hwa spread   James Smith
  Jul 23, 2007 06:44 PDT 

Okay, speaking here obviously as the layman that I am.

Over the past few years I've figured that hwa had pretty much all but
killed our hemlocks. All of the places that I visited either had
completely dead stands of hemlocks, or the trees were already heavily
infested and on their ways down to extinction. So I figured this must be
the case over the entire range of the eastern and carolina hemlocks.

However, most of my initial travels were done northward and southward
along the eastern corridor of their southern habitat. Shenandoah
National Park--I couldn't find a single living hemlock. BRP from
Shenandoah south to GSMNP--the hemlocks all along this ribbon of road
range from dead to very sick.

South Mountains State Park has a vast hemlock forest that is beginning
its downhill slide. The tree that are not showing the effects of the
sap-sucking monsters are totally infested. This is what I found in just
about every area where I looked where the trees still only "looked"
healthy.

However, over the past few months, I've made trips westward of the
corridor of death where I had been spending most of my hiking time.
Middle Tennessee showed me some fine stands where I couldn't find any
evidence at all of the hwa. This past week I hiked extensively in the
Allegheny highlands of West Virginia and saw only slight signs of hwa,
and many, many, many thousands of acres of healthy hemlocks.

Considering that the variance in latitude and distance from utterly
destroyed habitat and healthy habitat is slight, what is the cause of
this? I seriously doubt that groves father west are less susceptible to
the infestations--groves in north Georgia are dead and they're farther
west than the groves I saw in WV.

I've read that the prime vector of hwa might be birds. If so, are
certain species of birds carrying the adelgids up and down a migration
corridor? And lacking this certain vector, hwa might be slower to spread
westward by less reliable means (wind, etc.).

At any rate, just something this mailman has noticed in his journeys
peak-bagging and waterfall wandering and forest watching throughout the
South.
Re: hwa, spread   Kirk Johnson
  Jul 23, 2007 11:22 PDT 

Some think colder winters might be a limiting factor. In the Allegheny
National Forest in NW Pennsylvania, there is no hwa infestation (yet).
Although one affected tree was discovered (and destroyed) in SE Elk County
in 2005.

Kirk Johnson
Re: hwa, spread   Jess Riddle
  Jul 29, 2007 16:06 PDT 

James,

I've heard the same ideas on birds being the primary long distance
dispersal vector and responsible for the relatively rapid spread of
adelgid up and down the Appalachians. I have not heard a more
convincing explanation for the relative dispersal rates.

Jess