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.