Deriving nitrogen critical levels and loads based on the responses of acidophytic lichen communities on boreal urban Pinus sylvestris trunks

Abstract The deposition of reactive nitrogen (N) compounds currently predominates over sulphur (S) deposition in most of the cities in Europe and North America. Acidophytic lichens growing on tree trunks are known to be sensitive to both N and S deposition. Given that tree species and climatic factors affect the composition of epiphytic lichen communities and modify lichen responses to air pollution, this study focused on the impact of urban air pollution on acidophytes growing on boreal conifer trunks. The study was performed in the Helsinki metropolitan area, southern Finland, where annual mean nitrogen dioxide (NO 2 ) concentrations range from 4–5 μg m − 3 to > 50 μg m − 3 . In addition, background forest sites in southern and northern Finland were included. The results demonstrated elevated N contents (≥ 0.7%) in Hypogymnia physodes and Platismatia glauca at all the sites where the species occurred. In the Helsinki metropolitan area, a higher frequency of green algae +  Scoliociosporum chlorococcum and reduced numerical frequencies of other indicator lichen species (e.g. Pseudevernia furfuracea , Bryoria spp., Usnea spp.) were associated with elevated atmospheric concentrations of NO 2 and particulate matter containing N, as well as elevated concentrations of inorganic N in bark. The N isotope values (δ 15 N) of lichens supported the uptake of oxidized N mainly originating from road traffic. Sulphur dioxide (SO 2 ) also negatively affected the most sensitive species, despite the current low levels (1–4 μg m − 3  yr − 1 ). Critical levels of 5 μg NO 2  m − 3  yr − 1 and 0.5 μg NH 3  m − 3  yr − 1 , and a critical load of 2–3 kg N ha − 1  yr − 1 are proposed for protecting the diversity of boreal acidophytes. This study calls for measurements of the throughfall of various N fractions in urban forest ecosystems along precipitation and temperature gradients to verify the proposed critical levels and loads.