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.