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

Abstract

Abstract. Nitrogen (N) has emerged in recent years as a key factor associated with global changes, with impacts on biodiversity, ecosystems functioning and human health. In order to ameliorate the effects of excessive N, safety thresholds such as critical loads (deposition fluxes) and levels (concentrations) can be established. Few studies have assessed these thresholds for semi-natural Mediterranean ecosystems. Our objective was therefore to determine the critical loads of N deposition and long-term critical levels of atmospheric ammonia for semi-natural Mediterranean evergreen woodlands. We have considered changes in epiphytic lichen communities, one of the most sensitive comunity indicators of excessive N in the atmosphere. Based on a classification of lichen species according to their tolerance to N we grouped species into response functional groups, which we used as a tool to determine the critical loads and levels. This was done for a Mediterranean climate in evergreen cork-oak woodlands, based on the relation between lichen functional diversity and modelled N deposition for critical loads and measured annual atmospheric ammonia concentrations for critical levels, evaluated downwind from a reduced N source (a cattle barn). Modelling the highly significant relationship between lichen functional groups and annual atmospheric ammonia concentration showed the critical level to be below 1.9 µg m −3 , in agreement with recent studies for other ecosystems. Modelling the highly significant relationship between lichen functional groups and N deposition showed that the critical load was lower than 26 kg (N) ha −1 yr −1 , which is within the upper range established for other semi-natural ecosystems. Taking into account the high sensitivity of lichen communities to excessive N, these values should aid development of policies to protect Mediterranean woodlands from the initial effects of excessive N.

Plant-soil interactions in Mediterranean forest and shrublands: impacts of climatic change

“…This study did not refer specifically to Mediterranean ecosystems but in Mediterranean areas this could also occur, and the increases in available soil N:P ratios could further increase in some Mediterranean areas where the N deposition is increasing in the last years (Vourlitis and Pasquini 2009;Pinho et al 2012). In fact, experiments of N fertilization in Mediterranean terrestrial ecosystems have shown an increased water-use efficiency (WUE) (Guerrieri et al 2010), a response that could have positive effect on plant production under warming.…”

Interactions between leaf nitrogen status and longevity in relation to N cycling in three contrasting European forest canopies

“…70 %), whereas the Douglas fir stand had a re-translocation efficiency of only 37 %, meaning that it relocated only 53 % as efficiently as the two other species (Table 2). Inclusion of data on the N re-translocation efficiency from previously published studies (Staaf, 1982;Son and Gover, 1991;Näsholm et al, 1994;Reich et al, 1995;Escudero and Mediavilla, 2003) revealed that the longer the needles stayed green, the lower the N retranslocation efficiency (η r ) was (Fig. 5).…”

“…Forest ecosystems are generally considered to be adapted to N limitation rather than to N excess (Vitousek and Howarth, 1991;Rennenberg et al, 1998;Aerts, 1996;Xia and Wan, 2008). Due to deposition of atmospheric NH 3 , the critical N load for European forest ecosystems is often exceeded Pinho et al, 2012). This may in turn lead to elevated concentrations of NH + 4 and organic N compounds in the trees and to NH 3 emission events (Rennenberg et al, 1998;Fowler et al, 2009).…”

Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: The need for long-term and site-based research

“…The sensitivity of terrestrial biodiversity to the deposition of oxidized and reduced N provides the basis for setting critical loads for N deposition both in Europe and North America (Cape et al 2009;Bobbink and Hettelingh 2011;Pinho et al 2011Pinho et al , 2012. Independently derived critical levels for lichens and moss diversity have been found to be similar for northern and southern Europe, thus emphasizing the universal applicability of these plant groups as ecological indicators of N deposition.…”