Species and Sprawl: A Road Runs Through It

Species and Sprawl: Yellow Starthistle

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After taking root, invasive weeds can swiftly populate the roadside strip. For example, Gelbard analyzed 1,300 sq km of grassland near Napa County, California, and discovered yellow starthistle in 73 percent of areas 10 m from roads, but in only 21 percent of areas further than 1,000 m from roads. Starthistle disperses easily along roadsides by lofting on the high-speed, turbulent wind tunnels generated in road corridors by passing vehicles, or by riding vehicles themselves. After one Montana researcher drove through several feet of knapweed, a starthistle cousin, the car picked up 2,000 seeds and deposited all but 200 of them within 16 km.

The less improved a road, the less susceptible its edges are to weed invasion.

NASA

Roadside maintenance also aids one of starthistle’s growth strategies. In early spring, while grasses and other roadside plants are sending up young shoots, yellow starthistle lays low—literally hugging the ground with a fledgling, palm-sized rosette—and quickly sends down an extensive root system that can access deep water sources. “Roadside mowers come when the grass is still green,” says Gelbard. “The mowers will blade off the grass but leave the starthistle to flourish in the absence of competition.” By late May, starthistle bolts up into a tall prickly stalk with yellow flowers framed by 2 cm long spines. The dense spreads of starthistle have the potential to meander into adjacent wild lands, farms, and pastures, leaving landowners to bear the costs of controlling or eradicating the invader. “By putting off highway mowing for just two or three weeks they could help reduce the problem by mowing the starthistle stalks themselves, says Gelbard.”

Paving Paradise

To keep pace with development, which consumes 2 million acres of wild land per year, transportation agencies consistently add and expand the nation’s road network. Nearly 70,000 lane-kilometers were steamrolled in 2003 alone. But a great many are not part of the 6.4 million km estimate, including the 600,000 km of National Forest roads and untold hundreds of thousands of kilometers of improved and unimproved “ghost roads” for private, military, and ranch use, oil and gas drilling, and other uses. Gelbard singles out those unofficial tracks for four-wheel drive recreation as a serious yet underestimated vector for plant invasion. “Because off-road vehicles go far from roads,” he says, “they not only affect habitats near roads that are already disturbed, but also remote, roadless habitats that are a lot less invaded.” How far invasives are actually creeping from roadsides into interior lands is still unclear and little studied.

Gelbard is hesitant about road expansion as a whole. “By curtailing and limiting the spread of roads,” he says, “you are essentially sealing off corridors from multiple environmental problems and the economic impacts they bring.” While reining in the pavers is unlikely to happen anytime soon (In December 2004, Texas launched the first phase of construction of the Trans-Texas Corridor, a 6,500 km megahighway project with lanes up to a half a kilometer across), recent years have seen ecologists, geographical information specialists, transportation planners, and conservationists take notice of the new field of road ecology. Its science is cross-disciplinary, exploring roadside vegetation, roadkill mitigation, and water, sediment, and chemical flow. It gives roadside habitats their proper due as unique environments. State and federal agencies are also increasingly incorporating ecological concerns into transportation planning, such as the Transportation Research Board’s Task Force on Ecology and Transportation, which links leading road ecologists with transportation planners. Can our desire for mobility work in concert with the needs of the environment? The answer lies down the road.