Synthesis of lichen response to gaseous nitrogen: ammonia versus nitrogen dioxide

NH3 concentrations below the current critical level affect the epiphytic macrolichen communities - Evidence from a Northern European City

Acidophytic, oligotrophic lichens on tree trunks are widely considered to be the most sensitive biota to elevated concentrations of atmospheric ammonia (NH₃). We studied the relationships between measured NH₃ concentrations and the composition of macrolichen communities on the acidic bark of Pinus sylvestris and Quercus robur and the base-rich bark of Acer platanoides and Ulmus glabra at ten roadside and ten non-roadside sites in Helsinki, Finland. NH₃ and nitrogen dioxide (NO₂) concentrations were higher at the roadside than non-roadside sites indicating traffic as the main source of NH₃ and nitrogen oxides (NO_x). The diversity of oligotrophs on Quercus was lower at the roadside than non-roadside sites, while that of eutrophs was higher. The abundance and presence of oligotrophic acidophytes (e.g., Hypogymnia physodes) decreased with increasing NH₃ concentration (2-year means = 0.15-1.03 μg m^-3) especially on Q. robur, while those of eutrophic/nitrophilous species (e.g., Melanohalea exasperatula, Physcia tenella) increased. The abundance of some nitrophytes seemed to depend only on bark pH, i.e., their abundances were highest on Ulmus, which had the highest average bark pH. Overall, the results of lichen bioindicator studies may depend on tree species (bark pH) and lichen species used in calculating indices describing the air quality impact. Nevertheless, Quercus is recommended to be used to study the impact of NH₃ alone and in combination with NO_x on lichen communities, because the responses of both oligotrophic acidophytes and eutrophic species can already be observed at NH₃ concentrations below the current critical level.