Creating a forest zone

Hey!

I was looking for a way to create forest zone for strategy game purposes. There will be many very low poly trees. What I need is a modifiable zone that:
- I can modify bounds of
- Will be populated by trees that snap on to unity terrain
- Trees will not need colliders
- Ability to identify this zone as 'forest', so units can move slower, stay hidden etc.

I was thinking of three approaches:

1) Use collider(s) as triggers that limit the forest zone. Randomly spawn tree meshes inside the zone.
2) Place trees in the editor, each tree has 'radius' that it will look for surrounding trees to see if they form a forest area.
3) Use unitys terrain tools to drop in trees and set collider zones around the trees

What would be efficient way to approach this?
Any performance tips?