This means Pokemon in the branches like Primeape, Pokemon hanging from the branches like Pineco and Tarountula, and Pokemon on the tree like Spidops. I'd also like to know the areas where they spawn.
Pineco Forretress Bounsweet Mankey Primeape: Applin (in trees) (click on the applin) Tarountula (can be hanging on branches): (I guess you're making a list yourself) Spidops (can be on a tree): Grafaiai Murkrow (appears at night) Corvisquire (Whiteout for a shiny in a tree) Swovet(Can be found in trees, probably have to knock it down with dashing in the trees) ( Greedent (not sure about this one) (seems more comment on the ground for me) Slakoth (remember it myself and it makes a lot of sense as it is a sloth and why I keep not finding them) Ditto (mentioned by Ryter the greater (and honestly it does make a lot of sense) (link to be added I have to go one page at the time to save data)
Note that some of them on Serebii do not show that they can be in trees, so my search is not 100% at the moment and I'm doing it one by one. Didn't include Pachirisu because of this.
Edit: I think now that's it completed. That's everything serebii has and it's late. Edit: Also probably any bird Pokemon as I knocked down a fletchinder from a tree. Maybe except corviknight as it is way too heavy for branches.