The long skinny ones around the castle?
Perhaps Duff's method? He makes green royal icing and spreads it out to dry. Then scrapes it up to make fluffy green "powder". Then he makes the tree shape in fondant and rolls the shape in the powder to give it texture. (Sorry, I don't know how he gets the powder to stick.)
That's a really good idea, thanks! If I wanted to stick fondant to rice krispie treats, do I need to put buttercream on it first, or just stick the fondant to the rice krispie treats?
I think most people use chocolate or those candy melts, but you'll need a sticky of some kind.