Global Warming and Eastern Hemlock Forests

Eastern hemlocks grow on 19 million acres throughout the eastern U.S. and are the dominant tree species on 2.3 million acres, ranging from the Southern Appalachians of Georgia and Alabama north to Maine and the Canadian Maritimes as well as west to Minnesota.

The hemlock's shallow root system excels along riparian corridors, where the soil remains moist throughout the year. These shade-tolerant trees form dense canopies that provide cool refuge for fish and wildlife.

Rich in biodiversity, eastern hemlock forests are habitat for more than 120 different species, including:

  • black bear
  • marten
  • fisher
  • bobcat
  • white-tailed deer
  • snowshoe hare
  • red squirrel
  • porcupine
  • ruffed grouse
  • pileated wood pecker
  • yellow-bellied sapsucker
  • goldfinches, crossbills
  • grosbeaks

Benefits for Humans and Wildlife

The eastern hemlock is used commercially for pulp, paper, lumber and mulch. As an evergreen that loses water to the atmosphere year-round, hemlocks profoundly affect water dynamics across eastern mountain ecosystems, regulating stream flow and moderating water temperature. They also minimize nitrate and other nutrient runoff, thereby improving downstream water quality for human consumption and wildlife.

Hemlocks shelter white-tailed deer and other wildlife during New England's harsh winter storms. Cool and densely shaded in the summer, hemlock stands provide important wildlife habitat, as well as hunting, camping, and other recreational opportunities. Streams sheltered by hemlocks are more likely to contain brook trout and are therefore popular areas for trout fishing.

Threats from Global Warming

Global warming poses a threat not only by reducing suitable habitat for hemlocks but, perhaps more importantly, by facilitating the expansion of the hemlock woolly adelgid (HWA). The HWA is a sap-sucking insect accidentally introduced from Japan into the southeast in the early 1950s and has infested more than half of the eastern portion of the hemlock's range. Lacking natural enemies, the HWA can kill a hemlock tree in as few as four years. Cold hard winters lower the survival rate of HWAs, but, rising temperatures due to global warming will likely allow the HWA to expand northward throughout the hemlock's range.

The loss of the hemlocks from global warming's combined effects on habitat availability and HWA infestation threatens to destroy the entire ecosystem, leading to an irreversible loss of North American biodiversity. Forest canopies will become more open as hemlocks are lost, and cold water streams will become warmer and thus less suitable for brook trout and other aquatic and semi-aquatic species.

In the northeast alone, with nearly 23 percent of the total volume of softwood available for commercial use at risk, economic losses may be significant.

Conservation Investments to Minimize Global Warming Impacts

Forest managers are seeking methods to control the populations, spread and impacts of woolly adelgids. This is of greater importance now as global warming hastens the northward spread of this deadly pest. Scientists must research and implement integrated pest management to facilitate prevention and treatment of infestations. Efforts may include using chemical agents to protect healthy trees (until natural enemies of the pest can be identified) and setting standards for salvage logging that reflect best management practices.

Where hemlocks have died out and when woolly adelgid infestations are better managed, widespread efforts will be needed to restore hemlocks in their historic range so that they may once again provide their many unique ecological and economic benefits.

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