Thanks very much for this interesting and useful summary.  I would like to add a couple of thoughts.  That is that the relative palatability of oak and white pine vs. northern hardwoods species to browsers may affect the rate of colonization of the valley.  As the abundance of white tailed deer increases in the valley, and they are most abundance at lower elevations, they may be foraging preferentially on northern hardwood species: sugar maple, striped maple, hobblebush and perhaps even yellow birch.  If this is true, seedling and sapling growth and survival may be affected, perhaps tipping the advantage to the colonizing species.  In addition, it would not surprise me if American turkey are having some impact too as they feed on beech seeds and germinating seedlings.  Due to BBD, the abundance of beech mast, even in most mast years, may no longer exceed what can be consumed by its myriad seed predators.

All the best,

Nick