How roads are aiding our worst plant enemies

Japanese stiltgrass growing on abandoned railroad tracks along Ashokan Rail Trail

I was hiking the Ashokan Rail Trail in New York and noticed how the gravel path was frequently lined with masses of the invasive grass Microstegium vimineum (Japanese Stiltgrass), which crowded out other invasives, as well as the native flora.

This predilection of the species for roadsides and paths is one that I am familiar with, and so once I got home I looked up whether there was any reason for its behavior. My first thought was that roadsides created an environment with higher light levels than the interior of the canopy, but it turns out there are other reasons why such man made strictures aid invasive plants in general

The characteristic silver midrib of Japanese stiltgrass

Large scale studies have shown that M. vimineum presence in forests was strongly correlated with the presence and proximity of roads and other man-made paths. The probability of finding this species along the east facing sides of roads was as high as 83% in one study area. In addition, experiments revealed that the natural spread of this species was greatest in patches closer to roads, and that these patches also tended to have higher populations.  There was something in the areas around roads and paths that proved advantageous to the proliferation and spread of this invasive species,

Japanese stiltgrass lining the sides of a hiking path

One interesting and important finding from studies of M. vimineum is that the species if left to itself spreads very slowly, with seeds landing only 1 to 2 meters away from the parent plant. This was perplexing because land managers noticed that Japanese stiltgrass can quickly spread throughout an entire forest within a few years, which implied a rate of invasion that was orders of magnitude faster than it should be.

Japanese stiltgrass expanding from roadside into forest interior

It turns out that human activity along the forest roads is one of the major causes of the rapid and unnatural spread of stiltgrass in invaded forests, whether it's from hikers picking up seeds as they trudge along hiking paths, or vehicles doing the same along passable roadways.

In addition, the forest roads themselves can create conditions that are environmentally advantageous for this invasive grass. For example, the use of limestone gravel in many unpaved roads can raise the pH of the surrounding soil, which is favored by M. vimineum.

Unfortunately, it is neither possible nor perhaps even desirable to completely remove all human structures from parks and other forested areas in a quest to return the forests to their original pristine condition, But we can at least minimize the negative impact we have on these environments through more studies that delineate the many ways our presence in the natural world affect the denizens of the forests.