Critical loads of nitrogen deposition and critical levels of atmospheric ammonia for semi-natural Mediterranean evergreen woodlands

[摘要] Nitrogen (N) has emerged in recent years as a key factor associated withglobal changes, with impacts on biodiversity, ecosystems functioning andhuman health. In order to ameliorate the effects of excessive N, safetythresholds such as critical loads (deposition fluxes) and levels(concentrations) can be established. Few studies have assessed thesethresholds for semi-natural Mediterranean ecosystems. Our objective wastherefore to determine the critical loads of N deposition and long-termcritical levels of atmospheric ammonia for semi-natural Mediterraneanevergreen woodlands. We have considered changes in epiphytic lichencommunities, one of the most sensitive comunity indicators of excessive N inthe atmosphere. Based on a classification of lichen species according totheir tolerance to N we grouped species into response functional groups,which we used as a tool to determine the critical loads and levels. This wasdone for a Mediterranean climate in evergreen cork-oak woodlands, based onthe relation between lichen functional diversity and modelled N depositionfor critical loads and measured annual atmospheric ammonia concentrationsfor critical levels, evaluated downwind from a reduced N source (a cattlebarn). Modelling the highly significant relationship between lichenfunctional groups and annual atmospheric ammonia concentration showed thecritical level to be below 1.9 μg m⁻³, in agreement with recentstudies for other ecosystems. Modelling the highly significant relationshipbetween lichen functional groups and N deposition showed that the criticalload was lower than 26 kg (N) ha⁻¹ yr⁻¹, which is within theupper range established for other semi-natural ecosystems. Taking intoaccount the high sensitivity of lichen communities to excessive N, thesevalues should aid development of policies to protect Mediterranean woodlandsfrom the initial effects of excessive N.

[学科分类] 地球化学与岩石