How do you make a tree stump cake using buttercream? Like this cake...

http://cakecentral.com/g/i/2361856/a/2362856/vanilla-cake-with-chocolate-buttercream-frosting-decorated-with-buttercream-frosting-and-fondant-accessories/sort/display_order/

Also, how do you make the deer antlers? Do you start with ivory fondant & paint brown streaks all over? How do you make the "horns" on the antlers?

They just applied the buttercream rough. The top part is fondant....

I would make the antlers out of a light brown, and use an airbrush to add texture and colour.

Could I maybe use a fork to get the rough texture/bark look? Or just make lines with my spatula?
 

Absolutely.  When I make buches de noel at Christmas I always use a serving fork to make bark lines in the ganache icing.