While "choir" refers to a group of singers, a collective noun for trees would be something like:
* Grove - A small group of trees
* Forest - A large group of trees
* Stand - A group of trees of the same species
* Thicket - A dense growth of trees and shrubs
* Arbor - A single tree, or a group of trees, planted for shade or ornament
* Wood - A large area of trees
* Plantation - A large area of trees planted for commercial purposes
The best collective noun for trees depends on the specific context and the type of trees in question.