You can make roots, rocks, whatever. Acrylic paint, followed by a fishsafe clear epoxy paint to seal everything.
Here's the making of a tree off the Polygem website
http://polygem.com/zoo/treehow2.php
I would suggest some kind of basic framework, major build up with Pond Foam, and then the Zoopoxy. The Zoopoxy is the most spendy part. It's also quite heavy, solid, has the look and feel and weight of wood......if you drop your "tree" on cement, the Zoopoxy can crack. Chicken wire would have probably been easier to work with than what I used.
My picture is a little deceiving. The bottoms of my "trees" are very slightly flared out so that the base is larger than the top. Basic sculpting with Pond Foam, as messy as it is, is easier than dealing with the Zoopoxy. Our basic goal was to get the Pond Foam completely covered up. When dealing with the Pond Foam (remember that it really swells) it reaches a point that it can be manipulated with water and gloves and really smoothed out. It can also be easily filed/sanded down. The Zoopoxy also has a critical set point where it can be smoothed down with wet gloves. However, to get the texture we want we are using a sander.
Apparently, you can make a silicone mold of bark texture or whatever, and use the mold to give you perfect bark texture on your trees. In our case, we need smooth "bark" as our doofus fish would only injure themselves on anything remotely sharp. I really think the Styrospray product might be an easier way to go.
http://www.industrialpolymers.com/p.../styrospray/?gclid=CNfkvbTjn7ICFYaDQgodRB4AGg
