I've been looking for some tips on how to do a cake like this in buttercream! You did a great job! How did you add the darkness in the lines of the carving and bark? How did you do the little branch nubs? :) 

Thank you, Angel. 

I did the darkness with watered down black food color and a paintbrush after the icing had crusted.  I made the branch nubs by placing a larger amount of icing there and smoothing it with my fingers and making the indention with a ball tool.