Interesting article. I always thought the main driver for the PNW’s lack of diversity was the recent glaciation. But now that I think of it, wouldn’t Chicago of had glaciers around the same time period? Chicago and the rest of the Midwest have a huge diversity of
trees and plants. I use to study insects and the same is true there: minimal diversity in the PNW and tons in the Midwest and east coast at similar latitudes.
The Cascades could be a good barrier for recolonization to the west of the pacific crest trail but east of there, I’m not sure what the story is.