Custom Feature "Trees" Overwriting the main filler block (example netherrack in the nether).

I've been looking for a solution for a long time, but I haven't found it. I made a tree in my dimension that grows into the main filler block above it. I want to make it so that, like the Nether Crimson Fungus, the main filler block overwrites the feature, not the other way around.

How to fix it? <3