Impact Type: Direct exposure to pollutant
Calluna vulgaris, common throughout the British Isles on acid soils, is present on heaths, moors, bogs and open woods. Elevated concentrations of NH3 may have negative effects on C. vulgaris, as well as on heathland habitats in general. The effects are usually considered as occurring through the contribution of NH3 to N deposition, although direct effects may also occur. Van der Eerden et al. (1991) found that NH3 exposure increased biomass production in Calluna, but that it was at a competitive disadvantage relative to the grass Deschampsia flexuosa which grew faster. Van der Eerden et al. (1991) also raised the concern that NH3 leads to increased frost and drought sensitivity (Peason & Stewart 1993). Such damage to Calluna may result in its decline at the expense of grasses.
However, Calluna can hold its own against grasses except when abiotic stresses (e.g. winter dessication, or attacks by the heather beetle) open up the canopy enabling the grasses to get a foothold under the conditions of improved light. Per unit N, NH3 increases foliar N content more than NH4+ does(Leith et al 2001). In the UK Calluna growing on lowland dry heaths is more at risk form NH3 than Calluna on wet bogs and moors because of the proximity of inland dry heaths (e.g. Thetford and East Anglia) to intensive livestock units. N is likely to increase flower production, shoot growth, and to a lesser extent root growth and litter.
Although critical levels for NH3 have been set, Burkhardt et al. (1998) have shown that the critical load for N deposition is usually exceeded first.
|Habitat/ Ecosystem Type||Critical Load/ Level||Status||Indication of exceedance||Reference|
3 µg NH3 m-3 annual mean (uncertainty of 2-4 µg NH3 m-3)
Direct visible injury; species composition changes. Ecosystems where sensitive lichens and bryophytes are an important part of the ecosystem integrity, the critical level is set at 1 µg NH3 m-3.