That's some excellent hair-splitting.
In general I think it means without direct assistance from humans. What you would find somewhere before people moved things around. What evolved in that location.
For instance; I might plant a White Alder in a riparian area nearby, but I wouldn't plant a White Birch or even a River Birch. Around San Francisco I would plant a Sequoia sempervirens but not a Sequoiadendron giganteum. Near London you would plant Quercus robur but not Quercus nigra.
Can you give some examples of confusion?
