I was going to ask about what is killing the trees, too. They all seem to have succumb fairly recently.
We have two Phemeranthus spp. in Minnesota: P. parviflorus and P. rugospermus. P. calycinus jumps around happily in my pots.
I haven't been to real mountains in decades, and it never ceases to amaze me that trees and meadows grow simultaneously without any mid sized floral contingency.
