I was in Vermont this weekend for the Northern Woodlands conference (for those not familiar with the group, I highly recommend their wonderful magazine and impressive programs). Along the way I saw wonders: Healthy eastern hemlocks!
After seeing mountainsides of hemlock reduced to browned standing dead trees in the Southern Appalachians, these trees were a balm for my eyes and mind. The cause of tree mass die-offs is the hemlock woolly adelgid, an insect introduced from Japan. The adelgids pierce hemlock leaves with their needle-like mouthparts, drawing down the trees’ energy reserves and causing dehydration. In the space of a decade, hemlock went from being one of the more common trees in many eastern forests to being an ecological ghost.
Native birds and tree-dwelling insects depend on the hemlock. Less obvious are the dependencies of aquatic creatures. Hemlock shades mountainside streams, cooling the water. Cool streams hold more oxygen than warmer ones, so hemlock death can cause streams to become less welcoming to insects, salamanders, and fish. Hemlocks also soak up rainfall and evaporate moisture back to the air. Their loss has changed the rhythms and amplitudes of water flowing from the mountains.
Much of Vermont is too cold for the invasive hemlock woolly adelgid, so hemlocks continue to thrive. Warmer winters and an endless supply of new insects from the south will present a challenge in coming years. For now, the northern woods are the hemlocks sanctuary, and thereby a refuge for the many other species that depend on hemlocks.
US Forest Service range map of hemlock woolly adelgid (2016). The insect continues to move west but cold temperatures keep the invasive species out of northern forests.
David George Haskell 
I’ve seen pine, spruce and hemlock infestations around Albany, NY. and elsewhere. There’s also a root rot attacking maples along road sides. Everything is moving north because of warming temperatures. A lot of it has to do with human mobility where various creatures are brought from other places by hitching a ride on some form of human transportation, like wood pallets or ballast water in tankers. There’s a certain irony in the fact that with our exploitation of fossil fuels, we’re literally changing the composition of the world around us.
I live in SW Nova Scotia on a farm/forest where the predominant species of large trees (>80 inches in circumference) is hemlock. Magnificent trees, a grove is very quiet and peaceful. Hemlock woolly adelgid has just been detected in the southern part of our province and there is much interest in its progress.
David, I think you will appreciate Wendell Berry’s poem about the life of a woods, “the brotherhood of eye and leaf,” how we have to enter it, and be transformed by it. http://www.poetry-chaikhana.com/Poets/B/BerryWendell/Sabbaths1985/index.html
The eastern hemlocks have been hit hard in eastern Pennsylvania, but some persist in fairly good condition in scattered locations. In my natural area preserve in southeastern Pennsylvania, we have hemlocks that have hosted the adelgid for over a decade but are still thriving. I spoke with folks from the Cleveland Museum of Natural History’s natural preserves department at the recent Natural Areas Association conference and they are watching with trepidation as the “wave” heads their way. On a personal note, I had considered asking my wife (should I pre-decease her) to bury my cremains in a favored hemlock grove from my youth, but I’ve changed my mind because the grove likely will be wiped out.
David, I subscribe to Panhala, a free poetry service. Today this beautiful poem, Lost, by David Wagoner, arrived. I think you’ll also enjoy this perspective of trees, the forest, and our place in it. http://blog.bestamericanpoetry.com/the_best_american_poetry/2009/09/lost-by-david-wagoner.html
Stand still. The trees ahead and bushes beside you
Are not lost. Wherever you are is called Here,
And you must treat it as a powerful stranger,
Must ask permission to know it and be known.
The forest breathes. Listen. It answers,
I have made this place around you.
If you leave it, you may come back again, saying Here.
No two trees are the same to Raven.
No two branches are the same to Wren.
If what a tree or a bush does is lost on you,
You are surely lost. Stand still. The forest knows
Where you are. You must let it find you.