Aguillaume, L., Avila, A., Pinho, P., Matos, P., Llop, E. & Branquinho, C. (2017) The critical levels of atmospheric ammonia in a Mediterranean holm-oak forest in North-Eastern Spain.Water Air and Soil Pollution, 228(3), 93. DOI:10.1007/s11270-017-3286-8 (IF2017 1,769; Q2 Environmental Sciences)
Despite recent regulations, atmospheric ammonia (NH3) emissions have not changed much over the last decades and excessive nitrogen remains as one of the major drivers for biodiversity changes. To prevent deleterious effects on species and ecosystems, it is very important to establish safety thresholds, such as those defined by the Critical Level (CLE) concept, “the concentration above which direct adverse effects on receptors may occur, based on present knowledge.” Empirical critical levels of atmospheric NH3 have mainly been reported for temperate forests and there is a lack of information for Mediterranean forests. Here, we provide a case study on NH3 CLEs for a typical Mediterranean ecosystem, the holm-oak (Quercus ilex) forest. To derive the CLE value, we measured NH3 concentrations for 1 year at a distance gradient in the forest surrounding a point source (cattle farm) and used diversity changes of lichen functional groups to indicate the onset of adverse effects. We estimate a NH3CLE threshold of 2.6 μg m−3, a value that is higher than that reported in other Mediterranean ecosystems and suggests that the site has been already impacted by NH3 pollution in the past. In a more general context, this study confirms the validity of lichen functional groups to derive CLEs in Mediterranean forests and woodlands and contribute to the body of knowledge regarding the impacts of NH3 on ecosystems.