Ammonia :: Coniferous woodland

Effects and implications

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

• Direct damage to spruce and pine tree foliage, for example, leaf discolouration, premature senescence and loss.
• Increased sensitivity to drought and 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.
• Increased rates of litter loss
• Changes in the composition of groundflora, bryophyte and lichen communities. Changes in the under storey, increase in grasses and ruderal species.
• Nitrification will be stimulated, increasing soil acidity.

Overview: evidence, processes and main impacts

Ammonia is a reactive water soluble alkaline gas that will deposit to acid surfaces and where acidic gases are also present in the atmosphere e.g. SO₂ (through co-deposition). Coniferous woodlands tend to occur on acid soils and their vegetation also tends to provide an acidic surface, increasing their susceptibility to NH₃. Also the rough surface helps increase rates of NH₃ deposition especially at the woodland edge. The low N status of the foliage will also lead to enhanced deposition of NH₃ to coniferous woodland.

Ammonia exposure significantly increases foliar N concentrations predisposing trees to biotic and abiotic stress.  Stomatal control appears to be sensitive to NH₃ especially in Pines. 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, but can also occur in predominantly agricultural areas (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%). Scots pine, Pinus sylvestris, is a relatively low N demanding species growing on nutrient poor often sandy soils. NH₃ might stimulate growth but with some reduction in carbohydrates to support ectomycorrhiza, leading to negative impacts on fruit body formation.

Woodlands on more acid soils may be more at risk because of the greater likelihood of deposition In the drier east, water stressed conditions, trees may be less sensitive if stomata work efficiently and close, excluding NH₃ uptake. Invasion by ‘casual’ plants will be greater in areas where availability of a seed source for such plants is high. Although, depending of the age of the wood, opportunities for seedling establishment will be restricted by the ‘dark’ conditions.

Pollutant deposition type and risk areas

Indicators of NH₃ effects

These depend on concentration and deposition

• Needle % N increase and especially amino acid content.
• Increased litter production and canopy
• Decreases in bryophyte and herb species richness
• Under-storey with increased Ellenberg scores
• Changes in under-storey species, with sensitive mosses and lichens likely to decline e.g.  increases in acid tolerant nitrophilic species (e.g. Deschampsia flexuosa) in non-fertilised Scots Pine stands over the last 20 to 40 years and decreases in Calluna vulgaris and Vaccinium vitis-idaea (Rodenkirchen 1992).
• Algae growing on the trees
• Reduction in faunal and floral biodiversity as dominant species take over.
• Reduction in large sporocarps and diversity of mycorrhizal fungi.
• Change in soil pH but depends on level of nitrification.
• Increase in N content of upper soil horizons, with subsequent effects on soil processes, e.g. mineralization.

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.