How do I make plants generate on or beneath trees?

Hello! I am brand new to Mcreator and currently I am creating my first mod. It is based around real world fungi and I am adding a wide variety of mushrooms to the game. I would really like to create several fungi that grow on the side of logs, especially the fallen logs. I was thinking something similar to how coral fans spawn on the side of coral blocks. Is there any easy way to make something like this? What kind of procedures or creation tools would I need to use to make these spawn naturally?

I also created a special mycelium block that spawns just below the grass level and if possible I would love to make this block spawn primarily beneath trees if possible. Is there a way to do this as well?

Thank you!