Effects and implications
[Ammonia is one of the key pollutants that contribute to nitrogen deposition. Please read the Nitrogen deposition - Broadleaved, Mixed and Yew Woodland record to understand the full impacts effects of nitrogen deposition including ammonia.]
- Direct damage to foliage, e.g. leaf discolouration, premature senescence and loss.
- Increased sensitivity to drought and spring frost and increased risk of pest and pathogens attack
- Reduced ability of stomata to close under drought conditions, leading to plant water stress (Van Hove et al. 1991, Erisman and Draaijers1995).
- Loss of mycorrhiza and fruit bodies.
- Changes in the composition of groundflora, bryophyte and lichen communities. Changes in the understorey, increase in grasses and ruderal species.
- Nitrification will be stimulated, increasing soil acidity.
- Effects likely to be exacerbated if leaf area, canopy increases
Overview: evidence, processes and main impacts
Ammonia is a reactive water soluble alkaline gas that will deposit to acid surfaces and to a small extent, given the decline in SO2 emissions, where acidic gases are also present in the atmosphere e.g. SO2 (through co-deposition). The rough surfaces of woodlands leads to increased rates of NH3 deposition especially at the woodland edge. Ammonia exposure significantly increases foliar N concentrations predisposing trees to biotic and abiotic stress. Stomatal control appears to be sensitive to NH3. High NH3 concentrations promote rapid uptake and accumulation of toxic ammonium ions in plant tissue. Severe damage has been frequently found in woodland edges adjacent to fields with high manure applications in the Netherlands and Belgium (van Herk 1999, 2002).
Exceedance of the NH3 critical level is a local issue relevant near major sources such as intensive livestock farms, manured fields or even wild bird colonies (Sutton et al 2000). Damage to trees has been reported in the vicinity of farms in England and Scotland (UKCLAG 1996, Pitcairn et al. 1998). Woodland ground flora will also be at risk as the trees only take up a small proportion of atmospheric NH3 (maximum < 20%). NH3 might stimulate tree growth but with some reduction in carbohydrates to support ectomycorrhizas (fungi) leading to negative impacts on fruit body formation.
Typical under-storey flora might include Oxalis acetosella, Primula nemorosa, Anemone nemorosa, Succisa pratensis and Viola riviniana which are N sensitive (Pitcairn et al 2006).
Case study, Pitcairn et al 2005: The flora in Piddles Wood SSSI (broadleaved, mixed and yew woodland) is close to a significant ammonia source (~100, 000 birds). A survey of the groundflora showed a decline in mean Ellenberg N indicator moving away from the farm reflecting the vegetation composition. Close to the farm ivy, (Hedera helix), with chervil (Chearophyllum temelentum), yellow archangel (Lamiastrum galeobdolon), bluebell (Hyacinthoides non-scripta) and nettle (Urtica dioica) were common. Trees had moss cover but this was covered in algae. This flora was typically nitrophilic. Further away from the farm some vernal species e.g. Primula nemorosa, Anemone nemorosa were recorded but far less frequently than expected. Management activities influencing light levels can also produce similar changes for some species.
Pollutant type and risk areas
Type of pollutant
Form of N
Woodlands in rural areas with elevated background concentrations for example close to intensive livestock agriculture.
Woodlands on the more acid soils.
Woodlands in wetter areas where stomata are more likely to be open increasing the likelihood of NH3 uptake.
Indicators of NH3 deposition
These depend on concentration and deposition
- Increase in foliage % N, especially amino acid contents.
- Increased canopy LAI and litter production
- Decreases in bryophyte and herb species richness
- Increase in grass to forb ratio.
- Understory with increased Ellenberg scores
- Changes in under-storey species, with sensitive mosses and lichens likely to decline e.g. increase in acid tolerant nitrophiles.
- Algae growing on the trees
- Reduction in faunal and floral biodiversity as dominant species take over.
- Change in soil pH but depends on level of nitrification
- Loss of lichen communities which thrive on oak and trees with acid bark. Communities become dominated by nitrophiles at the expense and virtual loss of acidophytes as bark pH becomes less acidic.
Example evidence 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.
Primula nemorosa, Anemone nemorosa
Pitcairn et al 2005
|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.
|Lichens and Bryophytes||
1 µg NH3 m-3 annual mean
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.