That's an interesting question, actually.

I don't know forestry from my elbow, but I used to read a blog by someone who was pretty into saltwater fish tanks. Now, one property of these tanks is that they're really sensitive to a bunch of feedback loops that can most easily be stabilized by approximating a wild reef environment; if you get the lighting or the chemical balance of the water wrong, or if you don't get a well-balanced polyculture of fish and corals and random invertebrates going, the whole system has a tendency to go out of whack and die.

This can be managed to some extent with active modification of the tank, and the health of your tank can be described in terms of how often you need to tweak it. Supposing you get the balance just right, so that you only need to provide the right energy inputs and your tank will live forever: is that Nature? It certainly seems to have the factors that your ersatz German forest lacks, but it's still basically two hundred enclosed gallons of salt water hooked up to an aeration system.