"Roads disturb the soil, open the forest canopy and allow more light to reach the ground," explains Todd Hawbaker, a University of Wisconsin-Madison forestry graduate student who presented the findings. "These conditions allow invasive weeds to take hold and displace native plant life."
For his master's thesis, Hawbaker used historic aerial photographs of 17 townships in northern Wisconsin to track road density during the past 60 years. He found that between 1937 and 1999 road density doubled, which was more change than he expected. "However, it's probably a safe estimate for wooded areas in other parts of the country as well," he says.
He points out that these roads stretch beyond the state and county highways to include a vast network of local access and logging roads. In fact, in northern Wisconsin, an area considered relatively undeveloped, a visitor is rarely more than a mile from the nearest road, says Hawbaker.
One of the potential effects of building a road is the spread of invasive species. The Wisconsin researcher adds that generally the only question is how long it will take for invasive species to colonize a new road.
To help answer that question, Hawbaker used a computer model to simulate plant invasions along roads using a variety of dispersal patterns. Usually, seeds of invasive species are spread by animals or wind over short distances, but on rare occasions can also be spread over long distances by animals, wind or vehicles. When successful, these long-distance dispersal events allow invasive species to rapidly colonize new roads.