Nitrogen Oxides :: Broadleaved, Mixed and Yew Woodland


[See also: Nitrogen deposition:: Broadleaved, Mixed and Yew Woodland| Nitrogen deposition :: Coniferous woodland| Nitrogen deposition :: Hedgerows]

Effects and Implications

  • Effects are mainly on growth, photosynthesis and nitrogen assimilation/metabolism with few species showing visible injury.
  • Visible decline symptoms for example, leaf discoloration can occur at very high concentrations (> 400 ug m3)
  • Direct damage to mosses, liverworts and lichens, which receive their nutrients from atmospheric deposition, often leads to reductions in species diversity, but also leads to an increase in nitrogen-loving species.

Overview: evidence, process and main impacts

Direct effects of NOx (the sum of nitric oxide NO and nitrogen dioxide NO2) have been recognised in the setting of a critical level concentration (Ashmore and Wilson 1994, Sanders et al. 1995). The gases are considered together partly because their concentrations in air are inextricably linked through their atmospheric chemistry, and partly because little is known of the direct effects of NO alone. However, rates of NOx deposition are rather slow, so that local impacts are likely to be less than for other pollutant gases such as ozone or ammonia.

The major role of NOx is as a transboundary pollutant, and its conversion in the atmosphere to nitric acid (HNO3) vapour and nitrate particles which are deposited directly or in precipitation many hundreds of km from sources. NOx is also a key precursor for ozone production in the atmosphere. Emission controls (in addition to those set out to protect human health) are driven by its role as an ozone precursor rather than because of its direct effects. However, direct effects may occur in the immediate vicinity of major roads and in urban areas, caused by high NOx emissions from vehicles. NOx may also lead to ground flora changes related to eutrophication.

Nitrogen oxides are known to have greater adverse effects in the presence of SO2 or O3, and hence the critical level should apply where these pollutants are also close to their critical level.

A report prepared for English Nature by Bignal et al 2004 summarised the plant responses to the exposure of the motor vehicle exhaust and motor vehicle pollutants under controlled conditions (Table 1).

Table 1: Summary of plant responses to exposure to motor vehicle exhaust and motor vehicle pollutants under controlled conditions. This table shows the responses to NO2 and NO.

+ or - is an increase or decrease in the response, 0 is no effect. The results are based on a literature review of studies. (Derived from Bignal et al. 2004).





 +, -, 0


Visible injury

 +, 0



 +, -, 0

 +, -, 0

NaR or NiR enzyme activity

 +, -


Ethylene production



Leaf/tissue chemistry - nitrogen

+, -, 0


Chlorophyll concentration



Davies et al (2007) found a significant inverse relationship between the diversity of epiphytes and NOx concentrations in London. Diversity declined where NOx exceeded 70 µgm-3 and NO2 exceeded 40 µgm-3. It should be noted that ability to utilise the additional nitrogen input from motor vehicle pollution appears to be species specific.

Fuentes and Rowe (1998) showed similar relationships in Seville, Spain, where there was a strong correlation between lichen diversity and NO2 concentrations. 77.6% of the variability in the zonation established from air purity is attributed to the strong effect of NO2 on the lichen flora sampled.

A number of road-side experiments have also looked at the effect of NOX on trees. Sauter et al. (1987) exposed spruce for 20 weeks within a distance of 5 m from the edge of a motorway which   showed advanced degradation of waxes compared to the control (10% of the NOx concentrations). One third of the stomata were structurally obstructed in the polluted exposure plants.

Pollutant type and risk

Type of N deposition

Form of N

Risk areas

Dry deposition


NOx as NO2 or NO

  • Habitats adjacent to motorways and large highways (within 1km of the road)
  • Habitats in and around urban centres

The measured NO2 concentrations across the UK highlight the predominance of traffic and urban sources, with the largest concentrations in the large conurbations and adjacent to the motorway network, with annual mean concentrations in excess of 20 µg m-3 in these areas. Urban centres are hot spots due to the high density of traffic emissions and the smaller contribution from the residential and commercial sectors, as well as other area sources such as mobile machinery (e.g. excavators, bulldozers, and front and back loaders) (RoTAP, 2012). However, in rural areas, away from industry and roads NOx concentrations are usually low and not a concern to ecosystems.

Indicators of NOx impacts

Biochemical changes have only been used as additional indicators of potentially relevant ecological responses. In an ecological context, growth stimulation and reduction are both potentially negative responses. For instance, NOx (and NH3 and NH4 +) generally cause an increase in the shoot:root ratio, which may or may not be beneficial (WHO, 2000).

Nitrate reductase may be an effective indicator of NOx uptake by coniferous trees, as the two appear to be linearly related (Ammann et al. 1995)

Examples of species specific responses




Epiphytes on Fraxinus excelsior (European ash)


Davies et al. (2007)

What factors modify NOx impacts?

Responses to nitrogenous pollutants can be further modified and exacerbated by interactions with other environmental factors, including frost, drought and pest organisms. These interactions generally include increased susceptibility to these factors, which may in turn lead to major ecological changes.

Nitrogen oxides are known to have greater adverse effects in the presence of SO2 or O3, and hence the critical level should apply where these pollutants are also close to their critical level.

Environmental limit: 

Habitat/ Ecosystem Type Critical Load/ Level Status Reliability Indication of exceedance Reference
all vegetation categories

30 µg NOX (as NO2) m-3 annual mean; 75 µg NOX (as NO2 ) m-3 24-hour mean

UNECE 2004 Uncertainty: quite reliable i.e. the results of some studies are comparable

The concentration units are referenced as if all the NOX were in the form of NO2. (see Unit Conversion). The level for NOX should only be applied where levels of SO2 and O3 are close to their critical levels.



Ammann, M.; Ballmoos, P.; Stalder, M.; Suter, M.; Brunold, C. 1995 Uptake and assimilation of atmospheric NO2-N by spruce needles (Picea abies): a field study Water, Air and Soil Pollution 85 1497-1502
Ashmore, M.R.; Wilson, [Eds.] R.B. 1994 Critical levels of air pollutants for Europe Report of the Egham workshop
Bignal, K. ; Ashmore, M. ; Power, S. 2004 The Ecological Effects of Diffuse Air Pollution from Road Transport. English Nature Research Report
Davies, L.; Bates, J.W.; Bell, J.N.B.; James, P.W.; Purvis, O.W. 2007 Diversity and sensitivity of epiphytes to oxides of nitrogen in London Environmental Pollution 146 299-310
Sanders, G.E.; Skarby, L.; Ashmore, M.R.; Fuhrer, J. 1995 Establishing critical levels for the effects of air pollution on vegetation Water, Air and Soil Pollution 85 189-200