Ammonia :: Dwarf Shrub Heath

Effects and implications

[Ammonia is one of the key pollutants that contribute to nitrogen deposition. Please read the Nitrogen deposition - Dwarf Shrub Heath  record to understand the full impacts effects of nitrogen deposition including ammonia.]

  • Ammonia can be directly phytotoxic to vegetation (Krupa 2003).
  • Effects will vary depending on the exposure concentrations, the length of time of exposure has been emitting and whether P/K deposition has also been increased, e.g. dust from point sources.
  • Direct damage to sensitive species, e.g. bleaching and leaf discoloration, observed in Cladonia lichens e.g. Cladonia portentosa at high concentrations. Bleaching is a particularly likely consequence of NH3 exposure and quite distinct (cf component of hair dye).
  • Reduced ability of stomata to close under drought conditions, leading to plant water stress (Van Hove et al. 1991, Erisman and Draaijers 1995) highly visible as greatly increased amount of grey foliage in Calluna a consequence of winter desiccation(Sheppard et al 2008; 2011).
  • Changes in species composition, species swapping.
  • Subtle changes in plant morphology, physiology and biochemistry which not only increases growth, but also increases sensitivity to environmental factors such as wind, frost, drought and pests (e.g. increased tissue N concentrations can predispose plants to insect attack).
  • Increased likelihood of heather beetle attacks.
  • Change from heath to grassland

Overview: evidence, processes and main impacts

There is anecdotal evidence of significant declines in heathland ecosystems in East Anglia and Europe where background NH3 concentrations have been high, with very high concentrations close to point sources e.g. intensive livestock units, manure stores (Sheppard et al 2011). There are no in situ ammonia manipulation experiments to draw on, however many of the species that comprise dwarf shrub heath have been subject to experimentally manipulated ammonia concentrations on an ombrotrophic bog. Ammonia can affect these ecosystems directly via a concentration effect (addressed through Critical Levels) and through contributing to the overall N load.

 Gaseous ammonia deposition poses a significantly greater risk to sensitive species per unit N deposited than N forms in wet deposition (Sheppard et al 2011). Dry deposited ammonia causes detrimental effects rapidly and at lower cumulative N doses than wet deposited N forms and these effects appear to be mediated directly, aboveground rather than via the soil (Sheppard et al 2011).

Heaths tend to be found on acid soils and their vegetation, mostly composed of species with high surface area also tends to provide an acidic surface, increasing their susceptibility to NH3. NH3 exposure predisposes Calluna to winter desiccation and pathogen infections, the latter possibly associated with the reduction in acidity in the soil and on the leaf surface.  Ammonia can debilitate stomatal function, predisposing sensitive species to desiccation stress e.g. Calluna (Sheppard et al 2008). Even some of the more N tolerant species of lower plant plants can be sensitive to NH3 eg Hypnum jutlandicum (Sheppard et al 2011). Vaccinium myrtillus and Empetrum nigrum  appear to be less sensitive than Calluna to NH3 being able to maintain efficient stomatal functioning. High NH3 concentrations can promote rapid uptake and lead to accumulation of toxic ammonium ions in plant tissue.

Pollutant type and risk areas

Type of pollutant

Form of N

Risk areas

Dry deposition



  • Rural areas with elevated background concentrations, and close to point sources e.g. intensive livestock agriculture, wild animal and bird colonies.
  • Lowland heath more than upland.

Indicators of NH3 impact

These depend on the extent of the deposition and level of NH3 exposure, but relatively robust examples include:

  • Bleaching on sensitive species, usually occurs first on lichens e.g. C. portentosa. 
  • Damage, greying of Calluna foliage, breakdown of Calluna canopy.
  • Increases in graminoid cover.
  • Increases in nitrophilic species, including mosses able to capitalize on the increase in bare soil e.g. Campylopus introflexus.
  • Which nitrophilic species can establish and expand will depend on other factors e.g. soil pH and which other nutrients are potentially limiting.
  • Absence of habitat constants
  • Increase in Ellenberg N.
  • Increases in foliar N
  • More pest and pathogen damage

Examples of species specific responses

Some examples of specific responses are given in the table below. This does not represent a comprehensive review of all species impacts.




Cladonia portentosa,

Detrimental, potential to disappear

Sheppard et al 2004, 2011


Increased winter desiccation

Sheppard et al 2008


Detrimental - in Scotland, infestations by winter moth, linked to increased %N from N deposition.

Kerslake et al 1998

What factors modify NH3 deposition impacts?

  • Climate: NH3 increases risk of damage from summer droughts and winter desiccation in Calluna dominated heath (Sheppard et al 2008).
  • Pests and pathogens: because N can accumulate as amino acids, vegetation becomes a more attractive food source, with consumption or damage to shoots and roots debilitating the vegetation far more than the effect of NH3 alone, providing the greatest threat to sustaining Calluna dominated heaths.
  • Closeness to source of propagules of nitrophilic species.

Critical Load/Level: 

Habitat/ Ecosystem Type Critical Load/ Level Status Indication of exceedance Reference
Higher plants

3 µg NH3 m-3 annual mean (uncertainty of 2-4 µg NH3 m-3)

UNECE, 2007

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.

Lichens and Bryophytes

1 µg NH3 m-3 annual mean

UNECE, 2007

Loss of sensitive mosses and lichens communities. Communities become dominated by nitrophiles at the expense and virtual loss of acidophytes as bark pH becomes less acidic.



Erisman, J.W.; Draaijers, G.P.J. 1995 Studies in Environmental Science 63
Hove, L.W.A.; Vankooten, O.; Vanwijk, K.J.; Vredenberg, W.J.; Adema, E.H.; Pieters, G.A. 1991 Physiological-effects of long-term exposure to low concentrations of SO2 and NH3 on poplar leaves Physiologia Plantarum 82 32-40
Krupa, S.V. 2003 Effects of atmospheric ammonia (NH3) on terrestrial vegetation: a review Environmental Pollution 124 179-221
Sheppard, L.J.; Crossley, A.; Leith, I.D.; Hargreaves, K.J.; Carfrae, J. ; Dijk, N.; Cape, J.N.; Sleep, D. ; Fowler, D.; Raven, J.A. 2004 An Automated Wet Deposition System to Compare the Effects of Reduced and Oxidised N on Ombrotrophic Bog Species: Practical Considerations Water, Air and Soil Pollution: Focus 4 (6) 197-205
Sheppard, L.J.; Leith, I.D.; Mizunuma, T. ; Cape, J.N.; Crossley, A.; S., Leeson ; Sutton, M.A.; Fowler, D.; Dijk, N. 2011 Dry deposition of ammonia gas drives species change faster than wet deposition of ammonium ions: evidence from a long-term field manipulation Global Change Biology 17 (12) 3589-3607
Sheppard, L.J.; Leith, I.D.; Crossley, A.; Dijk, N.; Fowler, D.; Sutton, M.A.; Woods, C. 2008 Stress responses of Calluna vulgaris to reduced and oxidised N applied under real world conditions Environmental Pollution 154 (3) 404-413