I'm not saying it explains everthing they found (they may even have corrected their stats for it), but the area they studied seems closer to the east coast then to the west coast. Moving long distances east is hard there for trees. First, they would hit larger populations centers; then, the ocean.
Re:American Trees Are Moving West
by piotrkaminski on Sunday May 21, 2017 @03:45PM
(#14388870)
So basically they're moving west because there's more space there? ;)
If it is human-influenced, what are the determinative factors? Do humans favour certain seeds/trees in this selection? Are the seeds geographically distributed evenly, or is distribution biased towards the west? Is the westward growth bias caused by seed distribution, or locale viability? If humans weren't distributing the seed, are there other vectors that would provide an equivalent function?