits.

Likewise, moving the lumber or finished goods to consumers will also require some energy that might offset the carbon removed.

I'm confused...the numbers sucked for extensive farming in order to cut CO2, but only 1% of arable land would cancel CO2? What am I missing?

I initially thought it didn't look good when looking at the amount absorbed yearly per tree, but then was surprised by some estimates on how many trees you can fit per unit area. A more accurate estimate needs to be done though, different numbers I found put the final range at about an order of magnitude.

Also, most farming and stable forests are mostly carbon neutral. This would require continual seeding, harvesting and sequestration of the cellulose. Maybe it will be a good thing though that cellulose+lignins are so hard to breakdown.

Still plenty of room at sea. Is there a fast growing mangrove?