Where are the trees?

I want to put a new tree in my map as a doodad.

So I extracted Doodads\Doodads.slk
But I cannot find Trees in it. Where are they?
I tried making a unit look like a tree in WE, but it just appears as a white, skinless object. Where is the data for trees, and how many mdx's do I need to get them working correctly?