You can pipe buttercream around the cake and shape it. Paint it with different shades of brown. You can also use fondant on the sides, sculpting it to make it more realistic. Painting it with chocolate works too.
Wow I love your cakes
Haven't done it, but I would think you could put mounds of fondant against the iced cake then cover all with a sheet of fondant. Then you could shape the bark and paint.