Vehicle emissions increase the amount of nitrogen oxides (NO_x) in the air, which ultimately increases the amount of plant usable nitrogen. Nitrogen is typically a limiting nutrient that influences ecosystem processes, such as nitrogen sequestration by plants, plant quality for herbivores and decomposers, and ultimately, ecosystem productivity. Changes in ecosystem processes as a result of increases in nitrogen availability are at odds with the goals of protected wilderness areas that are typically accessed by vehicles.

In this study, we used data from 2003 to 2019 to determine if vehicle emissions had an observable impact on the ecosystem in a protected wilderness area, Kananaskis Valley, near a major city (Calgary, Alberta). We examined the concentration of NO₂ in the air and the abundance of combustion-derived nitrogen isotopes (d¹⁵N) in naturally-occurring moss (Hylocomium splendens and Pleourozium schreberi) within 20 meters of the highway as a function of traffic levels that varied along the highway and among years.

Within the valley, we observed a gradient in the number of vehicles that was greatest at the main access to the valley, ranging from 1100 to 2700 vehicles per day across sites and years. A gradient in the number of vehicles was observed over the years, which ranged from a daily average of 2210 vehicles per day in 2013, to 4000 vehicles per day in 2019. Within years, NO₂ concentration in the air increased with the gradient of traffic along the valley as expected, although it did not correspond to among-year variation in vehicle numbers nor to within valley vehicle numbers. D¹⁵N in moss increased with the traffic gradient along the valley, and was positively correlated with traffic both within and among years, signalling that nitrogen from vehicle emissions may have entered the local ecosystem. We conclude that seemingly small anthropogenic influences, such as vehicle emissions, in protected areas along roadways with relatively low levels of traffic, can be observed through impacts on the ecosystem.