"gregpresley" expounded:

I would say that root rot is rare. However, doug firs, expecially if they
have been part of a larger forest that has been thinned, are prone to being
knocked down by the wind during ferocious winter storms. Apparently their
root systems interlock, and when part of that connection is broken they are
more vulnerable to wind damage.

It's not just the root system. Trees that grow together grow in
certain patterns in the wind. When you take down some of the trees,
the ones left are left without the protection they grew up with.
Their wind strength was established with their partner trees (for lack
of a better word).

--
Ann, Gardening in zone 6a
Just south of Boston, MA
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