There currently is an issue with tree spawning in Gaia where the bounds radius setting is ignored and the trees don't produce a collider cache for spawn collision checks as the regular GameObjects do.

That issue is caused by an optimization for tree spawning which does not take the bounds radius of trees into account properly. We will fix this in an upcoming update for Gaia, for now you can re-enable the old bounds system (just like it works with the gameObjects) by opening

Gaia\Scripts\SpawningSystem\SpawnRule.cs

and removing line 689 and 708.

These lines should read /* and */ and removing them will comment back in an entire block of code that creates the bounds collider cache for the trees. After that change you should notice that trees now spawn with a sphere collider in the collider cache that represents the bounds radius from the spawn rule settings, and other trees will not spawn inside that collider.