If I had to try and replicate that cake I would cover it with brown buttercream, refrigerate, then cover very, very lightly with off-white buttercream. You could build up the brown buttercream for the "knots" and smooth some white over the "knots" lightly with your finger.
Then, after refrigerating again, I'd use a rough toothpick to "gouge" out the bark lines. You could make the "puffy" look by digging the toothpick slightly sideways under the gouged line to make it "puff out". (Similar to what you do when making nostrils on a gumpaste face.)
Does that make sense the way I tried to explain it?