Eastern hemlock (Tsuga canadensis) is a long-lived, late-successional conifer, native to Eastern North America, occurs from the Appalachians to southern Canada. Hemlock is a foundational species that has profound effects on the structure and function of its ecosystem, especially in the north where the species is dominant and can form almost monospecific stands. Hemlock has dense, evergreen foliage that casts the darkest shade of any northeastern tree (excluding most deciduous trees). This renders the forest underneath cool and moist. This dark shade benefits cold-stream species such as fish or amphibians, as hemlock often grows along streams and creeks. Hemlock needles are acidic and decay slowly, leading to acidic and nutrient-poor soils. Overall, forest with abundant hemlock often have different environmental conditions, and select for very different terrestrial and aquatic species compared to those in deciduous forests.
Populations of this foundational species have been plummeting due to an infestation by the exotic hemlock woolly adelgid (Adelges tsugae); a small flightless insect introduced from Japan in the early 1950s. This pest pierces into hemlock needles, siphoning vital nutrition from the tree. Woolly adelgid has been spreading rapidly since the 1980s, killing hemlocks in an affected stand within about ten years, with mortality rates of up to 99%. Hemlock declines have been especially massive in many parts of the US, while Canada so far has hardly been affected. As canopies of dying trees thin, the dark hemlock forest shifts to lighter, warmer and drier mixed hardwoods. Animals adapted to hemlock forests exhibit significant declines, including several birds, cold-water trout, aquatic invertebrates, and especially the eastern red-backed salamander. This salamander species typically loses about 50% of its population once hemlock is gone. Sadly, global warming threatens to exacerbate hemlock declines with warming winters. Currently, cold winters limit adelgid populations and slow the pest’s northward spread as temperatures below -25 degrees C cause insect death. While the first adelgid populations were sighted in Ontario in 2022, most of them insect still occur south of the Canadian border. However, taking future warming scenarios into account, woolly adelgid will most likely be a threat to hemlock throughout its total range in all of North America, including Canada.
Original text written by Shabnam Tejani, rewritten by Ivana Stehlik
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Photo credit: liz west from Boxborough, MA, CC BY 2.0 (https://creativecommons.org/licenses/by/2.0), via Wikimedia Commons