The impact of pollutants from a major northern highway on an adjacent hardwood forest

Abstract

Emissions of pollutants from highways can exert multiple stresses on adjacent ecosystems. In this study air concentrations of NO₂ and throughfall deposition of inorganic N (NO₃^- and NH₄⁺), SO₄^2-, Cl^-, base cations and several metals were measured in all four seasons along a 1.5km hardwood forest gradient extending away from a major highway (Highway 401) in southern Ontario, Canada. Soil and lichen chemistry and herbaceous plant and epiphytic lichen species composition were measured within the hardwood forest to evaluate impacts of these pollutants. Air concentrations of NO₂ and deposition of inorganic N, Cl^-, base cations and Cu and Zn in throughfall were significantly elevated within 100m of the road compared with the more distant sites. Concentrations of several pollutants including N (and δ¹⁵N), Na⁺, Al and Fe in epiphytic lichen tissue decreased with distance from the highway, and epiphytic lichen richness was lower at sites within 100m of the road. Despite high throughfall inputs of >15kgNha^-1y^-1 and 100kgNa⁺ha^-1y^-1 within 33m of the highway, for example, there was no significant difference in soil chemistry amongst sites. Plant community composition at sites within 80m of the highway differed from sites located further from the road, but it is unclear whether differences were due to highway emissions or were a result of natural forest edge effects.

Keywords: Highways; Lichens; Metals; Nitrogen; Nitrogen dioxide; Road salt.