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

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.

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.