I made a 2 tier birch cake last December and used all buttercream frosting. I frosted the cake roughly with white, then went back and added "globs" of white with a small spreading knife, but pulled the frosting back with a sweeping motion to create the peeled bark effect. I tinted some of the extra white frosting a brownish color. I used a "brush-like" stroke with a small spreader to blend it in to the white in random streaks, finishing some areas with an actual water color paint brush to blend. It's hard to describe, so I'm sorry if I'm not making any sense...but after the wedding I was told some of the guests didn't even realize it was a cake, they thought it was just a decoration. Good luck with your project, I hope you have as much fun as I did!! 
