Do Roads Drive Forest Plant Invasions?

Roads provide a means for moving people and products, but they can also allow hitchhiking organisms to spread. Some exotic invasive plants thrive on the disturbance created by road construction that displaces native plants.

road-and-farm
Researchers found that land use within ‘road effect zones’ is an important predictor of forest plant invasions. Photo by Larry Korhnak, InterfaceSouth.

However, a new study led by Kurt Riitters, U.S. Forest Service research ecologist with the SRS Eastern Forest Environmental Threat Assessment Center, found that the presence of a road may be less important than the presence of farms and other human activities.

“In the eastern U.S., a third of all forested areas are within 650 feet of a road, and invasive plants are found on half of the plots monitored by the Forest Inventory and Analysis (FIA) program,” says Riitters. “While there is little doubt that roads are linked to forest plant invasions at local scales, effective resource conservation at regional scales requires an understanding of other factors linked to both roads and invasions across the larger landscape.”

To gain this understanding, researchers developed a series of models that allowed them to see the incremental influences of land use (including agriculture and development), forest fragmentation, local site conditions, and regional ecosystem characteristics in comparison to road proximity effects in eastern U.S. forest plant invasions.

Their results were published in the journal Diversity and Distributions.

The team used FIA data collected on more than 23,000 forested plots — excluding urban and residential forests — between 2001 and 2011. The data indicated the presence or absence of invasive plants and classified plots as high, medium, or low productivity sites.

Researchers then used a 2006 land cover map to determine forest fragmentation and land use in four areas surrounding each plot ranging in size from about 11 acres to about 1,460 acres. They also measured distance ‘as the crow flies’ between each plot and the nearest road or railroad.

The first model only included distance from a road as an influencing factor in plant invasions. As expected, plots closer to a road were much more likely to have invasive plants than sites farther from a road.

In subsequent models that combined distance from a road with surrounding land features, the odds of invasion that could be attributed to roads alone were substantially reduced.

eco-provinces-map
Ecological provinces most influenced the odds of invasion. Only Northeastern Mixed Forest, Laurentian Mixed Forest, and Central Appalachian Broadleaf Forest/Coniferous Forest-Meadow contained plots at low risk of invasion due to road effects.

Regional ecosystem characteristics including terrain and vegetation cover—known as ecological provinces—best explained the odds of invasion. In the combined model, land use, site productivity, and forest fragmentation, respectively, were the next best predictors of plant invasion, followed by distance from a road.

Plots with the lowest odds of invasion were on low-productivity sites that were not fragmented nor located near development, but road effects were still detected in these plots up to 6,560 feet from a road. Because road density is so high in the eastern U.S., only three ecological provinces contained any forestland outside this 6,560-feet ‘road effect zone.’

Results of the study demonstrate the value of the modern FIA program for answering non-traditional forest inventory questions. Managers can use the findings to develop regional strategies to control or eradicate invasive plants. Scientists monitoring forests and developing risk assessments can also apply the model results.

“Because roads and human activities surrounding them are so pervasive in the East, we should be thinking of ‘human impact zones’ instead of ‘road effect zones,’” adds Riitters. “The risk of forest plant invasions cannot be evaluated only on the basis of distance from a road. The context of the road is key.”

Read the full text of the article.

For more information, email Kurt Riitters at kriitters@fs.fed.us.

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