Roadside weeds are the focus of this doctoral student's research -- specifically, the role of roads in the spread of invasive, non-native plants.

Roads, it seems, not only take people where they want or need to go, they carry and support certain botanical hitchhikers, too.

You've seen them: The brushy blades of a wild barley locally known as foxtail. The tall, coarse stalks of Italian thistle. The thorny stems of yellowstar thistle that prick so many ankles in summertime.

Looking at almost any stretch of highway, it's plain to see that weeds flourish and predominate.

This and similar observations have seeded a rising field of research called road ecology, which explores how roads alter the environment. As Americans drive more miles than ever, the subject is starting to spread beyond academia into government policy discussions.

On Monday, the UC Davis Institute of Transportation Studies is to co-host a workshop on road ecology that includes a talk by Jeff Morales, chief of the state Department of Transportation. The gathering is organized by Dan Sperling, transportation studies institute director and co-author of a new book, "Road Ecology: Science and Solutions," which is proving popular in transportation circles.

"Roads are a hot topic right now," said student Gelbard, who began seven years ago to examine the relationship between roads and invasive plant species. Working as an environmental consultant in Oregon, he surveyed noxious weeds in national forest areas.

"It became clear that noxious plants were mostly along roads, in clear-cuts and in power line corridors," he said.

While any unwanted plant may be deemed a weed, noxious weeds are those that tend to take over. These are foreign plants that haven't evolved to coexist with local flora, and for which few or no natural predators or competitors exist here.

Roads promote their spread in assorted ways:

* Seeds in dirt or mud get picked up in the cracks of tires, in radiator grilles and in the undercarriages of vehicles, later to be deposited far from the parent plant.

* Vehicles driven off the pavement disturb the soil, providing an entree for invading plants.

* The earthen material under the pavement may have come from another area, changing the soil chemistry, making it less-suited for locals and more welcoming to outsiders.

* The roadsides have their own environment. They are bathed in runoff -- water and chemicals, whether herbicides, salt for de-icing or hydrocarbons and heavy metals found in automotive fuels and fluids -- that is better tolerated by some plants than others.

* Passing traffic creates gusts that help push wind-borne seeds along.

While the seeds of any plant in theory may be distributed widely by roads, plants from Europe and Asia -- typical sources of imported weeds -- tend to do better than natives in disturbed environments, Gelbard said, because they have evolved over centuries alongside human activity.

Scientists around the world are documenting the role of roads and vehicles in disseminating plants. Richard Forman, a landscape ecologist at Harvard University and a pioneer in the study of road ecology, said one researcher in Nigeria collected mud samples from 75 vehicles, from which he managed to germinate 40 plants.

In the same vein, a researcher in Australia collected wastewater from a carwash, from which he retrieved 18,500 seedlings, which yielded 259 species of plants.

"The bulk of the seedlings were from the city and surrounding croplands and woodlands, but there were 20 species not even known from the region," said Forman, who co-authored the new road ecology book with UC Davis' Sperling.

Given the ease of world travel, is it even possible to impede the spread of exotic plants?

Many scientists and government agencies think it's important to try.

"They're displacing native plants, threatening threatened and endangered species, they upset biodiversity; some weeds are toxic to livestock and wildlife," said Dave Thomas, a U.S. Forest Service vegetation management specialist.

The agency is poised to place greater emphasis on exotic species. Forest Service Chief Dale Bosworth, in a speech at UC Berkeley last month, identified invasive species as one of his four top priorities.

Thomas said the weed issue will influence a national debate on road-building in forestland, though it will not be a deciding factor. "Invasive species will not be ... forcing roads closed," he said.

The agency will tackle the problem in other ways. For example, Thomas said, contractors entering forests will be required to do things such as set up wash stations to clean their equipment to stem the spread of seeds.

Thomas said public education is needed, too, since problem weeds can be carried in hikers' socks, shoes and pants cuffs.

Vehicles, which can go much farther than pedestrians, magnify the effect. And the better the road they travel, the greater the impact. Gelbard, working in Utah's Canyonlands National Park, found that the more improved the road, the weedier the roadside. So graded surfaces promoted weed growth more than four-wheel drive tracks, and paved roads more still.

In Davis, where Gelbard is wrapping up his doctoral work, roadsides offer plenty more fodder on the topic. On a recent drive around Yolo County, Gelbard talked about how road maintenance can unintentionally support the growth of noxious plants. He got out of the car on the broad median strip of Highway 113 to explain.

Gelbard leaned over the freshly mowed grass, rummaging in the thatch. "Here we go," he said, revealing a low-lying rosette of leaves. "Here's a yellowstar thistle."

A cousin of the dandelion, yellowstar thistle begins the season hugging the soil. Later, it bolts up, forming spindly, thorny stems bearing yellow flowers that later turn to seed. The thistle's growing pattern is such that it tends to be small when the grass is long and lush, and to bolt when the grass goes dry, Gelbard said.

Since road crews mow when the grass is long, the thistle doesn't get cut. By summer, Gelbard predicted, the median will be covered with it.

The state Department of Transportation is aware of the problem. But reducing highway fire hazards must come first, said Caltrans spokesman David Anderson. "Public safety has to be our top concern," he said.

The department hopes eventually to solve both problems by planting along highways low-growing, drought-and fire-resistant species that are stronger competitors against weeds.

Not every alien weed is a problem, and not every road brings invasive weeds. Gelbard said roads going through highly infertile soils -- such as serpentine soils found in California -- tend not to promote weeds.

Gelbard's attention these days is on the inner coast range, from St. Helena to Bear Valley. His research examines the flip side of roads -- how roadless habitats provide refuges for native grasses -- and the ways in which roads influence plant communities.

"What I'm trying to say is not just that roads are bad ..." Gelbard said. "I'm trying to take a more constructive approach. I'm trying to say, 'Under what conditions is it bad and under what conditions is it not bad?' "