Ozone :: Bogs, wetland and heath


Effects and Implications

  • Visible leaf injury and/or premature leaf die-back, which could result in reduced growth during a growing season;
  • Reduced growth of sensitive species;
  • Potential alterations of response to other environmental stresses, for example drought stress in heaths;
  • Alterations in emissions of greenhouse gases such as methane, but the direction of change is not conclusive;
  • 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 leaf-fall.

Overview: evidence, processes and main impacts

Effects of ozone have been studied using both individual species and plant communities, although many studies have been short-term (< 3 years). Ozone is very reactive; the reactive oxygen (molecular) species that form as ozone enters a leaf can 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.

Ozone exposure studies have shown that ozone-specific visible leaf injury symptoms can occur for some species e.g. Eriophorum anugustifolium, Vaccinium myrtillus and Potentilla erecta (Mortensen, 1994; Hayes et al., 2006). Enhanced leaf die-back has been shown for Molinea caerulea and Juncus subnodulosus (Williamson, 2009). Species can show reductions in biomass in response to elevated ozone 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). 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 bog, wetland and heath communities, indicating that elevated ozone conditions could contribute to changes in species composition. However, it is to predict and generalise about which groups of species are most ozone-sensitive.

Recently it has been suggested that ozone pollution may affect emissions of greenhouse gases e.g. methane from wetland communities. Many plants that are adapted to living in waterlogged conditions where methane formation occurs, have vascular tissue called aerenchyma that allows oxygen to reach the roots deep in the oxygen-free substrate (Greenup et al. 2000; Roura-Carol and Freeman 1999). An input of oxygen to the roots may inhibit the action of methane-forming bacteria and therefore impacts of ozone on these vascular plants can influence methane production. However, to date the limited experimental work has shown mixed results on methane production in response to ozone, with some showing increased methane production (e.g. Williamson 2009; Niemi et al., 2002), some showing reduced methane production (e.g. Toet et al., 2011) and some showing no change (e.g. Morsky et al., 2008, Kanerva et al., 2007).

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 including bogs, wetlands and heaths (see critical levels table).

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. Species of the genus Trifolium (clovers) were selected from the many species considered (see grasslands). The flux-based critical level for Trifolium is also considered to be applicable to other natural vegetation communities including bogs, wetlands and heaths; there is a lack of data to establish flux-based critical levels for typical species in these habitats (see critical levels table).

Pollutant type and risk

Ozone is a naturally occurring chemical in the troposphere. 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 released due to anthropogenic emissions (particularly from transport). Ozone concentrations are usually highest in rural and upland areas downwind of major conurbations.

In the UK large areas of bogs, wetland and heath coincide with areas where the ozone exposure is highest. It has been estimated that in England and Wales at least 75% of BAP priority habitat bogs, wetland and heath are in areas where ozone concentrations are moderate to high (>4750 ppb.h, based on 1999-2003 values), whereas this is at least1-25% (depending on habitat type) in Scotland (Morrisey et al., 2007). 

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. 

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.




Molinea caerulea

Increased leaf senescence (die-back)

Williamson, 2009

Juncus subnodulosus

Increased leaf senescence (die-back)

Williamson, 2009

Sphagnum (angustifolium, magellanicum and papillsum)

Decreased chloroplast area within cells of the capitulum

Rinnan et al., 2004

Eriophorum angustifolium

Visible leaf-injury

Mortensen, 1994, Hayes et al., 2006

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). It has been suggested that bog and wetland species may be more sensitive to ozone pollution in field conditions than some species of drier habitats because soil moisture is rarely limiting in these communities and therefore stomatal pores remain open, allowing uptake of more ozone.

Critical levels

Flux-based critical levels (Mills et al., 2011, 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 bogs, wetland and heath 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.



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