Ozone :: Grasslands

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;
• Alterations in numbers and timing of flowering and seed production;
• Alterations of response to other environmental stresses such as drought stress
• 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 most 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 and leaf-fall.

Overview: evidence, processes and main impacts

Effects of ozone on grasslands have been studied using both individual species and plant communities, although many studies have been short-term (< 3 years). 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.

Ozone exposure studies have shown that ozone-specific visible leaf injury symptoms can occur for some species e.g. Trifolium repens (Bungener et al., 1999; Wilbourn et al., 1995). 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). 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 grassland communities, indicating that elevated ozone conditions could contribute to changes in species composition. However, it can be 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. Usually these pores close during drought conditions so that the plants do not use as much water. However, 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), where the stomata did not close as expected, implying that soil drying during a prolonged drought would be further exacerbated by ozone pollution. 

A concentration-based critical level was originally 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. An AOT40 (Accumulated ozone concentration over a threshold of 40 ppb in daylight hours) of 5000 ppb.h, accumulated over six months was established for grasslands dominated by perennial species, and 3000 ppb.h accumulated over three months for grasslands dominated by annual species (see critical levels table), based on the concentration of ozone in the atmosphere at the top of the plant canopy (LRTAP Convention, 2011). The applicability of this concentration to protect grassland communities was verified based on exposure-response experiments (documented in LRTAP Convention., 2011).

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 (LRTAP Convention, 2011; Mills et al., 2011). 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 because: i) effects of ozone in ambient air on these species are widespread across Europe; ii) ozone exposure experiments have confirmed that these species are amongst the most sensitive to ozone of those tested in Europe (e.g. Hayes et al., 2007); and iii) Trifolium species have an important role as nitrogen fixers within grassland ecosystems. The critical level was determined for a 10% reduction in biomass of Trifolium species, the lowest percentage biomass reduction that was statistically significant (see critical levels table).

A multi-species, multi-level canopy ozone flux model for perennial grasslands (Emberson et al., pers. comm.) was included in the review process, but was considered insufficiently robust at that time for application across all of Europe due to the large ranges in species present, leaf area index’s and management practices across the region and the limited amount of associated effects data. Hence, the flux-based critical level for Trifolium species is currently the best one available to protect grassland species.

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.

In the UK large areas of grassland 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 grasslands are in areas where ozone concentrations are moderate to high (>4750 ppb.h, based on 1999-2003 values), whereas this is at least 10 to 25% (depending on grassland 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. 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.

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 grassland communities that are more recently formed are more responsive to ozone than older, established grassland communities (Bassin et al., 2007), which may be because the component species are growing faster due to reduced competition and increased nutrients available to each plant.

Critical levels:

Flux-based critical levels

Only experiments conducted in Europe under semi-controlled conditions have been considered for critical level derivation, in which the selected species were growing in competition with other grassland species (Temperate perennial grasslands) and as single species. Critical levels were determined for a 10% reduction in the effects that could change ecosystem dynamics (CLRTAP, 2017).

1 A: Atlantic; B: Boreal; C: Continental, S: Steppic, P: Pannonian; M: Mediterranean. Derived for species growing in regions not in brackets, but could also be applied to regions in brackets.
2 Represents the (POD1SPEC – Ref10 POD1SPEC) required for a x% reduction
3 Calculate the % reduction using the following formula: (POD1SPEC – Ref10 POD1SPEC) * potential maximum rate of reduction.

Concentration-based critical levels

Concentration based critical levels are based on accumulation of the hourly mean O₃ concentration at the top of the canopy over a threshold concentration of 40 ppb during daylight hours (when global radiation is more than 50 W m^-2.

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