Modelling help

Hi,
I’m making a cake for a competition however it’s based on appearance alone. The design is based on pollution: https://www.google.co.uk/url?sa=i&source=web&cd=&ved=&url=http%3A%2F%2Fmazendesignes.deviantart.com%2Fart%2FPolluted-530188046&psig=AFQjCNHaq1kIA_f-5CWdlM2ND3y-RAs37A&ust=1463055303947332&rct=j

A tree which in one side is polluted and on the other it’s not. A 10" cake will make up the ground and I’m wondering what I can use the make a 3D tree which I can decorate. I know I can make rice crispy models but since the cake is for appearance only I don’t see the point in making it out of cakey materials. What could I use for the trunk of the tree and branches?

Thanks :)