Hi..
What do I have to do so that the trees are automatically marked for destruction?

Ok...
It works now. But just extremely slow.
Is there a way that the trees are marked faster? If not, I have to think of something, because as it is currently it is clearly too slow.

Hello!
The trees are marked for deconstruction as soon as they reach maturity.
Probably that is why it seems slow.

Hope it helped, in case let me know

are there multiple growth stages?

Yes, you can even change it if you want to.
You can also change how much time it should a tree take to grow to full size.