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The aliens.

A litany of insects and diseases from beyond our shores is making deep inroads in woodlands across the country and shaping a "quiet crisis" in forest diversity.

From a distance--the eye of a soaring peregrine falcon, say--eastern forests look much the same as they did two centuries ago. They are greatly shrunken, of course, but the scalping and scalding of entire regions by rampant logging and successive brush fires through the 19th century have largely healed. Once again vast tracts of trees billow green in summer and stand bark naked against the cold of winter.

Look closer, though--from the viewpoint of a warbler or a woodpecker--and you see that these forests are utterly changed from the days when falcons and eagles ruled the skies. More disastrous than the holocaustic removal of trees, more relentlessly impoverishing even than massive erosion of topsoil, are the toilings of introduced insects and diseases.

Some of these pests were identified early and their impact was widely mourned (the chestnut blight, for example), but many have operated over decades or perhaps centuries without recognition, even by forest experts. Forest ecosystems are so complex, and the nature of their processes before European settlement are so poorly understood, that insidious change imposed from outside the ecosystem has been difficult to detect. Commerce, with its inherent tendency toward widespread distribution of goods such as lumber and nursery stock, has been the instigator of most of these changes, and as the 20th century moves toward a global economy, the world's ecology is also being globalized. While we struggle with insects from Europe and fungi from Asia, a North American pine nematode is eating Japanese forests, and a new form of Dutch elm disease originating in our Midwest is killing Europe's formerly resistant trees.

Colonizing insects and diseases have been successful in North America, as elsewhere, because native hosts have not evolved genetic resistance, and host ecosystems have not evolved checks and balances. Another reason is that in the past the United States imported little raw wood from abroad and thus "has an inadequate and fragmented program for the protection of native forests from introduced pests," according to Richard Smith, staff pathologist for the insect and disease research division of the U.S. Forest Service. "And we haven't placed a high priority on the reestablishment of tree species jeopardized by past introductions, even though the loss of these trees has had far greater impact on biodiversity than such controversial practices as clearcutting."

The most devastating of all colonizers was a fungus--carried by Asian nursery stock and discovered in the New York Botanical Garden in 1904--that by the 1950s had virtually eradicated the magnificent American chestnut (see "Tree in a Coma," American Forests, November/December 1992). Years ago I climbed on chestnut stumps in West Virginia that could have hosted a tea party, and it was then I began to dream about what had been lost.

The sad gaps in the forest left by dead chestnuts have been filled largely by oaks, and to a lesser extent by maples, hickories, and birches. Ironically, this bloom of oaks, whose acorns provide the main substitute for chestnuts, is now the culinary choice of an introduced insect, the gypsy moth. Native to Europe, the moth was brought to Massachusetts in 1869 for experiments in silk production. By 1991 the leading edge of moth invasion cut across my woodlot in Virginia. At the end of June, my hundred acres of white oaks, which the leaf-eating caterpillar stage of the moth prefers above all else, were stripped almost as bare as December.

"You can look forward to irregular outbreaks from now on," Mark Twery of the Forest Service's Northeastern Forest Experiment Station in Morgantown, West Virginia, told me. "But the first outbreak usually causes the greatest tree mortality."

Twery believes, as do most other scientists, that the moths will inevitably become naturalized wherever climate and vegetation offer hospitable range. The limits of that range are not yet clear--gypsy moths are causing problems in Colorado and Utah, and have even been found in Alaska.

In the short term, destruction of the canopy allows sunlight to flood the forest floor, stimulating berries and other foods of understory plants, but also drying and warming the soil. No one is quite sure how this affects salamanders and other ground creatures, or birds whose nests are suddenly exposed, or trout living in streams kept just cool enough by shade from overhanging branches.

In the long term, production of acorns is likely to decline--either from a reduction in the number of oaks or from recurrent defoliations that don't kill the tree but abort its acorn crop. Scientists speculate that loss of acorns has helped starve out woodrats in the northern U.S., and that bears dependent on acorns to build fat for the winter may suffer the same fate. Widespread spraying programs to suppress moth outbreaks blow their own ecological ill wind through the forest, killing much more than gypsy moths.

Like gypsy moths, the fungus that kills dogwoods operates in the West as well as the East, but unlike the slowly spreading moths, dogwood anthracnose was reported on flowering dogwoods from TABULAR DATA OMITTED Massachusetts to Alabama, and on Pacific dogwoods from British Columbia to Oregon, all in the 1980s. It is thought to be imported, although no one is certain of its origin. Laboratory studies that mimic precipitation conditions show dogwoods bathed in acid mist to be strikingly vulnerable to infection. In cool, wet places the disease is heartbreakingly lethal.

"We don't have a single dogwood left alive on our 50-acre homestead up in the mountains," said Robert Zahner, professor emeritus of forestry at Clemson University in South Carolina. "Before this epidemic, a healthy forest around here might have 100 mature dogwoods and thousands of seedlings in an acre." Throughout the dogwood's range, its white blossoms spangle the tender green of an unfurling forest in April and May. Those blossoms promise luscious red berries in autumn, repast for migrating and resident songbirds alike. More worrisome to Zahner even than loss of a major bird staple is the effect on the soil. "Flowering dogwoods accumulate calcium," he said, "raising the pH of the soil around them. This makes other minerals more available to nearby roots and also attracts earthworms, which assist organic recycling. The soil under dogwoods tends to be the richest in the forest."

Another fungus, native to Asia, collaborated with a European bark beetle (both immigrated on veneer logs from Europe) to kill almost all mature American elms. Beginning in the 1930s, Dutch elm disease swept across the eastern two-thirds of the country. Elms continue to sprout and even bear seeds in some places, but never reach the imposing stature for which they were famous. Mourned mostly for its gorgeous shape and shade, the elm played a largely unknown ecological role along creeks and rivers as well as in deciduous swamps, where it often grouped in thick stands. "Elm litter is one of the most rapidly decomposing leaf materials, and the whole set of invertebrates connected with it had to find other hosts in order to survive," said Burton Barnes of the University of Michigan, who has studied elm populations since 1973.

A different fungus thought to be introduced is also attacking the butternut tree. Once a source of sweet, fatty nuts in northeastern and midwestern forests down to Georgia, the butternut has so retreated since the 1950s that it's being considered for listing under the Endangered Species Act.

In the case of the beech tree, whose nuts are critically important for wildlife in the Northeast, native fungi that formerly had little effect are now debilitating and killing trees whose bark is perforated by the beech scale, introduced from Europe. A private forestry consultant in Vermont told me recently that she was hearing loggers talk about hurrying up the harvest of beeches before disease ruined them. This impulse, a common reaction to forest disease, culls potentially resistant trees out of the gene pool.

Another insect, the European mountain ash sawfly, seems to be killing the mountain ashes on high ridges in the Smoky Mountains, possibly abetted by air pollution. Red ash berries feed song and game birds throughout the winter.

Conifers as well as hardwoods are besieged. Fraser firs, crowning the highest mountains in the southern Appalachians, have been almost exterminated by the balsam woolly adelgid, an insect brought to Maine on nursery stock from Europe around 1908. The fir, which hosts a variety of rare bryophytes (mosses and liverworts) and at least one rare species of spider, is now a candidate for protection by the Endangered Species Act.

A related sap-sucking insect, the hemlock woolly adelgid (believed native to Asia), has been killing eastern hemlocks in Virginia for 40 years and is spreading north and south. Dense stands of hemlocks grow on shallow, steep soils near watercourses, offering a usually moist, deeply shaded habitat. White pine blister rust, an Asian fungus that was brought in on seedlings from Europe around the turn of the century, attacks most five-needle pines, including eastern and western white pines and the sugar and whitebark pines of the West. For part of its life cycle, blister rust depends on the 70 or so native species of currant and gooseberry bushes (Ribes genus) that are native to North America.

Throughout the first half of this century, a war of eradication was waged on Ribes. Old-timers where I live still tell stories of Civilian Conservation Corps crews in the 1930s pacing the woods to yank out bushes. Out West, bulldozers raked streams to get at tough Ribes roots. Animals from coyotes to gulls that utilized Ribes berries had to look elsewhere as the stock diminished drastically. Perhaps because of Ribes control, certainly because of climate and ecological factors, blister rust became truly disastrous in the East only in the great pineries of the Lake States.

In the West, too, where more varied and susceptible Ribes species grew, whole regions of pines died. Nuts and even needles and bark of pines are important wildlife food (the loss of whitebark pine nuts for grizzly bears is a particular concern). The long-lived pines that once offered the splendor of old-growth have been largely replaced by perennially pole-sized firs that succumb to root diseases by age 60, only to be replaced by more firs.

Blister rust notwithstanding, the West has not yet imported as many forest problems as has the East. Its turn may be coming, though, as patterns of international commerce change.

"Just as we feared," said Forest Service pathologist Richard Smith, "already in the 1990s several nonnative organisms have entered western ports on Siberian logs, and more logs are arriving from other Pacific rim countries."

Smith worked on a task force to identify potentially harmful insects and diseases on Siberian larch. Once such organisms are known, techniques such as heat treatment or debarking can kill them. Many species of pines and hardwoods from lumbering regions around the world--potential U.S. imports--need similar review. "We are in the process," Smith went on to say, "of improving our program for the protection of our forests from today's exotics." Although less than 15 percent of the Forest Service's research budget supports studies of all forest insects and diseases, introduced and native, Smith is optimistic: "The new science of biotechnology offers such promise for combating these exotic problems without upsetting other ecological relationships that we set up half a dozen biotech centers in the late 1980s." Half of these centers weren't fully funded in 1991, but increases in appropriations since then hint at what Smith calls "a philosophical change in public support from utilitarian to broader ecological values of forests."

One of the most heavily funded programs is gypsy-moth control. At a Forest Service biotech center in Delaware, Ohio, James Slavicek is exploring one of biotechnology's most promising aspects: enhancing organisms capable of controlling pests. Gypsy moths are susceptible to a specific virus that affects no other creatures; unlike every other pesticide, a viral spray would kill only gypsy moths.

"We're looking for molecular changes that make one viral strain especially potent, so we can engineer them into wild strains," Slavicek said. "We're also looking for cheap ways to mass-produce the virus through cell culture, so it can compete with the cost of other sprays."

Another promise offered by biotechnology is engineering disease resistance into the host tree. Paula Pijut, research physiologist at a Forest Service biotech center in St. Paul, Minnesota, began her career by inducing genetic variations in American elms by propagating tree tissue in a culture, then inoculating cells with fungus and growth hormones to assess resistance far more rapidly than was previously possible. A few years ago she started similar work on white pines. "This process of somaclonal variation has been commonly used for breeding resistant crops and ornamentals," she said, "but not many people are working on forest trees."

Included in this small group are a scattering of universities and a handful of private institutions. Perhaps the best known is the American Chestnut Foundation, a nonprofit organization that raises private funds for research. For decades the Foundation focused on traditional cross-breeding to capture the blight resistance of Chinese chestnuts in American trees, but both of the major biotechnological approaches--attacking the pest and improving host resistance--have opened new possibilities.

"A less virulent strain of fungus was found several decades ago, and we're trying to breed that hypovirulence into other strains," said William MacDonald, Foundation treasurer and forest pathologist at West Virginia University. "At the same time, molecular techniques could help us find the gene for resistance. If we could then combine even a slightly more resistant tree with a weaker fungus, we could start talking about reintroducing the American chestnut tree."

As I stand eye to eye with the innumerable gypsy-moth caterpillars I'm picking off a few favorite oaks, I think about those chestnut stumps in West Virginia, and dream about the forest that was, and could be.


In the late 1950s, English foresters began to notice that their elm trees, which had formerly been resistant to the European-born Dutch elm fungus that had practically wiped out American elms, were increasingly succumbing to the disease. The problem was most noticeable around seaports and sawmills.

English plant pathologists inspected the rock elms imported from Canada for shipbuilding and found what they initially thought was a new strain of Dutch elm disease, which they labeled "aggressive strain." After systematically sampling their countryside, the pathologists realized that this aggressive strain was spreading and killing English elms, which like the American version were much beloved for a great variety of landscaping purposes.

"When I went to England as part of a cooperative program in the 1970s, I wondered whether that new strain had originated here," said Dr. David Houston, principal plant pathologist at the U.S. Forest Service's Northeastern Forest Experiment Station in Hamden, Connecticut.

"And indeed, over years of sampling in several regions of the U.S. we found that the new strain was taking over and eliminating the older form of the fungus," Houston added. "We don't know how it originated, but we think it occurred in the Midwest, perhaps in the Illinois area, because of geographic factors of elm-stand isolation. We didn't notice the new form, because our American elm was already being devastated by the old one."

The new strain of fungus grows much more rapidly through the bark of elms, and thus contaminates many more of the bark beetles that carry the fungus to uninfected trees.

"Another new aggressive form has apparently arisen in the Near East and is moving west," Houston said. "The two appear to have met in Ireland. Europe is now thoroughly colonized by aggressive forms, and the prognosis for European elms is very bad. To date, about 90 percent of England's elms have died."

Recently, a British pathologist argued that the new strain from America should be considered a new species, and named it Ophiostoma novo-ulmi sp. nov.


American forests are under constant and increasing threat of exotic pest introductions. Since early settlers arrived here, over 2,000 exotic insects have been introduced to North America, and at least 20 percent of these are forest dwellers. Let me introduce you to two of the most serious recent unwelcome arrivals to our shores.

Asian Gypsy Moth: In 1991 the Asian strain of the gypsy moth (Lymantria dispar) was discovered in Washington, Oregon, and British Columbia. It arrived as egg masses on grain ships from eastern Siberia. Larvae that hatched when these ships docked in the Pacific Northwest were blown ashore. If this Asian strain becomes established, it could pose a threat more serious to native forest ecosystems than the European strain, because the adult females are strong flyers and could spread the range of this pest much faster than occurs with the European strain. It may also have a broader host range and could pose a threat to conifers.

Last year the U.S. and Canada spent $27 million on eradication efforts against this insect. The good news is that no Asian gypsy moths have been found in the Pacific Northwest since then. An intensive trapping and survey program this year will determine whether eradication efforts were successful and also keep watch.

Pine Shoot Beetle: Last summer another potentially serious pest was discovered. The common pine shoot beetle (Tomicus piniperda) could spell more trouble for pine forests in this country. It is native to Europe, where it is the most serious bark-beetle pest of pines. Unlike most native bark beetles, adults cause damage by feeding on shoots.

This insect was first discovered near Cleveland, and probably arrived from Europe on infested crating material on ships docking at Great Lakes ports. The Animal and Plant Health Inspection Service (APHIS) found infestations in New York, Pennsylvania, Ohio, Indiana, Michigan, and Illinois. The pine shoot beetle has been found primarily in Scotch pine Christmas-tree plantations but also in some native pines. Scientists are concerned that this pest could be very damaging, especially if it spreads to other regions such as the pine-growing areas in the South.

Much to the dismay of Christmas-tree growers, APHIS has imposed quarantines on the movement of Christmas trees, logs, and nursery stock from infested areas. APHIS will continue surveys this summer to evaluate the extent of the infestation. The Forest Service has initiated research on the pine shoot beetle at its laboratory in East Lansing, Michigan, to study the insect's life cycle and behavior in this country for use in developing treatments.

Chris Bolgiano is a freelance writer living in Fulks Run, Virginia.
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Title Annotation:Forest Health; includes related articles; insect infestation of US flora
Author:Bolgiano, Chris
Publication:American Forests
Article Type:Cover Story
Date:Jul 1, 1993
Previous Article:The President steps in.
Next Article:Life's a beech - & then you die.

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