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Lewis Ziska, a lanky, sandy-haired weed ecologist with the Agriculture Research Service of the , matches a dry sense of humor with tired eyes. The humor is essential to Ziska’s exploration of what global could do to mankind’s relationship with weeds; there are many days, he confesses, when his goal becomes nothing more than not ending up in a fetal position beneath his battleship gray, government-issue desk. Yet he speaks of weeds with admiration as well as apprehension, and even with hope.

It is easy to share the admiration and apprehension when you consider the site that Ziska planted with weeds in downtown Baltimore in the spring of 2002. Tucked in next to the city’s inner harbor, the site is part of a barren expanse of turf rolled out over a reclaimed industrial landscape. This unfertile scrap seems an unlikely choice for growing anything, but Ziska saw in it, ominously perhaps, a model of where the global habitat as a whole is headed.

“Ingenuity,” Ziska says, “may be the mother of invention, but poverty is definitely the father.” For some time, he had wanted to create in a laboratory setting the elevated temperatures and increased concentrations of atmospheric CO2 predicted for the mid-21st century by the Intergovernmental Panel on Climate Change, the leading international scientific authority on the subject. Carbon dioxide has received a lot of attention as a greenhouse gas, a major cause of global warming. But it is also, along with water, light and nutrients, one of the four essential resources for plant growth. The effect that boosting this gas’s concentration in the atmosphere will have on plants is very poorly understood.

The facilities for testing the effects of CO2 enrichment in Ziska’s lab on the U.S.D.A. research campus in Beltsville, Md., were limited. His best option there was a growth chamber, essentially an airtight, climate-controlled, artificially lighted aluminum box about as spacious as a walk-in closet. Ziska had something more ambitious in mind, but his budget, which has been cut repeatedly by an administration seemingly intent on minimizing attention to global climate change (his lab has been reduced to 3 researchers who study climate change and agriculture, from 10 in 1999), wouldn’t support the construction of special facilities. Then it occurred to Ziska that the complaints made by residents of nearby Baltimore about summer in their city — the exhaust-laden air and the way in which buildings and pavement soak up to create an abnormally warm “heat island” — could be put to good use. When he checked, he found that in fact the temperatures in Baltimore run 3 to 4 degrees Fahrenheit warmer on average than those of the surrounding countryside, and the concentration of CO2 in the local atmosphere (440 to 450 p.p.m., or parts per million by volume) is well above the current global average. This, coincidentally, matched almost exactly what the panel on climate change predicted for the planet as a whole 30 to 50 years in the future in its “B2 scenario,” a middle-of-the-road projection that envisions continuing greenhouse gas increases but also some success in abatement programs.

By comparing three sites — an organic farm in western Maryland, a park in a Baltimore suburb and the one by the inner harbor — Ziska planned to study three circumstances: the present (on the organic farm), the mid-century future as predicted by the climate-change panel (in Baltimore) and something in between (the suburban site). He took soil from the organic farm, which already contained seeds of 35 common weeds, and with it created uniform beds at each of the sites, urban, suburban and rural, so that the growing medium and weed population would be the same throughout. What happened over the next five growing seasons surprised even him.

Not only did the weeds grow much larger in hotter, CO2-enriched plots — a weed called lambs-quarters, or Chenopodium album, grew to an impressive 6 to 8 feet on the farm but to a frightening 10 to 12 feet in the city — but the urban, futuristic weeds also produced more pollen. Even more alarming was the way that the increased heat and CO2 accelerated and perverted the succession of species within the plots. Typically, a cleared area in the Eastern United States, if left to itself, returns to native woodland. This process varies with the site and circumstances, but in its archetypical form fast-growing annual weeds cover the soil first, playing the role of what ecologists classify as “pioneer plants.” These gradually give way to longer-lived perennial weeds, which are in turn replaced by shrubs and trees.

In the natural version of this process, the pioneers and their successors are species indigenous to the area, and the woodland’s restoration takes decades. But what Ziska observed in his urban plots was ecology on amphetamines, a nearly completed succession to trees by the end of five years, with a domination by invasive weed trees of the most troublesome sort: ailanthus, Norway maples and mulberries. Five years after the creation of the plots, the biggest ailanthus in the rural test site measured about five feet tall. The city site boasted a 20-footer. The suburban plot was following the city’s lead, though it lagged a couple of years behind.

As a scientist, Ziska was excited by his experiment’s striking outcome. As someone who has spent his career battling weeds, though, he was frightened by the implications. Weeds already cost U.S. farmers about 12 percent of their harvest, exacting an estimated annual loss of $33 billion. What would be the additional cost in the future, not only to farmers but also to foresters, land managers and gardeners, of beating back supercharged weeds? Still, even as he contemplated this, Ziska says he couldn’t repress a certain admiration. He traces his interest in weeds to an epiphany during his undergraduate years at the University of California at Riverside: noticing a weed springing up through a crack in the Southern California pavement, he was suddenly struck with wonder at any organism that could flourish in such a hot, dry, hostile environment. That may become an essential talent, it occurred to Ziska, given the way our planet is going.

Taking the long view, it becomes apparent that the events in Ziska’s plots were just another twist in the more than 10,000 years of joint history, ours and the weeds’. We have been intimately linked since Neolithic times, for in a fundamental sense weeds are a human creation. “Weed” is a subjective label applied as a matter of personal judgment, a point that becomes obvious when you consider how many “noxious weeds” — plants now marked for destruction by federal, state or county authorities — were deliberately introduced into North America by individuals convinced of their beauty or utility. The ailanthus tree, for example, currently regarded as one of the most troublesome weeds of our urban habitats, was brought from China to eastern North America in the 19th century for use as a fast-growing shade tree and is said to have been introduced into California by Chinese immigrants who valued its medicinal properties.

There are countless definitions of weeds, ranging from the hardheaded one necessarily observed by farmers, that a weed is any plant that interferes with profit, to the aesthetic (a popular gardener’s definition of a weed is “a plant out of place”), to ’s sanctimonious assertion that a weed is “a plant whose virtues have not yet been discovered.” But all agree on the central criterion: to qualify as a weed, the plant in question must be viewed with disfavor by humanity. Simply put, any plant, if we dislike it, becomes an intruder in our landscape and so a weed.

Arguably, then, there was no such thing as a weed until mankind developed the need to discriminate, which came with the development of agriculture in the Neolithic era, around 9,000 B.C. In fact, many of the wild grains like red rice or wild oats that are among our most troublesome agricultural weeds today were valued food sources until we graduated from the hunter-gatherer stage of our existence.

Much has been made of our scientific triumph in breeding modern crop plants from those wild ancestors. The transformation of an east Asian wild grass (red rice) into the crop that provides 20 percent of humanity’s caloric intake is extraordinary. What generally goes unrecognized, though, except among weed scientists, is the extent to which we also made weeds what they are.

Coexistence with mankind has given rise to the sort of tough plants that flourish despite the worst we can do — hoeing, pulling, burning and, more recently, spraying the fields with herbicidal chemicals. Weeds have adapted to every means we used to exterminate them, even turning the treatments to their own advantage. Attacking a Canada thistle (actually of Eurasian origin and a regular entry in “worst weeds of North America” lists) with hoe or plow, for example, may destroy the plant’s aboveground growth but leaves the soil full of severed bits of fleshy root, each of which may sprout a new plant.

A result of this history is that crops and weeds embody diametrically opposed genetic strategies. Over the centuries, we have deliberately bred the genetic diversity out of our crop plants. Creating crop populations composed of clones or near clones was an essential step in achieving higher yields and the sort of uniform growth that makes large-scale, mechanized cultivation and harvesting possible. Because weed populations live as opportunists, however, they must include individuals with the ability to flourish in whatever type of habitat we make available. They also need diversity to cope with the wide range of punishments we inflict. A patch of Canada thistles, if it is to survive when the farmer switches from hoeing to herbicides, must include individuals that develop a resistance to the chemicals over time. Weed populations that lacked the necessary genetic diversity faded from our fields, lawns and waste places; historians of agriculture can cite many such casualties.

The survivors are an astonishingly plastic group of plants. James Bunce, a plant physiologist with an office down the hall from Ziska’s, has been studying the effect on dandelions (that nemesis of the suburban greenskeeper) of atmospheres artificially enriched with CO2. He found in a series of trials that populations of the familiar weed evolve, changing physically to take advantage of this sort of resource enhancement, within the space of one growing season.

“When you change a resource in the environment,” Ziska said recently, sitting in his compact office, “you are going to, in effect, favor the weed over the crop. There is always going to be a weed poised genetically to benefit from almost any change.”

Ziska, together with Bunce, has been testing the effects of changing CO2 concentrations on a range of crop and weed species. Wending his way through a basement full of pumps, filters and boxlike aluminum growth chambers, Ziska showed himself to be a connoisseur of atmospheres. Peering at the instrument panel outside one growth chamber, he noted a CO2 concentration of 310 p.p.m. “That’s a 1957 atmosphere, the year of my birth,” he said. What he and his colleagues have found, he said, is that weeds benefit far more than crop plants from the changes in CO2 and that the implications of this for agriculture and public health are grave.

Tests with common agricultural weeds like Canada thistle and quack grass found them more resistant to herbicides when grown in higher concentrations of CO2, making them harder to control. Ziska hypothesizes that this may be a result of faster growth; the weeds mature more rapidly, leaving behind more quickly the seedling stage during which they are most vulnerable. This promises to be an expensive problem for farmers, who will have to spend more on chemicals and other anti-weed measures to protect their crops. (Herbicides already cost farmers more than $10 billion annually worldwide.)

But enhancing CO2 levels, Ziska has found, not only augments the growth rate of many common weeds, increasing their size and bulk; it also changes their chemical composition. When he grew ragweed plants in an atmosphere with 600 p.p.m. of CO2 (the level projected for the end of this century in that same climate-change panel “B2 scenario”), they produced twice as much pollen as plants grown in an atmosphere with 370 p.p.m. (the ambient level in the year 1998). This is bad news for allergy sufferers, especially since the pollen harvested from the CO2-enriched chamber proved far richer in the protein that causes the allergic reaction. Poison ivy has also demonstrated not only more vigorous growth at higher levels of CO2 but also a more virulent form of urushiol, the oil in its tissue that provokes a rash.

According to Ziska, the steady increase in atmospheric CO2 since the beginning of the Industrial Revolution may have already had a major impact on the growth of at least one supremely costly weed. Cheatgrass (Bromus tectorum), a native of central Asia, is believed to have been introduced into the United States accidentally, as seeds in soil used to ballast ships or as a contaminant in agricultural seed, in the mid-1800s. Since then, its ability to flourish in dry habitats and its prolific seed production (a single plant can bear as many as 5,000 seeds) has helped it to overrun 100 million acres of Western rangeland, an area larger than the state of Wyoming. In doing so, cheatgrass has displaced more nutritious native grasses, reducing the quantity of livestock a given acreage can support. Cheatgrass has also diminished the land’s value to wildlife, which also finds the introduced plant unpalatable.

The spread of cheatgrass has been widely attributed to the degradation of native grasslands by overgrazing — cattle prefer and selectively eat the native grasses — and more especially to its exceptional combustibility. Periodic fires are an integral part of the rangeland ecology, but when the rangeland is still dominated by native grasses, fires occur in some areas at average intervals of every 60 to 110 years. In areas overrun by cheatgrass, however, fire sweeps through every three to five years. While cheatgrass can tolerate such frequent burns, the native flora cannot.

Cheatgrass’s combustibility is inherent in the plant’s pattern of growth. Sprouting in the fall, it resumes growth at winter’s end to mature and set seed in early summer, whereupon the plant dies, leaving a tuft of dry, highly flammable leaves through the following dry season. Ziska and his colleagues discovered, though, that the weed’s flammability seems to have been greatly augmented by the increases in atmospheric CO2 that occurred during the period of cheatgrass’s spread through the West.

The scientists grew the plant at four concentrations of CO2: at 270 p.p.m. (the ambient level at the beginning of the 19th century, before the Industrial Revolution), at 320 p.p.m. (a 1960s level), 370 p.p.m. (a 1990s level) and 420 p.p.m. (the approximate level predicted for 2020 in all the climate-change panel’s estimates). What they found was that an increase of CO2 equivalent to that occurring from 1800 until today raised the total mass of material (the biomass) each cheatgrass plant produced by almost 70 percent. In addition, the composition of the cheatgrass changed as the CO2 level increased, the tissues becoming more carbon-rich so that the plant leaves and stems are less susceptible to decay. In a natural setting, this would mean that the dead material would persist longer, adding yet more fuel for wildfire.

More fuel, with a longer life — Ziska says that the rise in greenhouse gases we have already achieved may have played a decisive role in the spread of a weed that has already transformed the ecology of the Western United States. The situation seems likely to worsen too. The cheatgrass that Ziska grew at the CO2 level equal to that projected for 2020 increased the plant’s biomass by another 18 percent above current levels. Global climate change, it seems, will further stoke the rangeland wildfires.

There’s no such thing as natural selection,” Ziska confides. He is not, he hastens to explain, a creationist. He is merely pointing out that the original 19th-century view of evolution, the one presented by and Alfred Wallace, is obsolete. Their model presented evolution as a process taking place in a nature independent of human interference. That is almost never the situation today — even at sea, where less than 4 percent of the oceans remain unaffected by human activity, according to a recent article in the journal Science. This interference with nature has set the stage for the success of a growing category of weeds, one exemplified by cheatgrass: invasive plant species.

These are plants that evolved outside a local or regional ecosystem but were at some point released into it, typically by human action. Some invasives, like cheatgrass, arrived as hitchhikers and stowaways; others, like kudzu, were introduced deliberately. (A Japanese species, kudzu was planted by state and federal agencies to control soil erosion throughout the Southern states in the 1930s and ’40s.) In any case, the invasive plant species share a quality of aggressive, explosive growth in their new homes and the ability to outcompete the native vegetation of forests, grasslands and wetlands — areas that we are accustomed to think of as outside the sphere of human influence.

Popular opinion has treated the invasive plants as botanical illegal aliens. The has labeled them as the second-greatest threat to the continent’s biodiversity, exceeded in their impact only by outright destruction of habitat. Major resources have been devoted to the spraying and rooting-out of invasive plants in the belief that their removal would enable an ecological revival. Roughly $45 million, for example, is spent every year in the unsuccessful attempt to stop the spread of a single European wetland weed, purple loosestrife (Lythrum salicaria).

New research, however, suggests that invasive species, at least in some instances, aren’t so much the causes of environmental degradation as eco-opportunists taking advantage of disturbed habitats. Or, as the biologist Andrew MacDougall of the University of Guelph, Ontario, puts it, the invasives may behave more as “passengers” than as “drivers.” This is the conclusion he reached in a pair of studies, one of an oak savanna in British Columbia and the other of degraded prairie in southwestern Saskatchewan.

MacDougall had not intended to focus on invasive plants when he began studying a Canada property on Vancouver Island. An 86 acre remnant of oak-studded grassland, this sanctuary exemplified a type of open savanna habitat that was once common in the area but that was nearly eliminated by agriculture and sprawl. MacDougall’s original interest was in the native flora; this Nature Conservancy sanctuary is a biodiversity hot spot, hosting more than 100 species of plants and animals at risk in British Columbia or nationally.

Despite this land’s protected status, MacDougall found that the native plant community was failing, the rarities becoming rarer. The young ecologist blamed an invasion by several foreign grasses for this decline. Initially, he supposed that simply removing the foreigners would prompt a renaissance of the native grasses and wildflowers.

The actual response was quite different. For three years MacDougall removed the invasive grasses from plots he outlined within the reserve. In some plots, he did this by mowing or burning; in others, he removed the weeds entirely. Yet the native flora didn’t rebound significantly. In some cases, the decline of the native plant species instead accelerated, and the fundamental character of the flora within the plots began to change, with woody plants encroaching on the formerly open, grassy areas.

MacDougall concluded that rather than serving as drivers of change, the foreign grasses were functioning more in the role of passengers, merely filling in as the natives disappeared. In fact, the foreigners seemed to be serving a stabilizing role. By blocking light from reaching the soil, they inhibited the germination of tree and shrub seeds. Keeping the brush at bay in this fashion preserved the open character of the savanna habitat so that the remnants of the original savanna wildflowers, grasses and wildlife could at least survive. In light of these findings, MacDougall says, he came to believe that the primary cause of the native flora’s decline was human intervention. Before European settlement, fire periodically cleansed the soil surface of dead plant material. Suppression of fire since settlement had allowed a thick layer of litter to accumulate, and the foreign grasses cope better with this than do the natives.

The relevance of this discovery to an era of global climate change has become apparent in MacDougall’s subsequent research in the Saskatchewan prairie. These grasslands were infiltrated with crested wheatgrass, a species from the Eurasian steppe. Again, the foreign grass was blamed as the driver in the decline of the native flora. MacDougall, however, says he believes the invader’s success is largely derived from climatic change over the last half-century. Weather records reveal that spring warmth in this semiarid region is coming earlier than it used to, and the season’s rain is more consistent. The wheatgrass, which awakens from winter dormancy earlier than the native grass species, has gained a competitive advantage from this change.

MacDougall says he believes that a North American grass species could be found that could compete successfully in the altered climate and would also (unlike the exotic) interact beneficially with native wildlife. He admits, though, that replanting this prairie would be a big endeavor, that it would require as much effort as the 19th-century pioneers gave to taming the prairie habitat. Whether the will and resources exist for this seems questionable, especially as habitat disturbance spreads around the globe, creating many similar situations.

MacDougall says he is hopeful that the climatic changes projected for this century won’t exceed the tolerance of most native plant species. He admits, though, that the spread of the exotics suggests that they are more genetically diverse and thus better able to cope with environmental change. MacDougall clearly doesn’t like the prospect, but he admits he can imagine a future so generally disturbed that we may well be grateful for what he calls the “positive services” — the aggressive adaptability — of the botanical aliens.

It was a Tuesday in early January, but the temperature in center city Philadelphia had reached 65 degrees, and rosettes of dandelion leaves were starting to sprout flower buds in the neat bed of mulch outside the Sheraton Society Hill hotel. Inside, in a meeting room set up with chairs, screen and PowerPoint projector, the membership of the Northeastern Weed Science Society was equally disturbed. These are, by necessity, conservative people. A mixture of university researchers, county agents and representatives of the herbicide industry, the attendees had the look of farmers or foresters temporarily off their land — clean-cut, tanned, tending toward the wiry. Most looked distinctly uncomfortable in crisp sport jackets and polyester blazers that, you suspected, had spent the 12 months since the last annual meeting in a closet. If the members looked like farmers, that was because it is farmers they serve, and they had clearly absorbed the wary ethic of that profession in which sudden change, whether of weather, markets or government policies, is almost always for the worse.

The day’s news surely confirmed that prejudice. The second day of this year’s annual meeting was devoted to a symposium on weeds and global climate change, and the speakers were outlining a future in which many of the members’ current strategies will be irrelevant or ineffective.

The keynote speaker, Cameron Wake of the ’s Climate Change Research Center, did little to put the audience at ease. Wake is a charismatic man who has traveled the colder regions of the world — the Canadian arctic, the Greenland ice sheet, Antarctica and the high mountains of Central Asia. On these trips, he collects ice cores, whose analysis enables him to reconstruct histories of past atmospheric and climatic changes. His soul patch, pink shirt and pink tie made him a minority of one in this room. He dealt firmly with an audience member who asserted that the climatic warming is nothing new, that records from imperial Rome indicate that citrus and other warm-weather crops were then far to the north of their current ranges. Wake pointed out that local archaeology can’t change the global data set, which proves that the level of CO2 in the atmosphere is at its highest point in more than 650,000 years and that the rate of increase is accelerating.

Subsequent speakers got down to cases. Andrew McDonald, an agricultural scientist at , had used the Intergovernmental Panel on Climate Change’s high projections for CO2 levels at the middle and end of the century to create an atlas of potential weed migrations in cornfields in the Eastern United States. If these projections prove accurate, Kentucky, by the end of the next one to three decades, should have a climate (and weed flora) resembling that of present-day North Carolina; by century’s end, it will have shifted to a regime more like that of Louisiana. Delaware, over the same period, will be transformed to something first like North Carolina and then Georgia, while Pennsylvania will metamorphose into West Virginia and then North Carolina. Florida will become something unprecedented in this country. Field observations indicate that these transformations are already under way: another speaker pointed out that kudzu, “the weed that ate the South,” has already migrated up to central Illinois and by 2015 could be extending its tendrils into Michigan’s Upper Peninsula.

Even more sobering were the figures that the biologist Brent Helliker of the flashed on the screen. First, he used maps taken from an ecology textbook to show the way the last ice age drove various forest types southward. Then came a map Helliker created, suggesting that the current warming seems most likely to change the ranges to which forest trees are adapted — the areas where the black spruce, for example, grows now, are likely to become better suited to broadleaf trees. He asked the question that was on the lips of every one of his listeners: Can the forest adapt so drastically in a space of just decades? Helliker announced that he had no answer to that question, and his talk was over.

During a break between talks, Lewis Ziska was surprisingly upbeat. With the challenges, he insisted, come opportunities. Kudzu, for instance: Ziska has been seeking financing to study its potential as a source of biofuel. Kudzu roots, as much as 50 percent starch by weight, seem ideal for ethanol production, while the plant’s supercharged vines, which can grow a foot a day, would be an abundant source of alternative energy. This would be win-win: we develop an alternative to fossil fuels and, at the same time, create a financial incentive to root out a particularly troublesome weed.

Developing techniques for managing weeds in a time of global climate change will be essential to the world’s agricultural future, and the U.S.D.A. researchers, though they have been starved of essential financing, lead the world in this field. (There is one exception, Ziska admits; his Web searches have revealed that marijuana growers have an amazingly detailed knowledge of how CO2 enrichment affects their crop. But as Ziska points out, they don’t publish in scientific journals.) Possession of this expertise could be a great economic asset to the United States, both for the protection it could provide to our own harvests and as an intellectual export that is sure to be much in demand in other countries.

Ziska says that he worries about mankind’s ability to feed itself in a fast-changing future. Paradoxically, it is weeds, he says, that can provide solutions. They have helped us deal with lesser crises in the past. When diseases and pests overwhelmed our domesticated food crops, it was to their wild relatives — plants that mankind has been battling for millennia — that plant breeders turned. Because weeds have more diverse genomes, it is easier to find one with the proper genetic resistance to a given threat — and then to create a new hybrid by breeding it with existing crops. An answer to the Irish potato blight of 1845-6 was eventually found among the potato’s wild and weedy relatives; a wild oat found in Israel in the 1960s helped spawn a more robust, disease-resistant strain of domesticated oats.

Weedy ancestors of our food crops, Ziska predicts, will cope far better with coming climatic changes than their domesticated descendants. Coping, after all, is what weeds have always done best. As last year’s climate- change panel report, Climate Change 2007, made clear, we have already set in motion far-reaching and unstoppable changes in regional temperatures and precipitation and in the composition of our atmosphere. No matter what actions we take, these changes will continue for decades. If we are to avoid disaster, experts agree, we will need to be tenacious but flexible, ready to identify and exploit any opportunity in what will be a challenging, even hostile situation. In this new world that we have made, weeds, our old adversaries, could be not only tools but mentors. At which point, if Ralph Waldo Emerson is to be believed, weeds by definition will cease to exist.

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