Forest managers can fight invasive species that come with roads
Road density in northern Wisconsin has doubled during the last 60 years, but forest managers have a time window to fight the non-native plants that often come with construction and overwhelm native plant life, according to new research to be discussed on Thursday, Aug. 5, at the annual Ecological Society of America conference in Portland, Ore.
“Roads disturb the soil, open the forest canopy and allow more light to reach the ground,” explains Todd Hawbaker, a 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.
“We found a lag time of up to 60 years between when a road is built and when a road is completely covered by invasive species,” explains Hawbaker, referring to the results. “The actual time lag depends on the invasive species’ dispersal capabilities and the density of roads. The time lag decreases substantially over time as new roads are added.”
That window is both a problem and an opportunity, says Volker Radeloff, a forestry professor who supervised Hawbaker’s work. “On one hand, people won’t see invasives immediately, and they may assume it’s not an issue and underestimate the ecological impact of roads. But on the other hand, there is a window of time to do something.”
The best defense against invasive species is good monitoring by forest managers and quick action against new satellite populations, according to Radeloff and Hawbaker. “It actually pays to be proactive in this case,” says Radeloff.
And, as the window period gets shorter when road density increases, another solution is to carefully consider whether or not to build new roads. “Areas without roads are quickly becoming treasures,” adds Radeloff.
These findings, he says, lend support to a federal policy aimed at conserving roadless areas in national forests and grasslands; the rule has been the target of litigation in several states, and has recently been changed to allow state governors to build roads in formerly roadless areas for certain purposes.