I’ve been searching for a bit and figured I’d ask y’all.
How about “copse” (a small group of trees)
https://dictionary.cambridge.org/us/dictionary/english/copse
Copse was my initial thought, but there is also the word Spinney.
It would depend exactly how big/substantial this ‘gathering’ is, but I could imagine that “Grove”, “Stand” or “Thicket” might be appropriate.
They aren’t exclusive to your definition, but could be applicable.
Came here for grove.
One of the surnames on my mom’s side of the family means “grove of trees near a bog” and comes from the same area as my best friend’s surname that means “evil bog goblin”
I like to think that his family was evil bog spirits, and my family were good tree people, and he and I have mended the feud.
This has nothing to do with OP’s question, I just thought of it when grove came up, and thought I’d share.
Copse
I agree with others saying copse, as being my first thought as well, but I’m really commenting to say I love the imagery the description, “a gathering of trees” produces.
Ent moot
The only reply which takes the “gathering” aspect into account. But wouldn’t Ents tell you they are not trees? Still, we don’t have to cede to their demands here.
Let them present themselves and their objections to being called trees and I will listen.
