Forests in the Gas

If you're a plant, carbon dioxide is your favorite greenhouse gas. And you'll get a lot more of it in the future. But is twice as much too much of a good thing?

By Yvonne BaskinOct 1, 1994 5:00 AM


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In the shimmering heat of the North Carolina summer, Boyd Strain climbs a tower of steel rising 60 feet above a patch of forest. A third of the way up, his feet clear the spires of the young pines growing around the tower. Strain, a plant ecologist at Duke University, stops to survey the scene around him. He has emerged into a sort of time warp where he can peer into both past and future.

Below him he can see the loblolly pines and sweet gums that sprang up after the area was clear-cut 12 years ago. These young trees are helping him catch a glimpse of the future. A few hundred yards away, just beyond the young pines, looms the great hardwood forest of the past. Huge 180-foot red oaks stand beside ancient hickories that probably sprouted well before the Civil War. These trees are some of the oldest in North Carolina's Duke Forest. Nineteenth-century cotton and tobacco farmers felled nearly all the ancient forests of the Southeast, leaving only pockets of trees like these on land too swampy or rocky to plow. Sooner or later a hurricane or tornado will topple these old forest giants too, and then the cycle of renewal will begin again.

But will the new forest resemble the old one? Typically, weeds, wildflowers, and sedges are the first to take advantage of a new sunlit clearing, followed by an invasion of quick-growing trees--loblollies and sweet gums in this case. These plants would normally be succeeded and overshadowed by the slower-maturing hardwoods, like oaks, that traditionally dominated the Southeast. But the next time the forest regrows, the past may be no guide to the future. That's because humans are changing the composition of the air, tampering with the very gas that fuels and nurtures plants. No one can tell how forests will fare in this atmosphere of our creation--but the young plants shooting up around the tower may give Strain a preview.

The crux of the problem is this: When Duke's hardwood forest started to come back from the last blowdown more than 200 years ago, the atmosphere contained only about 280 parts per million (ppm) of carbon dioxide (CO2). Human industry has rapidly changed that. By 1993 our habit of burning carbon-rich fossil fuels to run power plants and cars had pushed the global average to 355 ppm and climbing. By the time the young forest under the tower reaches middle age in 2050--when oaks and other hardwoods should start shading out the pines--CO2 concentrations will be pushing 700 ppm. In other words, in 60 years or so CO2 levels will be double what they are now.

It's not yet clear which species will gain a competitive edge from that extra breath of carbon. But the fledgling forest around Strain's watchtower is an answer in progress. A persistent roar emanates from the forest, like the sound of a giant blowtorch. It comes from a circle of 16 pylons surrounding the pines and sweet gums, each pylon supporting two vertical tubes perforated with a series of half-inch holes. Put a hand to one of these holes and you can feel a cool jet of invisible, odorless CO2. As the gas spews from the tubes, it propels the young forest--with its loblollies and sweet gums, its weedy undergrowth of dogbane, honeysuckle, wild ginger, and liverwort--into the double-CO2 world of the mid-twenty- first century.

The experiment is called Free Air CO2 Enrichment (FACE), and it is the most ambitious attempt yet to preview the manner in which an entire ecosystem, with all its competing plants, invasive weeds, soil microbes, and insect pests and pollinators, will respond to levels of CO2 that no organism alive today has ever known. This is not to say that Earth hasn't experienced high CO2 before; 135 million years ago, when dinosaurs still roamed the planet, CO2 levels were in the thousands of parts per million. But CO2 has been nowhere near those levels for ages. The last time global CO2 reached current levels was more than a million years ago, long before humans appeared. It hasn't climbed to 700 ppm for some 100 million years.

The goal of the current experiment, Strain explains, is to fast- forward the forest into that 700-ppm future: "What does a woods that has grown in high CO2 look like? We'd like to know--but instead of waiting a lifetime, we'd like to know in ten years." In truth, the experiment ought to run for decades, but by the middle of the next century it will already be too late--Mother Nature herself will be doing the double-CO2 experiment worldwide. As head of a task force for the International Geosphere- Biosphere Program, Strain is pushing hard for FACE fumigation studies in all the world's major biomes, including grasslands, tundra, shrub lands, and boreal and tropical forests. Two other FACE projects are already under way--one in an Arizona wheat field, and one in a Swiss meadow. Strain hopes we can learn enough to prepare for and perhaps mitigate the change in the atmosphere.

That the atmosphere will change is now a foregone conclusion. You can still debate just how much global warming will result from rising CO2-- plenty of experts do--but there's no doubt whatsoever that the gas itself is building up. Even stabilizing carbon emissions at present levels would only slow the rate of CO2 buildup, not reverse it. Nevertheless, it has taken Strain and like-minded ecologists two decades of research to persuade federal policymakers that the atmospheric shift will directly alter familiar ecosystems, even without attendant changes in global temperature, rainfall, and other weather patterns.

Strain, upbeat and soft-spoken, sometimes betrays frustration as he describes how little we can predict about our future high-CO2 landscapes. But he is hardly a prophet of doom. Carbon dioxide, after all, is the fuel plants use for photosynthesis. Using sunlight as their energy, plants combine CO2 from the air with water from the soil to make carbohydrates, the basic units of Earth's food chain. Rising CO2, to put it simply, is free fertilizer. Horticulturalists have been piping it into their greenhouses for a century to produce better roses and tomatoes. In fact, Strain believes that 10 to 15 percent of the bumper-crop yields ascribed to the green revolution of the fifties and sixties might actually be due more to CO2 than to clever plant breeding. Since then hundreds of experiments with wheat, soybeans, oranges, sugar beets, and other crops have shown that doubling the gas raises yields by an average of 30 to 40 percent. So the rosiest scenario is a lush world where farms flourish and plants grow exuberantly, consuming all the extra carbon we produce.

But a completely free lunch is hard for ecologists to swallow, and with good reason. Those bumper yields occur when crops are not only lavished with fertilizers and water but also protected by herbicides and pesticides from weeds,insects, and disease. The Third World can't afford this sort of chemical-intensive farming, and the developed world is ever more troubled by it. Besides, says Strain, CO2 experiments with crops--most of which are conducted in labs or enclosed spaces--tell us next to nothing about the future of forests and rangelands, where plants must compete with one another for light, water, and nutrients.

Of course, outdoor experiments such as the one at Duke are much harder to do. It's no mean feat to pipe CO2 into a plot of forest that grows higher by the year--at the experiment's start last summer, the 12- year-old loblollies were already 20 feet tall, and they're shooting up at the rate of 3 feet a year. To keep up with the trees, George Hendrey, an ecologist at New York's Brookhaven National Laboratory and the original designer of the FACE system, had to hang the gas-dispensing tubes from tall pylons whose height can be extended as needed.

Then there's the challenge of maintaining a constant artificial atmosphere in the open air. Hendrey's system is programmed to release gas only from the upwind tubes, relying on the wind to carry a curtain of CO2- rich air across the plot. A computer controls the flow of CO2 from a refrigerated tank through a series of pipes to the gas dispensers on the pylons. The computer adjusts gas levels as needed, based on continuous feedback from 70 gas sensors dispersed among the trees. To demonstrate, John Nagy, a physicist on Hendrey's team, picks up a sensor--a white plastic cup dangling from a coil connected to an infrared gas analyzer. "Your breath consists of 40,000 ppm of CO2," Nagy notes, as he exhales quickly into the cup. Fifteen seconds later, the analyzer's digital readout flashes to 1,000--and then zooms off the scale.

If all goes well with this system, Hendrey and Strain plan to build six more in Duke Forest, including some in freshly razed woodlands. "The time we're going to see the most dramatic changes," predicts Strain, "is when a catastrophic event--a fire, a hurricane, or clear-cutting--wipes out an entire ecosystem. As the forest recovers, we'll see plants that do well in CO2 get their chance to take over more land and claim more resources."

Such experiments could provide some sorely needed answers. Most important, they could help us calculate just how much of our profligate carbon emissions the biosphere can really consume. Forests act as a carbon sink. They perform two-thirds of terrestrial photosynthesis and account for 90 percent of the carbon locked up in land plants. But even this amount of photosynthesis may not suffice, according to some estimates. One 1989 study concluded that if no further deforestation occurs, the world would still need a new forest twice the size of Europe to completely mop up current levels of human-produced carbon emissions.

For the time being, though, most of what's known about plant responses to elevated CO2 comes from short-term experiments performed in artificial settings. At Duke's campus, five miles from the forest plot, Strain shows off one of several greenhouse rooms at the university's plant- growing laboratory, whimsically named the Phytotron. To temper the fierce summer sun, cool water is sprayed over the room's glass roof, sending ripples of shadow across the plants inside. Here hundreds of potted loblolly and ponderosa pine seedlings (two of the most important timber trees in the United States) are consuming air enriched to 700 ppm of CO2. At the Phytotron's cavernous center, Strain points out rows of aluminum growth chambers that look like walk-in freezers. Inside one, someone is growing cotton grass from the Alaskan tundra. In another, desert creosote and mesquite are breathing extra CO2. The creosote is turning yellow, a sign any home gardener would recognize as a plea for nitrogen.

The take-home message from such experiments with natural vegetation is much the same as the one for crops: high CO2 often revs up photosynthesis and causes faster or lusher growth in plants--at least for a time. The catch is that for many plants the gains are short-term: photosynthesis later levels off or even drops.

The plants most likely to do well in this CO2-rich atmosphere are C3 plants--so-called because the first step in their photosynthetic process turns out a three-carbon molecule. This is the original and more common style of photosynthesis, which evolved more than 600 million years ago when atmospheric CO2 was extremely high. At today's CO2 concentrations, this carbon-capturing mechanism is somewhat inefficient, with plants losing as much as half the carbon they garner. Raise the CO2 back up again, however, and the carbon loss is all but eliminated. C3 plants include key crops such as wheat, rice, and beans and virtually all trees.

C4 plants, on the other hand, have a more efficient form of photosynthesis that creates a four-carbon molecule in the first stage and prevents carbon loss. This type of photosynthesis evolved later, probably at the end of the Cretaceous Period (some 65 million years ago), when CO2 began to drop. These plants are all but oblivious to increases in atmospheric CO2. Though fewer in number, they too include important crops-- corn, sorghum, sugarcane, pineapples--as well as prairie and savanna grasses and many shrubs.

Theoretically, then, C3s should do better than C4s when carbon dioxide rises, but it's not that simple. Even when plants respond by revving up photosynthesis, a lack of some other factor--water, sunlight, nitrogen and phosphorus in the soil--can throw a wrench in the assembly line. Take nitrogen, for example, which plants need to make protein to build more roots, stems, and leaves. Without adequate nitrogen, a plant can't use the carbohydrate it makes to stoke the building process. Instead the plant may stockpile carbohydrate as starch grains, clogging its photosynthetic machinery and turning its leaves a chlorotic yellow like those of the creosote plant in the Phytotron chamber. Even if the plant keeps growing, starch deposits raise the ratio of carbon to nitrogen in plant leaves, reducing their protein content and nutritional quality.

These effects will radiate up and down the food chain. For example, plants that are poor in nutrients may force herbivores to eat more to get the same level of protein. This indeed was the result when Strain and his colleagues fed soybean leaves to pests called soybean loopers: when leaves were grown in double CO2, they found, the creatures gobbled 80 percent more of them. No one has yet tried the experiment with cattle and the grasses they graze on, much less with vegetables bound for human tables.

Even microbes in the soil could be affected by the atmospheric change. Some ecologists believe the microbes will decompose organic matter less efficiently, returning less nitrogen and other nutrients to the soil. But some of their colleagues think the soil's fertility might actually increase. They point out that plants can respond to high CO2 by growing more roots, and all roots ooze organic acids into the earth. These organic acids eat away at bits of sand and rock and make them give up their enriching minerals to the soil.

Studies so far provide only the most general clues to how all these interacting factors will play out in the natural world. In earlier experiments in the Phytotron, for instance, Strain's sweet-gum seedlings did better in high CO2 than his loblolly pines did, thus raising the possibility that the pines might be outcompeted in the future. But seedlings in pots say little about forest dynamics and future landscapes. And the few field studies done in natural vegetation have shown that different environments produce quite different results.

One experiment in Maryland's Chesapeake Bay has been tracking C3 sedges and C4 cordgrass growing in a coastal salt marsh, an environment swimming in plant nutrients. These grasses have been enclosed in open-top chambers and exposed to double CO2 since 1987. In this nutrient-rich setting, the C3 sedges have responded with increased photosynthesis and growth. In contrast, the C4 cordgrass has shown little response to the increased gas and seems to be losing ground to the sedges.

Another field study in the eighties piped high CO2 into greenhouses erected over the Arctic tundra near Prudhoe Bay, Alaska--a nutrient-poor environment. In this case, high CO2 slightly boosted carbon uptake in the tundra's C3 cotton grass and shrubs during the first growing season, but the response faltered completely by the third year.

Shifts in strategy and species dominance should show up much more clearly in long-term studies, such as the one at Duke, that examine whole ecosystems. Some species, for example, may opt to use their extra carbon to grow more roots to absorb more nutrients. Others will invest in a denser canopy or taller growth that shades out competitors; taller plants, however, might be toppled more easily by wind. Some annual plants may extend their life cycles and be killed by frost before they seed; others might flower earlier and fall out of sync with insect pollinators. Experiments in natural settings may also provide a chance to monitor more capricious natural checks on growth. California chaparral, for instance, seems to grow more prolifically at higher CO2--but the extra biomass may simply fuel more frequent brush fires.

Soaring CO2, of course, is also expected to disrupt climate, entailing further environmental stress and change. Predictions range from shifts in temperature and rainfall to more hurricanes, floods, and other natural disasters. Frequent upheavals favor the appearance of plants early in the regrowth cycles of woods and other ecosystems, giving fast-growing opportunistic plants the edge over slower-maturing ones. One category that usually benefits from disturbance is invasive weeds. Most likely C3 weeds will be the biggest winners. Strain's previous Phytotron studies show that the weedy vegetable okra may become a bigger nuisance in cotton fields; cheatgrass will most likely edge out native grasses on the western range; pondweeds like hydrilla and water hyacinth could clog wetlands; and, if warmer temperatures prevail, kudzu and Japanese honeysuckle vines could blanket the landscape from the Deep South to the Great Lakes.

"The net result," says Strain, "could be ecosystem simplification and loss of diversity, with increasing dominance of weedy types and aggressive C3 plants." Other ecologists have even predicted that ecosystems might turn over too quickly to allow slow-maturing "climax" vegetation-- such as Duke's oaks and hickories--to gain a proper foothold.

Thus the gas now spewing into that fumigated patch in Duke Forest could reveal a future community dominated by sweet gums, kudzu, and honeysuckle vines, where even pine trees have a hard time competing.

"People say why worry, because ecosystems have always changed," Strain says. "But it's the rate of change that's the troublesome thing. Can natural ecosystems make corrections that fast and still continue to function?

"I suppose we could just tailor the plants we grow to suit the prevailing atmosphere," he goes on. "But we have to ask if relandscaping the entire globe is an option. That would assume we know what we're doing-- that we can engineer the world to suit ourselves."

True, in our gardens and parks, farms and timber plantations, we already select our favorite plants. We pamper and protect them; we even breed hardier varieties to suit the landscapes we create. But, Strain adds, "we don't have enough energy to garden the whole Earth. There's a good part of the Earth that takes care of itself, and we'd like that to continue."

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