Trees and how few of them there are

For the opening of my wad it'll be a really foggy forest and then opens up to the rest of the level. I need some thick greenery for that purpose. Any suggestions?

Skyewood includes some trees and gives permission for use of its resources.

If you have Hexen you can extract some greenery from there. It's fairly full of trees and shrubbery plus animations for leaves blowing off a tree into the wind if you so desire. If you don't have it I'm sure someone will happily extract them for you - just make sure you credit the source when you include them.