On a mountainside overlooking Red Butte Canyon, Barbara Bentley points out a dense swath of sweet clover, so peppered with tiny yellow flowers that it looked like a single yellow blaze. In a way it is a wildfire - the fire of a conquering invader.

How many species of plants are in that patch? She asks, then answers her own question: "Maybe one."While no other type of plant grows inside the swath, just outside it is a profusion of native grasses. The fast-spreading clover moves into new regions, grows thickly, blocks the light and kills off the natives.

This worries Bentley, director of environmental programs for Red Butte Garden and Arboretum. The garden's visitor center is just below the trail where she stood last week while on a walk to show examples of her favorite plant, the lupine, indigenous to Utah and possibly threatened by the invader.

A California native, for 20 years Bentley taught at the State University of New York at Stony Brook. She retired as a professor two years ago and came to Utah along with her family. A research botanist with the University of Utah department of biology who teaches field botany, next year Bentley will launch an upper-division course about biology and environmental issues.

She is concerned not only about the lupines that she loves but about the ecology of the whole world.

Bentley is finishing a study at Duke University on the ways that lupines react to different levels of carbon dioxide. Man's burning of fossil fuels is believed to be responsible for a steadily rising level of carbon dioxide in the air, and her study could help pinpoint the consequences.

In her seven-year study, funded by the National Science Foundation, Bentley has been looking at the way different carbon dioxide levels affect photosynthesis in lupines. Through photosynthesis, plants convert air, nutrients and sunlight into sugar, starch and protein. While this is happening, microscopic chlorophyll "factories" in their leaves take in carbon dioxide and exhale oxygen.

"Carbon dioxide is the starting point for photosynthesis," she said. The implications of changes in the CO2 level are heavy, since "every green plant on this Earth is affected."

In a lab at Duke in Durham, N.C., she has been growing lupines at three levels of carbon dioxide: 350, 500, 650 parts of the gas per million parts of air.

The three sets represent, first, today's level of CO2 in the atmosphere; then the level to which the gas will rise in 25 or 50 years if mankind continues to burn fossil fuel at the same rate; and finally the level in 75 or 100 years "if we don't have the political will to reduce fossil fuel use," she says.

She chose lupines because they are legumes, the type of plants (such as beans) that have nitrogen-fixing bacteria that live in nodules in their roots. Unlike beans, they produce a poison to ward off insects, and that's important to her study, too.

The bacteria take nitrogen from the air and convert it into nitrogen fertilizer. This can benefit an entire community as the plants drop leaves that decay and enrich the soil with nitrogen.

"You can see patches of high nitrogen soil underneath lupine plants," she says.

After seven years, this phase of her project - which is sponsored by the National Science Foundation - is about to end. She has arrived at some conclusions:

- When carbon dioxide hits 500 ppm, lupines grow far better than with today's atmospheric levels. "They're almost blue, they're so green. They have just incredible amounts of chlorophyll. They have thicker leaves, bigger leaves."

More CO2 allows them to step up photosynthesis. They were able to make more sugar and distribute it to the bacteria in their roots. Better-fed bacteria work harder fixing nitrogen. The plant uses this nitrogen to make alkaloids, poisonous chemicals in its leaves that deter insects.

"They become more toxic and less tasty," adds Bentley, who has actually tasted some of the poison produced by lupines. The higher level of alkaloids is better at preventing insect damage. "From the lupine's perspective, global change's just wonderful."

- At 650 ppm, the trend has reversed. Production has dropped back to its present levels. "The whole system breaks down."

High levels of carbon dioxide stimulate the plant to make so much sugar and starch that it can't move all this food quickly enough to the roots. Starch granules build up in storage structures in the leaves called chloroplasts. Eventually, the granules physically break down the chloroplasts.

What would happen if the air got even more carbon dioxide? She doesn't know. The answer requires more research.

But even at 500 ppm, what's good for lupines isn't necessarily good for people.

Most plants that don't house bacteria that make their own fertilizer, the way legumes do. They must get nitrogen from the soil. If they are stimulated to greater growth because of increased carbon dioxide, they use up more of the soil nitrogen.

"Even though photosynthesis rates go up, in general for plants the percent of nitrogen goes down in the plant tissues," she said.

If the world's carbon dioxide rises to 500 ppm, "the yield of wheat will go up - we'll get more pounds of wheat per acre," Bentley said.

"But the quality of wheat will go down because it's lower in protein." Wheat needs nitrogen to make protein, and nitrogen is leached out of the soil by the stimulated growth.

Closer to Red Butte, if CO2 rises dramatically, perhaps alien species like sweet clover will thrive. Sweet clover is also a legume, able to make its own nitrogen, and it is extremely aggressive. If soil is depleted of nitrogen, it can move right in.

Already, fields of sweet clover glow like a yellow haze on distant mountainsides.

"It means that we may very well have shifts in relative abundance of different plants," she says. "Are we creating conditions here that would allow this invasion to go faster?"

Scientists from federal agencies and universities throughout the West will meet in November to discuss a possible baseline study to track changes in plant communities as global warming continues. The same areas would undergo intense plant population surveys now, in 10 years, in 20 years.

Bentley says Red Butte Canyon would be a perfect place for one of the studies.

The canyon, only a few minutes from downtown Salt Lake City, has been off-limits to livestock grazing since Fort Douglas acquired it in the 1860s. After it was handed over to the University of Utah and the U.S. Forest Service a few years ago, it has been managed as a research natural area to preserve its pristine environment.

"This is the last remaining ungrazed watershed in the Intermountain West," she said. With no impacts from ranching, it would serve as a good control for the overall studies.

Meanwhile, the sweet clover continues to march across the foothills.

"And this will invade," Bentley says, swinging her arms to show how the patch will spread into adjacent native plants. "The lupines could be out-competed."