Okay, I found somethingThanks to Chris P. : https://www.youtube.com/watch?v=UpvNJvsq-FU
The Ground gets a Particle System to spawn the trees, but set the Render to None:
Not sure if this also works with Hair, but this should do the trick.
Then make a Particle Instance on the Tree, which makes the Trees Meshes that can emit other Particles:
and add a Particle System after that, making sure to tickUse Modifier Stack:
