It still needs manual upkeep. Nature has a way of planting things that don't even necessarily belong on the same continent in spaces where they are undesirable.
See, for example: Kudzu. We introduced it intentionally, and nature's own mechanisms have it literally growing beyond control -- blanketing (and typically killing) anything in its path in the US South, including mature trees.
Like, if nature itself needs upkeep, something other than lack of upkeep is the issue.