Here’s a tutorial on making trees from scouring pads.. They’re very nice looking, and it looks like a fun project, but frankly you can get nice pre-made trees from a variety of sources. Still, they’re bound to be a lot cheaper, and if you’re a do-it-yourselfer ….
Ryan Scow has a tutorial on making miniature wargames trees from sage brush. They are very good looking.
WarFactory has a tutorial on making African Acacia Trees.
Here’s some advice on modeling trees from air conditioning filters.
Ironhands has some evil trees he built from kids toys, and a page explaining how he used an electric fog machine to add some great effects to his tabletop games.
Now how about some fog through the spooky trees.