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Threatened Trees

Pennsylvania is well known for its trees. It is in our name, after all. Our state holds about 200 different species of trees and a number of them are under threat due to invasive pests or disease. Many of these iconic woody wonders are the focus of specific conservation efforts that will, hopefully, prevent us from losing their contributions of diversity to our forests.

For those interested in learning more about the other native species of trees we have a topic dedicated to them on our website. It offers an excellent introduction to identifying some of the more common trees you are likely to encounter while out exploring. It also provides a primer on the anatomy of our leafy friends and outlines some of the vocabulary associated with identifying trees.


Pennsylvania has been a changing land for the last 200 years. When Europeans first arrived they found dense forests dominated by White Pine and Hemlock anywhere there was water (and there’s a lot of water in our forests), and enormous stands of Chestnuts, Oaks and Maples everywhere else. Over the course of colonization almost all these original stands were cut down, despite the state’s name “Penn’s Woods” remaining the same.

While we’ve been wildly successful in reforesting our state from complete devastation, threats to many of our returned trees became widespread in the new 20th century. These threats came along with the shrinking of the world due to reduced travel times from far off locales, allowing invasives to be introduced easier and faster.

The American Chestnut, once so plentiful throughout the state that entire regions were named after it, suffers at the hand of one of these introduced species. A fungus, colloquially referred to as chestnuts blight, was accidentally introduced into North America in the early part of the 1900’s. The fungus attacks wounds in chestnuts and parasitically draws nutrients and energy from the tree. Interestingly it does not kill off the root system, meaning that plenty of “living stools” regularly send out sprouts that grow for a few years before failing.

While the damage done to the American Chestnut has already taken its toll on their populations, the American Ash is currently under threat by a small beetle. The emerald ash borer, appropriately named, bores into the ash’s inner bark where it disrupts the flow of nutrients throughout the tree. The beetle can destroy the entirely of the cambium, or the growing portion of the tree, leaving behind tracks and causing the bark to slough off. Affected trees are almost always killed by the beetle and while there are still billions of ash trees across the eastern United States the emerald ash borer has the potential to kill every one. Since its first sightings in 2002, It is widely considered to be the most destructive invasive forest pest in North America.

Another invasive insect is the hemlock wooly adelgid which, like the ash borer, is named its food source, hemlocks. Our state tree is under considerable threat due to this minuscule mangler as the adelgid sucks the sap out of the leaves of the hemlock tree and often causing the tree to defoliate, or lose its leaves. This causes extreme stress on the tree and often kills weakened or at-risk trees. The bug leaves behind small fuzzy balls on the underside of hemlock branches that contain the wooly adelgid’s eggs. The wooly adelgid has spread to cover the entire range of eastern hemlock species and can drastically affect the survivability of hemlocks which grow slowly and make up much of mature forests in Pennsylvania.

Fortunately for Pennsylvania’s trees, there are active control methods for the current threats to our forests. In the cases where control methods are no longer viable, such as the American Chestnut where billions of trees are already gone, rejuvenation efforts are underway to re-introduce the species across our forests.

The American Chestnut Foundation is the primary organization driving the return of the once dominant chestnut. Through a process of hybridization, breeding blight-resistant chestnuts with the American varieties, they are slowly creating a cultivar of the original American Chestnut that is resistant to the blight that is still dormant across our forests. Locally the ACF has partnered with the Arboretum at Penn State and other local landowners to plant hundreds of hopeful seedlings. This new generation, while only about 50% viable, makes up the beginning of a comeback for the effectively extinct tree.

There are also active control measures designed to curb the spread of invasive pests and begin to fight back against their devastation, especially for the hemlock wooly adelgid. We are also lucky that very cold and dry winters often have detrimental effects to the viability of the adelgid’s eggs, so think about that the next time your car doesn’t start on that -5 degree day! Individual trees can be saved by applying a systemic pesticide directly injected into the trunk of the tree or by administering it to the soil to be absorbed by the tree. The pesticide is highly targeted in that way and only affects the adelgid as it feeds on the tree. Biologic methods are used at a larger scale by introducing a natural predator to the adelgid that feeds almost exclusively on the eggs and larva of the pest. Both control methods are very successful and have helped keep our trees alive and pest-free for years.

Many of our threatened trees continue to thrive due to ongoing conservation efforts. Visitors wanting to see these tree species can find them in many locations across our region.


Alan Seeger offers a glimpse into the past. Many of the trees in the protected natural area are over 200 years old and represent what most of the forests in Pennsylvania would have been like before the 1800’s. Several trails lead past stand of truly ancient hemlocks that have weathered hundreds of years and thanks to active management of invasive pests still stand today.

The trails up Mount Nittany pass by several examples of the “living stool” chestnuts. Visitors looking to make the journey to the top can also find small copses of ash trees while taking in some of the best views in Centre County.

The Spring Creek Canyon is home to diverse and variable forest system that includes multiple efforts to save some of our threatened trees. Many of the trees in the area are still first generation regrowth from the iron and timber industries, but in that time many ash and hemlocks have begun to come up through the fast growing oaks and maples.


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