N deposition :: Heather or Ling

Impact Type: Deposition of pollutant

Key Concerns:

Heather, Calluna vulgaris, is an ericoid common throughout the British Isles on acid soils from heaths, moors, bogs to open woods. Experimental spraying in the field with ammonium nitrate and ammonium sulphate has been shown to reduce frost hardiness and affect water conservation of Calluna, although concentrations used were frequently larger than ambient levels (Caporn et al. 1990, Uren et al. 1997, van der Eerden et al. 1991). In the Breckland heaths which receive an estimated 30-40 kg N ha^-1 year^-1, high foliar N content (2.6% N) may predispose Calluna to damage by heather beetle, frost and drought (Pitcairn et al. 1995). At Leende Heide in the Netherlands, a declining heathland receiving 39 kg N ha^-1 year^-1 (Duyzer et al. 1992), foliar N content of senescent Calluna was found to be 1.9% (Sutton & Fowler 1995). Foliar N contents of Calluna at Moor House NNR Cumbria have increased from 1.4% in the 1960s (Heal & Smith 1978) to 1.9% in 1989 and similarly high concentrations were found in some areas of Scotland. Clearly, high foliar N levels are contributing to Calluna decline in Europe and current concentrations in some important Calluna heaths and moors in the UK must be viewed with concern (Pitcairn et al. 1995).

Additional Comments:

Heather is generally managed through burning or cutting and management practices can restrict N accumulation in the system. The decline of heathlands and replacement of Calluna by grasses in the Netherlands has been well documented (Heil & Diemont 1983, Bobbink & Roeloffs 1995). Both increased N deposition (largely in the form of NH₃ from intensive stock rearing units) and heather beetle damage (Brunsting 1982) have been implicated. Calluna decline has also occurred in a number of Breckland heaths in East Anglia (Pitcairn et al. 1991, 1995) and has been attributed to catastrophic death of Calluna caused by frost, drought or heather beetle attack. Calluna can usually outcompete grass species, unless the canopy is opened up. N induced c hanges in root shoot ratios will increase sensitivity to drought and winter dessication. Effects on the ericoid mycorrhizas are unclear with both positive and negative effects of N deposition being observed (Caporn et al 1995). Effects of N deposition tend to be exacerbated in the soil because mineralization rates are enhanced (Caporn et al. 1995).