Tree Ground Decorator Issues

Hi all, I'm still experimenting with trees and trying to explore what they can do. I'm wanting to have a ground tiles have a chance be altered from the current floor to blocks like grass or gravel; however for whatever reason I'm not able to have these blocks be replaced (In this case, my biome's ground is light gray concrete powder). I'm using decorators for this as I'm aware that this is what should probably be used.  Screenshots included for code of the tree.