Observer doesn't detect when a grass block changes to dirt if a tree grows on top

Having a observer facing away from a grass block does not detect when a tree grows, changing the grass to dirt. it does detect if a block placed causes the grass to change and it does detect if the grass grows back. 

What if you wanted to build a fully hipster tree farm but Mojang said:

NO

 __ 

also rip Ybuds. they died quietly