How to creat a forest where trees have a collider?

I want to make a foggy forest(camera far clipping : 50) where each tree have a collider ,but when I add too much trees (manually) my computer almost dies. The built game has 120 avarage fps but when I turns quickly it drops to 10 then back to 120. Any idea what should I do?

Which collider do you use on your trees? If it is a mesh collider it is bound to impact performance. In that case try using a capsule collider instead.