passive afforestation

I've come across spontaneous afforestation if that helps. Other terms include natural regeneration and natural reforestation.

By the way, succession is supposed to be spontaneous.

Rewilding

2.

to return (land) to a more natural state: rewilding an unpopulated island for use as an animal preserve. from dictionary.com

It still doesn't guarantee a passive process, but doesn't have the strong implication of human action that afforestation does. It's (of course) not specifically about trees, though most uses seem to carry that implication.

"Natural wild growth" is the idea you seem to be looking for but that's a phrase, not a single-word.

As opposed to cultivation/ afforestation, it is disuse that leads to natural wild growth.