This species succession, or climate induced vegetation change, is being observed all across western North America where drought pulses have become more severe. It’s not the overall moisture that matters, it’s the extremes. Precipitation can increase, but if drought pulses are severe enough, they cause stress that allows insect and disease infestation and even kill mature trees. These dead forests are then replaced by young trees that are more drought tolerant, until they get a little older. Then the process repeats as climate change weather extremes become even more severe, resulting in ecological chaos where forests never mature.
The young trees are moving westward because during non-drought periods, moisture is more abundant, making the environment more suitable to eastern species with greater moisture needs. When drought recurs, the younger trees are more drought tolerant than the mature trees in the area, allowing these “invading” species to persist.
The vegetation just can’t keep up.