Surveys for ozone injury

Ozone pollution can cause visible injury to develop on the leaves of sensitive plant species such as legumes. Typically injury is present on the older leaves as small bronze, brown, or yellow flecks on the upper surface (see photos). In severe cases, the flecks can join to form large lesions covering most of the leaf surface. Ozone can also cause leaves to turn yellow and die-off earlier than normal.



Ozone injury on white clover, brown knapweed and violet respectively.


One of the roles of the ICP Vegetation has been to monitor the frequency of occurrence of injury-causing ozone episodes in Europe. Annual biomonitoring experiments have shown that ozone injury has occurred at least once on clover species at every site since the monitoring network was established over fifteen years ago. At more than half of the sites, injury has occurred during each 28 day interval assessed during the summer months, in every year since 1994. The highest ozone injury scores are usually found at sites in western and eastern Mediterranean, with the lowest scores occurring at sites in Atlantic Central Europe and northern Europe (see Figure).


Median ozone leaf injury score on white clover (1998– 2006) in five climatic regions of Europe. NE, Northern Europe; ACE, Atlantic Central Europe, CCE, Continental Central Europe; WM, Western Mediterranean and EM, Eastern Mediterranean. Ozone injury score: 1 = <1% of leaves affected, 2 = 1-5%, 3 = 5-25%, 4 = 25-50%, 5 = 50-90% and 6 = 100%.

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Studying ozone damage to crops

The participants of the ICP Vegetation study the effects of ozone on crops using a variety of ozone exposure methods (e.g. closed chambers, open-top chambers, solardomes, field release systems). Their results are collated at the Programme Coordination Centre. Participants are studying ozone damage to crops such as:

  • Injury to the leaves which can cause a loss in value in e.g. salad crops (see photos).
  • Reductions in the weight of seeds e.g. grain yield in wheat.
  • Changes in crop quality e.g. potato processing characteristics.
  • Cumulative yield reductions over several seasons for perennial crops such as grape vine.


Ozone injury on lettuce, tomato, and soybean.


A comprehensive analysis of the published data from ozone exposure experiments conducted at the Programme Centre has indicated that some of the most sensitive crops are wheat, soybean, cotton, pulses, water melon, lettuce, tomato (Mills et al., 2007a). Participants from approximately fifteen countries per year have conducted biomonitoring experiments with ozone-sensitive and –resistant biotypes of white clover. These experiments have indicated that the ambient ozone climate has the potential to reduce growth in many countries, especially those in southern and central Europe (see photograph). Many participants are currently studying the factors (climate, soil conditions and plant growth stage) influencing the uptake of ozone through the pores on leaf surfaces. A new biomonitoring system with green beans has been developed in the USA using ozone-sensitive and –resistant clones. This new biomonitoring system is currently being tested at several ICP Vegetation sites.


Ozone –sensitive (left) and ozone-resistant (right) white clover after exposure to ambient ozone for four weeks in Greece.

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Studying ozone damage to (semi-)natural vegetation

Ozone damage to natural and (semi-)natural vegetation is much harder to study because of the complexity of natural ecosystems. Through controlled ozone exposure experiments, participants are studying effects such as:

  • Early-die back of leaves.
  • Reduced seed production.
  • Reduced growth of over-wintering organs.
  • Reductions in competitive ability and resistance to stresses.
  • Consequences of ozone impacts on vegetation for concentrations of greenhouse gasses in the atmosphere and carbon sequestration.


Early die-back in Viola lutea exposed to ozone


The ICP Vegetation Programme Centre has analysed published data on the responses of individual species to ozone (Mills et al., 2007b) and has identified the following communities to be at risk of damage from the pollutant:

  • Dry grasslands
  • Mesic grasslands
  • Alpine grasslands
  • Woodland fringes
  • Dehesa grassland
  • Heathland


These results are primarily based on the outcome of chamber studies, field-based studies on plant communities are required to validate these results. Studies are ongoing to identify how growth in a naturally occurring competitive environment modifies the sensitivity of individual species, and how this information can be used to improve predictions of vegetation at risk.  Participants are also studying the factors influencing ozone uptake in naturally occurring vegetation with the long-term aim of developing a complex dynamic model simulating changes in uptake with species.

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Impact of nitrogen deposition on ozone damage to (semi-)natural vegetation

Nitrogen availability has been identified as an important factor in the ozone sensitivity of (semi-)natural vegetation. Nitrogen availability has the potential to modify both ozone exposure and ozone uptake by vegetation. Although there are many potential mechanisms by which nitrogen and ozone interactions may arise, to date the body of knowledge on the impacts of nitrogen pollution on the ozone-sensitivity of communities of (semi-)natural vegetation is relatively small. Communities that may be sensitive to both elevated ozone and nitrogen deposition are:

  • Dry grasslands
  • Alpine grasslands
  • Heathlands

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Ozone critical levels for vegetation

During the 1980s, the Convention on Long-range Transboundary Air Pollution (LRTAP Convention) adopted the critical loads/critical levels approach to defining sensitivity of receptors to pollutants. Much progress has been made since then in establishing critical levels of ozone for vegetation, above which adverse effects could be expected. Several Convention workshops have been held in which the critical levels of ozone for crops, (semi-)natural vegetation and forest trees were established.  Two main types of critical levels have been developed:

  • Concentration-based critical levels that are based on the concentration of ozone in the air immediately above the plant.
  • Flux-based critical levels that are based on the amount of ozone entering the leaves of plants. Ozone fluxes are calculated using knowledge of how climate, soil factors and plant factors influence the opening and closing of the stomatal pores on the leaf surface. 

In addition, methods have been developed for mapping the risk of damage to a generic crop and two generic tree species using a simplified flux model that can be applied within integrated assessment modelling.  Ozone critical levels, their scientific basis and generic flux -based risk assessments are described in detail in Chapter 3 of the LRTAP Convention Modelling and Mapping Manual. This manual was substantially revised in 2004 and has been updated with the decisions made at the most recent ozone critical levels workshop in Obergurgl (‘Critical levels of ozone: further applying and developing the flux-based concept’, Obergurgl, Austria, 15-19 Nov 2005) and discussions at following ICP Vegetation, ICP Forests and ICP Modelling and Mapping Task Force Meetings.


Ozone critical levels can be found in Chapter 3 “Mapping critical levels for vegetation” of the LRTAP Convention Mapping Manual.

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Evidence of ozone damage to vegetation in Europe

The ICP Vegetation has been collating and analysing all available information in Europe on incidences of damage to vegetation caused by ambient ozone pollution during the period 1990 - 2006. The results were presented in the Evidence Report. Types of evidence include:

  • Results of ICP Vegetation biomonitoring experiments with clover species.
  • Nationally-funded surveys and biomonitoring activities.
  • Experimental comparisons of responses in ambient ozone and filtered air.


The report provides summary of the adverse effects of ozone on vegetation across Europe during the period 1990 to 2006 and compares these effects with modelled ozone concentration and flux data. Although many experimental studies have demonstrated that current ozone concentrations have the potential to impact on crops and (semi-)natural vegetation, until now there has been no comprehensive assessment of ozone effects in fields of crops and natural ecosystems. This report has established that effects of ambient ozone on crops and (semi-)natural vegetation are actually occurring in the field, with leaf injury and reductions in biomass or crop yield developing under ambient ozone conditions. Compared with ozone concentration maps, maps of stomatal fluxes of ozone were found to be better at predicting the occurrence of ozone damage on vegetation. At about one third of the sites where adverse effects of ozone on vegetation were observed, ozone concentrations were below the current critical level. These sites were mainly located Northern, Atlantic and parts of Continental Central Europe, i.e. regions in Europe where climatic conditions were conducive to considerable ozone fluxes.


EMEP modelled mean ozone concentration-based (left) and generic flux-based metric (right) for crops for 1995 to 2004, superimposed with the location of all sites where an effect of ozone has been detected during those ten years. Map data source: EMEP/MSC-West

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