Trees have been around for a very long time, hundreds of millions of years. Much longer than we have been. Possibly something they "know" at a genetic level is a sort of risk assessment where species are calculating better long term odds for the species based on local trend. They could be migrating toward areas where less adaptation is needed to survive; or maybe even a substantial but available path to adaptation via recessive genes.

This migration isn't the first. They've been doing this all along.