Recently, on a tip from somewhere on the internet, I bought some pine tree bark to use as rocks in model terrain and basing.
I picked up some trees from Michael's and a few wooden oval cutout to act as bases, and started gluing. I have to say, I'm really pleased with how pine bark looks. Even without painting, the bark really looks like layers of natural rock.
You can see in the photos that a couple of layers of brown and grey make for some nice additions to some sparse tree patches.