America's trees are under attack. Species by species, they're being invaded by insects and fungi, native and foreign. Scientists fear their loss will devastate suburban streets and upset the delicate ecological balance of many woodlands.
"Invasive species are a real threat to the nation's forests," said Dale Bosworth, chief of the U.S. Forest Service, at a recent conference in New Orleans. "There are so many things, it just seems too big to talk about. ... Like a slow-moving fire, they're going everywhere."
In the past 150 years, fungus-based diseases known as chestnut blight and Dutch elm disease have virtually wiped out the American chestnut and American elm.
These days, scientists point to dozens of newer invaders, from the well-known European gypsy moth to the newly discovered emerald ash borer.
"We're seeing plague after plague come in," said Faith T. Campbell, head of the invasive species program at the American Lands Alliance and co-author of two definitive reports on the loss of the nation's forests.
Native pests that have long been part of the forest ecosystem are suddenly gaining an upper hand in some areas of the country, as expanding development and extreme weather leave trees more vulnerable to attack.
Just last month, the forest fires of Southern California were fueled by more than 400,000 acres of dead ponderosa pine trees. Those trees were killed by Western pine beetles that are native to the forests and typically co-exist with the trees.
"Our native insects are always present, usually in non-outbreak or non-epidemic levels," said Michael J. Raupp, chairman of the entomology department at the University of Maryland, College Park. "A tree gets struck by lightning, or one tree falls on another tree, and the insects come in and take over that tree.
"But when we impose a drought over a large geographic area, instead of having just a few trees, you have thousands of trees that are susceptible, and this is the condition that leads to the outbreak of the native pest," Raupp said.
Among forest watchers, perhaps the biggest concern over the past decade has been an influx of new pests that seem to emerge from nowhere. Often arriving from foreign countries, they hitch a ride on live horticultural imports or sneak in through wood packaging in container freight.
"This is probably the most under-reported threat facing America's trees," said Phyllis Windle, an invasive species expert at the Union of Concerned Scientists. "There's almost something for every kind of tree.
"If we don't get a handle on this problem, we could lose a third or more of the trees in this country over the next several decades."
Expanded trade with various parts of China has brought in a new set of pests that live in areas with climates similar to ours -- but without the native enemies that usually keep the pests in check overseas. Advances in just-in-time shipping, which speeds up international cargo traffic, make it easier for pests to survive trans-Atlantic or trans-Pacific journeys.
In some areas of the country, previous battles with invaders have left communities and forests more vulnerable. For example, the elms wiped out by Dutch elm disease were frequently replaced along suburban streets by ash trees -- which are now under attack by a new pest.
State and federal officials who want to save their trees have to mount a rapid and heavy-handed response to just about any possible infestation.
Some pests aren't fatal by themselves -- they just weaken trees and make them more susceptible to disease. But other invaders attack until they cut off the supply of food and water, killing every tree they infest.
Scientists can spray some trees to drive off pests and inject others with chemicals. But those are expensive propositions -- viable for individual homeowners but impractical for large parks or woodlands.
Consider the emerald ash borer, a deadly exotic beetle from Asia discovered in the United States last year. Raupp described the insect as "the biggest threat to our natural forest, in my opinion, since chestnut blight."
"It kills every ash it sees, from the healthiest to the weakest. It has the potential to basically eliminate ash trees as a component of our natural forest stands here," Raupp said.
The emerald ash borer, which most likely arrived through shipping crates from China five to 10 years ago, has infested at least 5 million trees in Michigan and a portion of Ohio.
But this spring, a Prince George's County nursery unknowingly received a shipment of ash trees from Michigan. When Maryland officials discovered the infested imports, their reaction was swift and deadly.