Forest soils adjacent to urban interstates: Soil physical and chemical properties, heavy metals, disturbance legacies, and relationships with woody vegetation

While urban ecology is an expanding field of study, some natural areas within the urban environment remain under-examined. These include naturally regenerating forest communities adjacent to urban interstates. In addition, the status of interstate soils and their relationships with the community composition of forested interstate verges has received little ecological study. The purpose of this study was to examine variation in soil conditions along forested interstate corridors in Louisville, KY and to explore the extent to which soil characteristics (e.g., bulk density, pH) and heavy metals (e.g., Pb, Zn) vary with respect to three factors: interstate (e.g., traffic density), surrounding urban environment (e.g., industrial land use), and interstate construction legacies. Additionally, we explored the relationships between several edaphic factors and woody vegetation structure in these forested verges. We found that the degree and direction of the slope of land towards the interstate and the distance to the interstate pavement were strong determinants of soil characteristics and heavy metal concentrations, suggesting that the movement of de-icing salts, heavy metals, and other pollutants from the interstate was important in determining forest soil conditions along urban interstates. Since within our study area these highways did not extend into rural lands, variation in urban land uses and cover within 26 km of the city center was not large enough to explain variation in soil characteristics or heavy metals, except for a positive correlation between chromium and surrounding industrial land use. We did find that past physical soil disturbance caused by interstate construction (e.g., imported fill) left an important legacy on soil characteristics, heavy metal retention, and woody plant growth patterns in forests adjacent to urban interstates. The legacy of interstate construction on the current forest community structure (e.g., lower species richness) and the future forest (e.g., reduced tree regeneration) may further alter ecosystem productivity and ecosystem services provided by these forests and their soils.