NH3 :: Heather or Ling

Impact Type: Direct exposure to pollutant

Key Concerns:

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.

Additional Comments:

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.