The disturbance and connectivity associated with forest roads can facilitate the spread of invasive plants. However, little is known about how soil changes caused by roads might influence plant community invasibility. Many unpaved forest roads in Pennsylvania are surfaced with limestone gravel, which raises the pH of roadside soils. The invasive grass Microstegium vimineum (Japanese stiltgrass) is commonly found along forest roads. Previous studies have shown its presence to be correlated with elevated soil pH. We conducted a field survey and greenhouse experiment to investigate whether conditions created by use of limestone gravel increase the abundance and growth of M. vimineum. We selected four pairs of unpaved forest roads, consisting of one road with native shale surface and one with limestone. For 1000 m along both sides of each road, we measured M. vimineum occurrence in five abundance categories. We used the distance measured in each category and the midpoint of each abundance category to calculate an average M. vimineum abundance for each road. We measured extent of M. vimineum away from the road at 25 points, and sampled soil pH along four transects across each road. In the greenhouse, forest soils were amended with powdered limestone to create a range of pH levels and M. vimineum biomass accumulation in these treatments was measured.
Results/Conclusions
The average abundance of M. vimineum on limestone roads (35.1%) was significantly greater than on shale roads (11.8%; p=0.001). The average extent of M. vimineum away from the road was also significantly greater along limestone roads (7.6 m) than along shale roads (2.5 m; p<0.001). The pH of undisturbed forest interior soils was 4.5 or lower; beside shale roads it ranged from 5.0 to 6.2. At the edge of the limestone roads pH was 7.3 or higher; 3 m away from the road edge it was between 6.3 and 7.2. In the greenhouse experiment, M. vimineum biomass accumulation was greatest in the pH 6.3 treatment, a soil pH considerably less acidic than native forest soils but within the range measured beside limestone-graveled roads. Our results suggest that use of limestone gravel facilitates invasion of roadside vegetation by M. vimineum.