Ozone :: Coastal and Rocky Habitats


Effects and Implications

  • Visible leaf injury and/or premature leaf die-back, which could subsequently result in reduced growth during a growing season;
  • Reduced growth of sensitive species;
  • Potential alterations in numbers and timing of flowering and seed production;
  • Alterations of response to other environmental stresses such as drought stress;
  • Potential enhanced susceptibility to pests and diseases.

Habitat structure could be altered via changes in growth of species, particularly as the extent of growth reduction would vary between species. There may be seasons in which the structure is more influenced as changes in leaf die-back could result in a more open canopy.  In addition, habitat function could be impacted via changes in timing of flowering, seed production and leaf-fall. In some rocky habitats soil is stabilised by plant roots. It is therefore possible that reduced root growth in response to ozone could reduce colonisation by plants in rocky areas.

Overview: evidence, processes and main impacts

Ozone is very reactive and the reactive oxygen (molecular) species that form as ozone enters a leaf can quickly cause damage to cell components e.g. cell membranes. Reactive oxygen species are also produced by plant cells in many processes involved in growth and development and from abiotic stresses, e.g. high light and drought. This is one of the reasons that some symptoms of ozone pollution are not specific to ozone.

There is a small amount of information about species of coastal and rocky habitats. Much more information is known available for grassland species (some of these species also occur in coastal and rocky habitats). Ozone exposure studies have shown that ozone-specific visible leaf injury symptoms can occur for some species e.g. Vaccinium myrtillus (Mortensen, 1994) and Potentilla erecta (Hayes et al., 2006). Species can show reductions in biomass in response to elevated ozone (Hayes et al., 2006; Thwaites et al., 2006) and alterations in partitioning of biomass between different parts of the plant, for example leaf biomass can sometimes be maintained at the expense of roots, particularly if leaves are damaged following ozone exposure and new leaves are developed to replace these (e.g. Cooley and Manning, 1987; Franzaring et al., 2000; Reiling and Davison, 1992).

Elevated ozone has also been shown to have carry-over effects the following spring in species that did not respond to summer ozone exposure, with changes in growth and flower number observed (Hayes et al., 2006). Reduced flower numbers with increasing ozone exposure have been observed for some species including Campanula rotundifolia (Hayes et al., 2012). The date of peak flowering was earlier with increasing ozone exposure for Lotus corniculatus (Hayes et al., 2012). In addition, ozone can affect seed production, viability (Black et al., 2000) and germination (Bender et al., 2006; Bergmann et al., 1999). There is a wide range in sensitivity to ozone of the component species of coastal and rocky communities, indicating that elevated ozone conditions could contribute to changes in species composition. However, it is difficult to predict and generalise about which groups of species are most ozone-sensitive.

It has been demonstrated that ozone can impair the functioning of leaf pores (stomata) responsible for gas exchange (e.g. water, carbon dioxide, ozone) between the plant and the atmosphere. A reduced response to drought has been shown for Leontodon hispidus (Mills et al., 2009) and Ranunculus acris and Dactylis glomerata (Wagg et al., 2013), implying that soil drying during a prolonged drought would be further exacerbated by ozone pollution. 

A concentration-based critical level was established for the protection of the vitality of pasture and fodder quality in productive perennial grasslands, and the vitality of natural species in perennial grasslands of high conservation value. This critical level is also considered to be applicable to other natural vegetation communities and it is considered that the critical level may also be appropriate for coastal and rocky vegetation communities (see critical levels table), based on current knowledge, although there is currently no specific information for this habitat.

More recently it has been shown that the impacts of ozone depend on the amount of the pollutant reaching the sites of damage within the leaf and stomatal (leaf pore) flux-based critical levels were developed to address this. The flux-based critical levels for effects on (semi-)natural vegetation are based on flux to the upper canopy leaves of individual species frequently found in grassland communities across Europe, i.e. species of the genus Trifolium (clovers), and are also considered to be relevant to vegetation of coastal and rocky habitats (see critical levels table). 

Pollutant type and risk

Ozone is a naturally occurring chemical in the lowermost layer of the Earth’s atmosphere. Natural sources of the precursors of ozone such as oxides of nitrogen and non-methane volatile organic compounds ensure that there is always a background concentration of ozone.  Additional ozone is formed from complex photochemical reactions of precursors, which include NOx, carbon monoxide and non-methane volatile organic compounds, released due to anthropogenic emissions. Ozone concentrations are usually highest in rural and upland areas downwind of major conurbations.  However, some regions with more moderate ozone concentrations also have high risk of effects where climatic conditions favour ozone uptake by the plants.

Indicators of ozone impacts

These can be difficult to identify in ‘field’ conditions. Often the symptoms of ozone injury are those of a general stress response, and in addition a rapid turnover of damaged leaves can make attribution to ozone pollution difficult. Some species exhibit ozone-specific visible leaf injury symptoms in controlled studies and these ‘typical’ symptoms of visible leaf injury attributed to ozone have occasionally been observed in natural conditions. Legumes are one of the species groups on which these symptoms are most likely to be observed.

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.




Armeria maritima

Reduced shoot biomass

Hayes et al., 2006

Festuca rubra

Reduced biomass

Thwaites et al., 2006

Campanula rotundifolia

Reduced flower numbers

Hayes et al., 2012

Lotus corniculatus

Accelerated timing of flowering

Hayes et al., 2012

Molinea caerulea

Increased leaf senescence (die-back)

Williamson, 2009

Vaccinium myrtillus

Visible leaf-injury

Mortensen, 1994

Potentilla erecta

Visible leaf-injury

Hayes et al., 2006

What factors modify ozone impacts?

Climatic (e.g. temperature, humidity) and soil factors (e.g. water content) that influence stomatal uptake of ozone can influence ozone impacts in some species (see above section on stomatal fluxes). There is very little information on possible interactions between salinity and ozone. It is possible that plants of saline habitats are more sensitive to ozone than those of less saline habitats (Jones et al., 2007). Experimental studies on interactions between salt stress and ozone have tended to focus on crop plants (e.g. tomato, Maggio et al., 2007) rather than those native to saline habitats.

Critical levels

Flux-based critical levels (Mills et al., 2011b, LRTAP Convention, 2011)



(per cent reduction)



Critical level

(mmol m-2 PLA4)

Conservation grasslands (based on clover)1

Biomass (10%)



Concentration-based critical levels (LRTAP Convention, 2011)






Critical level

(ppm h)

(Semi-)natural vegetation communities dominated by perennials1

Growth reduction



1 Considered to be applicable for coastal and rocky habitats too.

2 POD1 = Phytotoxic Ozone Dose above a flux threshold of 1 nmol m-2 PLA s-1.

3 AOT40 = Accumulated ozone above a threshold of 40 ppb during daylight hours.

4 PLA = Projected leaf area.

Environmental limit: 

Habitat/ Ecosystem Type Critical Load/ Level Status Reliability Indication of exceedance Reference
Semi-natural vegetation

AOT40 3000ppb hours over 3 months or AOT40 5000ppb over 6 months

UNECE, 2010 expert judgement i.e. only limited or no data are avaliable for this type of receptor

AOT40 is the Accumulated concentration Over a Threshold of 40 ppb. If an hourly average ozone concentration exceeds 40 ppb the difference between the concentration and 40 ppb is added to a running total. The units are therefore ppb multiplied by hours. For natural vegetation, the AOT40 is summed for the daylight hours for a period of three months. Daylight hours are defined as when solar radiation exceeds 50 W m-2. The daylight hours are when plant stomata are normally open.

Flux-based critical levels, based on biomass reduction, are also available for local and regional assessment but are not yet incorporated into APIS. See critical levels chapter of the UNECE Mapping Manual.



Bender, J. ; Muntifering, R.B. ; Lin, J.C. ; Weigel, H.J. 2006 Growth and nutritive quality of Poa pratensis as influence by ozone and competition. Environmental Pollution 142 109-115
Black, V.J. ; Black, C.R. ; Roberts, J.A. ; Stewart, C.A. 2000 Impact of ozone on the reproductive development of plants. New Phytologist 147 421-447
Cooley, D.R. ; Manning, W.J. 1987 The impact of ozone on assimilate partitioning in plants – a review. Environmental Pollution 47 95-113
Franzaring, J.; A.E.G., Tonneijck ; Kooijman, A.W.M. ; Dueck, T.A. 2000 Growth responses to ozone in plant species from wetlands. Environmental and Experimental Botany 44 39-48
Hayes, F.; Mills, G.; Williams, P.; Harmens, H. ; Buker, P. 2006 Impacts of summer ozone exposure on the growth and overwintering of UK upland vegetation. Atmospheric Environment 40 4088-4097
Hayes, F.; Williamson, J. ; Mills, G. 2012 Ozone pollution affects flower numbers and timing in a simulated BAP priority calcareous grassland community. Environmental Pollution 163 40-47
Jones, L. ; Hayes, F.; Mills, G.; Sparks, T. ; Fuhrer, F. 2007 Predicting community sensitivity to ozone, using Elleberg indicator values. Environmental Pollution 146 744-753
Maggio, A. ; De Pascale, S. ; Fagnano, M. ; Barbieri, G. 2007 Can salt stress-induced physiological responses protect tomato crops from ozone damages in Mediterranean environments? European Journal of Agronomy 26 454-461
Mills, G.; Hayes, F.; Wilkinson, S. ; Davies, W.J. 2009 Chronic exposure to increasing background ozone impairs stomatal functioning in grassland species. Global Change Biology 15 1522-1533
Mills, G.; Pleijel, H.; Braun, S.; Buker, P.; Bermejo, V. ; Calvo, E. ; Danielsson, H. ; Emberson, L. ; Fernandez, Gonzalez ; Grunhage, L.; Harmens, H. ; Hayes, F.; Karlsson, P.E.; Simpson, D. 2011 New stomatal flux-based critical levels for ozone effects on vegetation. Atmospheric Environment 45 5064-5068
Mortensen, L. 1994 The Influence of Carbon Dioxide or Ozone Concentration on Growth and Assimilate Partitioning in Seedlings of Nine Conifers. Acta Agriculturae Scandinavica, Section B — Soil & Plant Science 44 157-163
Thwaites, R.H. ; Asmore, M.R. ; Morton, A.J. ; Pakeman, R.J. 2006 The effects of tropospheric ozone on the species dynamics of calcareous grassland. Environmental Pollution 144 500-509
Wagg, S. ; Mills, G.; Hayes, F.; Wilkinson, S. ; Davies, W.J. 2013 Stomata are less responsive to environmental stimuli in high background ozone in Dactylis glomerata and Ranunculus acris. Environmental Pollution 175 82-91