True - that's why I use a definition that is based around how well the plant has integrated with the local ecosystem. Ultimately, everything growing here now will fully integrate and become "native".
I like this definition because it has nothing to do with some past point in history. It is just a measure of how other life living in the same environment as the specific plant has adapted to it and how well that plant has adapted to the other life it shares the environment with.
Now measuring this is challenging but one measurement is are there any other life forms that have specialized to rely only on that plant. Any plant represents a niche for other life to fill - at first only generalists can take advantage of this niche. But eventually some will specialize and come to depend on that one plant. Once this happens then in my book the plant is fully integrated into the local ecosystem and has become "native".
Really native and non-native is not the ideal terms but they are the words we currently have.
But this definition is why I fully believe that planting native plants provide a benefit that non-native ones can't provide at least at this time in time. But eventually all the non-native plants will make the transition and become "native" once other life has come to depend on it.
Cultivate abundance for people, plants and wildlife - Growing with Nature
It is impossible for me to observe a plant, growing in the wild, and draw any conclusions about whether or not it is endemic to a particular place/time. Every plant feeds microbes, insects, fungi, and other plants -- in death if not in life. Regardless of where/when that plant was previously growing. Every living plant shades the soil, and provides refuge to animal life, regardless of whether or not animals feed on it while living.
Something that I've been thinking about too is that sometimes it's possibly for us to save plants that are native to other ecoregions. An example is the American chestnut and Butternut (also known as the "white walnut" to some). Both of these trees have been struck down in their native ranges (the eastern half of the continental US) by diseases coming in from Asia. The hot humid summers were similar enough to Asia's monsoon seasons and humid summers that the diseases (chestnut blight and butternut canker) were able to spread. Butternut and American Chestnut are now very endangered in their native ranges. But here in the Pacific Northwest, it is too dry during summer for either disease to proliferate for long (unless you're using overhead irrigation). It's something to think about, in the interest of preserving biodiversity
When you reach your lowest point, you are open to the greatest change.
James - I like your thinking. I have planted quite a few chestnut hybrids from Burnt Ridge Nursery, as well as butternuts (actually the unfortunately-named "buartnut") and shagbark hickory from same. In the area we found a huge, beautiful Juglans that I'm pretty sure is a heartnut, but it doesn't bear and is seriously distressed -- rotten in the middle to the point of sprouting mushrooms from its splitting base. There are many black walnuts around here as well, but none of them bear either. Mine may reach adulthood only to remain single! At least they're not alone.
I also planted a grafted elm (weeping, grafted to American rootstock) to have an elm away from the elm borer crisis. In the novel "The Overstory," the first vignette is about a chestnut that, because of its freak placement by some homesteaders outside of the natural range, escapes the blight, which the story tells in great detail. It also grows in a bizarre and striking habit -- another potential upside to unusual plantings. Unfortunately I only really liked 3-4 stories in the book and so didn't finish it.
And Daron, your post really changed my life and thinking. I had several natives planned, but didn't know why, until I read Tallamy's book at your rec. It's great, every permieshould read it. I had intended to plant garry oaks agogo for various reasons, but in light of Sudden Oak Death syndrome, I'm even more chuffed. I'm sure they will grow dismayingly slowly. There are some burgambel oaks in there too but they're already showing less vigor than the garrys.
Anyway I also bought a bunch of natives from a tiny backyard Seattle nursery, because they were a little easier to find than where I am. This is all because of you and Tallamy! I'd love to volunteer at the nonprofit you work for as well, which is it? Now that my eyes have been opened I see the lack of diversity on my land and around me, and if I can find a local native plant expert (Skeeter? he's kind of far though) I hope to walk this land with them. I bought this place in part because of all the birds, more than many local properties, but I see how many more there should be and my neighbor, a bird enthusiast, has lent me volumes, expertise and her binoculars.