Alien invaders reshaping forest landscape of U.S.

Dogwood fungus can change Eastern woodlands in just weeks

February 10, 2002|By Carol Kaesuk Yoon | Carol Kaesuk Yoon,NEW YORK TIMES NEWS SERVICE

NEW YORK - In the summer of 1904, Hermann Merkel, the chief forester at the Bronx Zoo in New York, noticed that a few of the majestic American chestnut trees lining the zoo's walkways had developed a mysterious new disease.

The next year, nearly every chestnut tree in the parks of the Bronx had the disease. And by the 1950s it had spread from Maine to Georgia, killing billions of chestnut trees and changing the East's wooded landscapes forever.

Merkel had discovered the disaster known as chestnut blight, giving scientists their first bitter taste of the imported diseases that have been sweeping through American forests ever since.

The best known are chestnut blight and Dutch elm disease, both of which are believed to have come from Asia and are still attacking trees today. But scientists say a host of devastating forest pathogens have continued to arrive. Among the most recent is sudden oak death syndrome, which has killed thousands of oaks and other trees in California and may threaten redwoods as well.

Fast-moving and usually hard or impossible to cure, these exotic diseases have destroyed countless trees in forests, cities and suburbs. The results can be seen not only in landscapes stripped of some of their most beautiful species but in changes to how forest ecosystems work and in the economic value of this natural resource.

As world trade intensifies, scientists predict that more and more tree diseases will find their way into the country (and out of it). And while most pathogens die on arrival, unable to find a suitable victim or climate, a small portion turn into devastating blights.

According to a recent report from the National Academy of Sciences, about 13,000 plant diseases a year are intercepted by inspectors at international ports of entry - and these inspectors are able to examine perhaps 2 percent of the incoming cargo and baggage.

Dogwood fungus

The effect of such invasions is clear. A fungus called dogwood anthracnose has killed millions of flowering dogwoods in the southern Appalachians alone, essentially wiping out the species in many areas.

In the Southeast, butternut canker has hit butternut trees so hard that the species has been listed as threatened in at least one state, Tennessee, and has been declared a species of special concern, a prelude to consideration for federal listing as threatened or endangered.

In the Northwest, an imported root disease is killing off Port Orford cedars, whose wood can be worth as much as $50,000 for a single, mature tree. And the list goes on. If imported insects are included among the pests, the casualty list grows even longer; some species are attacked by both kinds of imported pest.

Don Goheen, a plant pathologist with the U.S. Forest Service in Medford, Ore., who works with Port Orford cedars, echoed the comments of others, saying: "These introduced diseases are bad news. We're run ragged by them."

Though the devastation goes on around the country, it has received little notice outside the community of forest researchers - largely, scientists say, because a diseased tree can take years to die and its loss is not always evident to the casual observer.

"When you look out there you don't have a barren moonscape," said Scott E. Schlarbaum, a forest geneticist at the University of Tennessee. "You have a forest. But that forest is very different than it was 100 years ago."

The loss is most likely to be noticed when the tree is prized for its beauty. The rapid spread of dogwood anthracnose, a fungal disease that begins with spotting leaves and soon ends in the death of the tree, has robbed many communities of flowering dogwoods, long a joyful herald of spring.

Mark Windham, a plant pathologist at the University of Tennessee, says the disease is attacking Pacific dogwoods in the West and flowering dogwoods in the East, with the southern Appalachians hardest hit.

In 1989, Windham said, he saw 20,000 healthy trees on Lookout Mountain in Tennessee go under. "They became fully blighted in two weeks," he said. "In three years most were dead."

Researchers estimate that in the Great Smoky Mountains National Park, 60 percent of the dogwoods have been killed by the disease. But even this well-traveled area may not look too bad to the casual visitor: roadside dogwoods do well, since they are exposed to sunshine, which the fungus dislikes. "Tourists don't tend to get out of their cars, so they might not see much," Windham said. "Several hundred feet into the woods they'd see a different picture."

Likewise, dogwoods in sunny yards can escape unscathed.

Besides being beautiful, dogwoods play a number of roles in the forest. The leaves have high levels of calcium, making them a primary food for lactating deer. In addition, the calcium in decaying leaves keeps soils from becoming too acidic, a growing concern now that dogwoods have disappeared from so many areas. And the outer coat of the berries is 20 percent fat, making them a crucial source of energy, for example, for songbirds.

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