eastern hemlock

Why save the Hemlock?

Why save the hemlock of Cook Forest from the invasive insect Hemlock Woolly Adelgid (HWA)?  Why not let them go the way of the American Chestnut, once the predominant species in eastern forests, which died out almost entirely following a blight that swept America in the early 1900s?  Why go to such lengths to protect this one species, tsuga canadensis, which has little timber value?

Why are Hemlock trees so important?

Hemlock are a keystone species, without which the forest cannot function as it does:

  • Hemlock provide crucial shade for streams, maintaining the cold water required for trout and other aquatic life;
  • Hemlock provide food and habitat for ≈ 120 species of vertebrates, as well as more than 90 type of birds – Cook Forest is an Important Bird Area, as recognized by the Audubon Society;
  • Aesthetic beauty!
  • Hemlocks provide abundant shade necessary to many native plants such as various sedges, trilliums, lady slippers, jack-in-the-pulpits, etc.;
  • Hemlocks help maintain the rich diversity of plant life in our forests and help prevent the spread of invasive plant species;
  • Hemlocks contribute greatly to air quality, filtering pollutants from the air, removing tons of CO2 and other greenhouse gases from the atmosphere, creating oxygen for us to breathe, and supporting human respiratory health – year-round;
  • Hemlocks are integral to maintaining water quality; their extensive root system prevents stream bank erosion, filters out pollutants before they reach the waterway, and prevents build-up of harmful bacteria;
  • Hemlocks prevent erosion and, on our steep slopes, prevent rapid downhill runoff into rivers and streams, and protect against flooding and landslides.


Cook Forest Conservancy nonprofit for hemlock trees & forest health

Hemlocks of Cook Forest

Cook Forest State Park has some of the finest stands of old-growth hemlock trees in the Eastern United States.  These hemlocks have shaded  the trails and waters of Cook Forest for 350 years — but they are now under attack.  A tiny invasive insect, the hemlock woolly adelgid (HWA), infects them, weakens them, and kills them.

Though our infestation here is currently light, and there is time to save these graceful giants, the hemlocks of Cook Forest are known to be infected with HWA, and these insects reproduce exponentially. A 90% winter kill is wonderful, when the weather is cold enough for long enough – but it won’t stop HWA except momentarily. 

Without prompt treatment, infected hemlocks will die.
Eventually, all hemlocks will be infected.

Hemlock HWA deaths in Great Smoky Mountains TN
Hemlock mortality due to HWA in the Great Smoky Mountains

But Cook Forest has some time, we have plans, and we have hope.

Since Cook Forest is recognized widely as an important site for old-growth hemlock, the PA DCNR has been treating our trees for several years, and they presently are in good health.  Unfortunately, since the reproduction rate of HWA is exponential, these insects will be a problem for the trees until an ecological balance is reached.  Scientists are working on establishing populations of bio-control agents (primarily insects from the native range of HWA, in the Pacific Northwest and Japan), but they’re costly to collect and raise, slow to colonize, and killed by harsh winters.

We’re very fortunate that the state park has been treating the state park trees – but this is an expensive and difficult effort, without guaranteed future funding.  It also doesn’t help hemlock outside the park borders, which can host HWA, which will continue re-populating the park trees with the insect.  The CFC and Bureau of Forestry host seminars for landowners on how to monitor the health of their hemlock, and treat them before it’s too late for the tree to recover.  The CFC is also part of the Allegheny Forest Health Collaborative, and works with its many other organizations to train citizen scientists to track and report HWA populations across the Allegheny plateau.  

You can help us provide all these programs – you can help ensure our forest stays healthy and biodiverse, our streams stay clear and cold for trout and our water supply, and our beloved hemlock trees continue to stand in Cook Forest for future centuries —

Please join us in saving the hemlocks of Cook Forest State Park!

Help fund the fight the Hemlock Woolly Adelgid (HWA) – help keep this last great stand of Eastern Hemlock uninfected:




Thank you for your interest in our hemlocks — if you haven’t seen Cathedral:  The Fight to Save the Ancient Hemlocks of Cook Forest by Wild Excellence Films, please watch the trailer here, and subscribe to our newsletter for information on upcoming screenings of the feature film:
Cathedral Film Trailer from Wild Excellence Films on Vimeo.