Eastern hemlock is a defining tree of the Northeast. On slopes and along mountain streams, hemlocks cast deep shade with dense, feathered foliage, creating a unique climate in the forest. During the nineteenth century, tannin-rich hemlock bark became a prime resource for processing animal hides into leather. The abundance of this tree in the Catskills and Hudson Valley made the region a center of the tanning industry. Tanneries were constructed in nearly every valley. They drove the development of towns, villages and hamlets as the surrounding forests were denuded. Overharvest nearly obliterated the species, deeply altering the ecosystem.

Over the past century, Eastern hemlock has been slowly reestablishing. Before any forest has had the time to stabilize, however, hemlock has entered collapse once again. Hemlock woolly adelgid (Agelges tsugae), a tiny insect native to East Asia, is bringing about the mass mortality of Eastern hemlock in its native range. HWA feeds on hemlock at the junction of needle and bough, sucking the sap and depleting the nutrient stores of a tree over the course of several years. The insect leaves a distinct mark on infested trees in the form of white, woolly sacs that accumulate along the underside of branches.

Hemlock woolly adelgid was introduced to the United States in the 1950s, but populations have exploded in recent years as climate change makes winters warmer. Higher temperatures permit the proliferation of HWA into the northern part of the tree’s range. According to a comprehensive report on the biology and ecology of Eastern hemlock published in September 2025, “...continued infestation [of hemlock woolly adelgid] is likely to cause a range-wide decline or elimination of this ecologically, culturally and economically important tree species. Cold temperatures are the only factor that can cause widespread mortality of HWA over broad areas.” (Orwig and Thomas).