Here's why:
* Collective nouns refer to a group of people, animals, or things considered as a single unit. Examples include "flock" (birds), "herd" (cattle), and "army" (soldiers).
* "Dump" typically refers to a place where waste is discarded.
While you could use "dump" to refer to a place where trees are disposed of (like a lumberyard), it wouldn't be a collective noun for the trees themselves.
Here are some collective nouns for trees:
* Grove: A small group of trees
* Forest: A large area of trees
* Wood: An area of trees, often for timber
* Stand: A group of trees of the same species
* Arbor: A single tree or a group of trees in a specific location