In California that's somewhere between 1400 and 1500. It's really hard to be site specific about the time or what plants were there in much of California as native plants in some areas were completely replaced by weeds, fire, and other activities of man.All we grow are California native plants. Have for almost 4 decades. That means we grow plants that grew within the boundaries of the state line in the year 1400 A.D. The Native Americans didn't move plants about much; a little, but not much. The first Europeans brought wild oats, filaree and other weeds and contaminated vast areas of the state in a few decades. The hillsides could then burn much easier. Many of the images of California taken by the first cameras show that in the late 1800"s the hillsides were nothing but black from fires. No trees or shrubs left. It's hard to figure how anything has survived our onslaught on nature.
Native plants are commonly misunderstood and have all sorts of misinformation applied to them. Think about when you walked through a truly wild area with no weeds, wasn't it nice?
Science is not absolute; it is the best description of the facts. "The only things certain in life are death and taxes" (Benjamin Franklin), and they're now beginning to question what is death.