Random example: blue jays plant oak trees. They like acorns, pick them off trees and bury them for later, and forget some. If you have an ecosystem that supports blue jays your forest will expand without fundraisers and government programs.
Ecosystems fix carbon in more active biomass than just tree trunks. If you get soil building ecosystems and ecosystems that put more carbon in living creatures, you have less carbon in the atmosphere. The extra CO2 in the atmosphere is like fertilizer, and you can get life to utilize it more and grab more of it out of the atmosphere by supporting it in small ways so it can go on to support itself.
I disagree. We also want to avoid ecological cascade effects in our ecosystems.
And perhaps the best way to create a sustainable ecosystem around the trees is to plant them and see what else develops, rather than try and curate a specific balance of organisms.