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

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⁺), SO₄²⁻, Cl⁻, base cations and several metals were measured in all four seasons along a 1.5 km 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 100 m 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 100 m of the road. Despite high throughfall inputs of > 15 kg N ha⁻¹ y⁻¹ and 100 kg Na⁺ ha⁻¹ y⁻¹ within 33 m of the highway, for example, there was no significant difference in soil chemistry amongst sites. Plant community composition at sites within 80 m 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. - Graphical abstract: Epiphytic lichen richness and normalized annual pollution deposition with distance from a major northern highway. - Highlights: • Inorganic N, base cations and metal deposition are elevated close to a busy highway. • Pollution gradient is evident in epiphytic lichen chemistry. • No impact of N or road salt on soil chemistry. • Altered lichen and plant communities are evident up to 80 m from the highway.