American Chestnut trees once dominated the hardwood forests of eastern North America. Now these trees are gone, and two other popular species appear to be facing similar fates.
The chestnut trees, mixed with several species of oak, hickory, maple, and birch, accounted for as much as 40 percent of the cover in some areas. Besides nuts, the trees were prized for their superior timber -- a rot-resistant wood used extensively for cabins and fences. A traveler to the Appalachians can still see structures made of chestnut. But from Maine to Georgia, no living examples of the tree remain. The species was wiped out by a disease called chestnut blight, which came from Asia a hundred years ago in a nursery shipment of Chinese Chestnuts.
A shipment of logs from Europe brought another new disease to America in the 1930s: Dutch Elm disease. Caused by a fungus and carried by native insects, it spread rapidly, killing an estimated 42 million trees by 1960. Like the chestnuts before them, our Dutch Elms had no natural resistance to the foreign pathogens that attacked them.
And now trees in California and Oregon are dying by the thousands. First noticed in Tanoak trees in Northern California, the pathogen quickly spread to several species of oak. It was soon dubbed Sudden Oak Death. Where did it originate? Researchers suspect it's a foreign invader. Worse, it's been found in a variety of trees and shrubs, including some species unrelated to oaks. The fear now is that if it proves deadly to even half of these species, whole ecosystems will be threatened.
In the East, meanwhile, Flowering Dogwoods are dying, victims of yet another blight of mysterious origin. This quickly spreading disease has killed as many as 90 percent of the dogwoods in moist forests from Connecticut to Georgia and as far west as Indiana. While some observers are quick to assume that the disease is also non-native, others assert that human suppression of fire may be altering the natural balance and creating situations that allow native pathogens to erupt -- with devastating effects.
The encouraging news from the forests where Sudden Oak Death and Dogwood Blight are wreaking havoc is that even in areas where most of the trees are dying, one can still find healthy trees. Thus some genetic strains may be resistant to these diseases, and the occasional massive die-off may simply be part of some long-term process of natural selection.