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Vol. 13 No. 2 - April 2007
‘Golden Jubilee Number’

Effect and Risk Assessment of Ozone Air Pollution on
Forest Vegetation in Switzerland

By: Marcus Schaub*


Many studies investigating the negative impacts of ozone on biomass production and physiological functions have demonstrated the relationships between ozone exposure and reductions in both growth and physiological gas exchange. Such studies have led to an increasing interest in effects of ozone exposure expressed as a critical cumulative exposure of 10’000 ppb hrs above the threshold of 40 ppb (AOT40). Ozone effects on plants mainly depend on atmospheric transport and stomatal uptake. Thus ozone risk assessments should not only use measured ozone concentrations, but should also account for the influence of atmospheric conditions and soil moisture on stomatal conductance and non-stomatal ozone deposition. Following the Level II approach, our on-going studies aim to provide a model to estimate the ozone flux for forest ecosystems throughout Switzerland.


Ozone pollution leaves no elemental residue that can be detected by analytical techniques. Therefore, visible injury on the foliage of broad leaf and needle bearing species is basically the only easily detectable evidence of ozone-induced injury in the field. The evidence we have today strongly suggests that ozone occurs at concentrations which cause visible foliar injury to sensitive plants. Even though visible injury does not include all the possible forms of injury to trees and natural vegetation (e.g. pre-visible physiological changes, reduction in growth, etc.), observation of typical symptoms on aboveground plant parts in the field has turned out to be a valuable tool for the assessment of the impact of ambient ozone on sensitive plant species.

During the 1990s, the potential impact of ground-level ozone on plants and human health has come into focus within the European co-operation on reductions of air pollution emissions within the United Nations Economic Commission for Europe (UNECE) and the European Union (EU). This has led to a request from policy-makers to the scientific community for quantitative information concerning ozone effects. The International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests’ (ICP-Forests) operating under the UNECE Convention on Long-range Transboundary Air Pollution (LRTAP) requires the assessment, validation, and mapping of visible ozone injury on the Long-term Forest Ecosystem Research plots (Level II plots). Furthermore, the Task Force of ICP-Forests agreed that data on air

concentrations of tropospheric ozone need to be implemented in the ICP-Level II monitoring.

International framework

In 1997, based on ten years of monitoring forest condition in Europe for estimating direct effects by air pollution on forest trees, the UN/ECE with the LRTAP has concluded that ozone is the main air pollutant to be considered. The Task Force of ICP-Forests further agreed in June 1999 – based on a report prepared by the working group for ‘Ambient Air Quality’ of the expert panel on deposition – that data on air concentrations of ozone need to be implemented in the ICP-Level II monitoring.

In December 1999, ministers from more than 20 countries signed the UN/ECE Gothenburg Protocol to abate acidification, eutrophication and ground-level ozone. A revision on the UN/ECE Gothenburg protocol is expected to take place around 2004-2005. The Sub-Manual of ‘Ambient Air Quality’, Part II on The Assessment of Ozone Injury on European Forest Ecosystems, was adopted at the 17th Task Force meeting in Ennis, Ireland in May 2001 and first coordinated assessments of visible ozone injury were conducted across Europe during the season of 2001.

State of the Art

Current levels of tropospheric ozone have been shown to cause damage to forest trees, agricultural crops and semi-natural vegetation. A rise in ozone concentrations has steadily occurred over the past decades, and as a result of the likewise continuing rise in the emission of precursor pollutants of nitrogen oxides and reactive hydrocarbons further increases in ozone are expected in many parts of the world. In order to address this, the UN/ECE has adopted an effects-based approach, using the critical loads/levels concept. This effects-based research has resulted in the establishment for critical levels for meeting the Level I standard. The current critical levels are expressed as cumulative ozone exposures using the index AOT40, i.e. the sum of hourly ozone concentrations above a cut off of 40 ppb (nl l-1) during daylight hours when global radiation exceeds 50 Wm-2. The critical level for ozone above the threshold of 40 ppb has been established in the range of 10 ppm.h for a seasonal period of 6 months during the daylight hours to protect ozone-sensitive forest tree species from a 10% biomass loss. This critical level is based on a significant regression of biomass decrease in 0-3 year-old Fagus sylvatica L. versus AOT40, established by normalizing data from three separate open-top chamber projects. However, it remains to be determined, whether other deciduous tree species show a similar biomass decrease. Furthermore, it needs to be tested whether a possible biomass decrease with other tree species is indicated by visible foliar injury on a macroscopic as well as on a micro-morphological level, and by changes within their physiological gas exchange process.

It is widely acknowledged that the impacts of ozone are more closely related to the ozone dose absorbed through the stomata than to ozone exposure in the atmosphere. The Level I standard is attractive because of its simplicity, but it is limited by the fact that any factor which may influence a plant’s response to ozone is largely ignored. These include but are not limited to plant and site-specific characteristics such as soil moisture content, vapor pressure deficit, wind speed, radiation, and temperature. Hence, a realistic estimate of the actual ozone dose and its impact is not feasible using the Level I approach. In 1999, it was agreed that ozone flux, the instantaneous rate at which surfaces such as leaf- and soil surfaces, and stomatal openings absorb ozone, would lead to the biologically more relevant estimates of ozone risks. This Level II approach proposed several additional environmental parameters to be included due to their modifying influence on tree responses. It was also stated that there was a scarcity of data upon which to base an ozone dose-response function, especially for Level II plots. Thus, the development of a realistic flux-based approach should be the long-term goal of our Level II investigations in order to gain additional data for an accurate ozone risk assessment.

There is evidence that ambient ozone concentrations as they are monitored in Europe can cause a variety of effects to vegetation such as visible foliar injury, growth and yield reductions, and altered sensitivity to biotic and abiotic stresses. Our surveys at the Level II plots throughout Switzerland and within the WSL open-top chamber research facility at Lattecaldo, southern Switzerland have recorded ozone-like symptoms on numerous native tree, shrub, and forb species since 1997.

Open-top chamber studies (filtration studies) and research using Continuously Stirred Tank Reactors (CSTR’s) (fumigation studies) have confirmed that ozone is the cause of the visible injury seen on seedlings of a variety of plant species. We have found that seedlings varied markedly in symptom severity and several species were found to develop ozone-induced symptoms at exposures below the current short- and long-term European air quality standards. However, there is still a lack of adequately documented evidence of ozone-induced foliar injury to tree and shrub species across the whole of Europe. Using the biomonitoring approach while looking for symptoms on native ozone-sensitive plant species, is the main objective of the ICP-Forests Sub-Manual to provide information on the distribution of ozone injury within the European forest ecosystems.

Recent studies have shown that increasing ozone concentrations not only have a negative effect on wood production, but may also lead to unstable conditions in forest ecosystems, which could result in a lowered adaptive capacity to new stresses in the future. Long-term effects of elevated ambient ozone concentrations on trees may weaken the function of forest ecosystems with respect to water and energy balances and soil protection against erosion particularly in the alpine regions. Some of the most important impacts may be the possible shifts in species composition and loss of biodiversity especially in areas with large numbers of endemic plant species with unknown sensitivity to ozone. However, much more detailed and defined site and species exposure/response research is required in order to confirm such hypotheses.

Environmental factors

The negative impacts of ozone on biomass production and physiological functions have been investigated in a number of experiments. Many of these experiments demonstrated relationships between ozone exposure and reductions in both growth and physiological gas exchange. Studies investigating physiological sensitivity to ambient ozone concentrations indicate that ozone stress may reduce carbon fixation, increase foliar and root respiration, shift the partitioning of carbon into different chemical forms, and disrupt carbon and nutrient allocation patterns. There is general agreement that ozone must enter the leaf interior through stomata to cause leaf tissue injury. Therefore, stomatal regulation must be an important factor in controlling ozone sensitivity of plants. The stomata also control carbon uptake, a crucial process for plant growth. When plants close their stomata, thereby avoiding further ozone uptake, they are additionally stressed through the detrimental effects on both plant growth and photosynthate allocated towards the repair of cells injured during ozone exposure. Therefore, it has been suggested that the ratio of net photosynthesis and stomatal conductance is a better indicator of plant sensitivity to ozone than either parameter alone.

Changes in environmental conditions such as light, temperature and humidity influence stomatal aperture and hence also the potential for ozone flux. It has been demonstrated that foliar injury responses were associated with site differences; trees on wetter sites experienced higher visible foliar injury compared to trees on drier sites with two thirds of the trees on wet sites being consistently more symptomatic than the trees on the dry sites. Various other studies have shown greater visible ozone injury on plants grown in moist soil over those grown in drier soil. Differences in the soil water regimes within an OTC-study with four-year-old seedlings of Prunus serotina L., Fraxinus americana L., and Acer rubrum L. seems to be a controlling factor in affecting interactions with ambient ozone and subsequent physiological differences leading to alterations in ozone uptake. Stomatal conductance has also been reported to be affected by tree age and tree size. However, these findings support the hypothesis that negative correlation between ozone uptake and net photosynthesis indicates that water stress and nutrient deficiency may prevent ozone injury by reducing stomatal conductance and ozone uptake. Making matters even more complicated, these findings also confirmed the hypothesis that greater visible foliar injury in the lower versus upper crowns of dominant black cherry trees and saplings was a result of higher ratios of ozone uptake and net photosynthesis when measured within the lower-crown which had shaded leaves.

In conclusion, it is widely accepted that the general receptor-specific maximum leaf conductance is modified by environmental and phenological parameters: soil factors, such as soil moisture deficit and irrigation, plant development factors such as phenological stages, and factors influencing the instantaneous ozone uptake by plants, including temperature, leaf-to-air vapor pressure deficit, global radiation, wind speed and positions within mature tree crowns. In order to improve our ability to identify areas potentially at risk for ozone impacts, an extension of the existing database and a more detailed understanding of the flux-response relationships are needed.

Below-ground plant response

A number of studies have demonstrated that ozone would adversely affect net photosynthetic rates. Consequently, physiological imbalances in the above-ground parts of the trees exposed to elevated ozone may alter below-ground carbon processes. However, quantification of below-ground pools and fluxes of C has proven to be a difficult task, because of the high spatial heterogeneity and difficulty of observation within the soil.

Decreased allocation of photosynthates to the roots is assumed to be responsible for a reduced root growth, a change in the root-to-shoot ratio and decreased fine root production in various tree species under elevated ozone exposures. Furthermore, a reduced supply of photosynthate to the roots was suggested as a consequence of decreased soil respiration of field-grown aspen trees under ozone exposure. A decline of root exudation of organic compounds due to reduced translocation of photosynthate to the roots under ozone exposure could result in a reduced nutrient supply to soil microorganisms, leading to a lower microbial metabolism and a lower total soil respiration. However, it is still unclear whether the decrease in total soil respiration is mainly attributed to the reduced heterotrophic (microbial) respiration or to the reduced autotrophic (root) respiration, or to both respectively or interactively. It is difficult to distinguish between heterotrophic and autrotrophic respiration, but both components are dependent on the carbon sources transported from the above-ground biomass into the soil.

Since the utilization rate of photosynthates is reflected in the respiration rates above and below ground, more research should be directed towards a better understanding of respiration in the below-ground portions. The soil including roots and soil microorganisms not only play a major role in the carbon budgeting of the ecosystem but also in regulating water and nutrient supplies to the tree.

Large-scale risk assessment

It is often difficult to assess the sensitivity of mature trees to ozone, especially when considering the diversity of sites occupied by a species across its entire natural range. In addition, the accuracy of using seedlings to predict responses of mature trees and forest ecosystems to ozone is questionable, considering the long life span of trees and the phenological differences. Although controlled experiments with seedlings have been valuable for revealing the ‘principles’ of ozone action on woody plants, these findings would lack ecological significance as long as validation at forest sites is missing. Differences in canopy structure and microclimate, exposure to ozone, gas exchange, and ransport of water and carbohydrates, nutrient allocation, and recurrent flushing between seedlings and mature trees are some of the complications that must be taken into account.

The current Level I critical level values, which are being used in developing European ozone control strategies both by UN/ECE and the EU, are intended to protect the most sensitive vegetation types under the most sensitive conditions. Exceedance of these critical levels, however, only provides an indication that some risk exists of negative effects to vegetation from ozone; the degree of exceedance cannot be used to provide a measure of the relative risk of negative effects to vegetation in different areas of Europe.

A risk assessment based on ozone flux to receptor sites within the leaf, rather than ozone exposure, could provide an improved estimate of the relative degree of risk of ozone damage to vegetation across Europe. Existing models of ozone flux require detailed micrometeorological information, and are applicable to only a limited number of species, and cannot be readily applied at a large geographical scale. Alternatively, models designed to model ozone deposition on a regional scale have a limited description of the stomatal responses, which are not species-specific and which do not consider the effects of vapor pressure deficit and soil moisture deficit. By limiting the existing data pool to hardwood species, 81% of the variation in observed photosynthetic response could be explained by cumulative ozone uptake alone. Given the strength of this relationship and uncertainties surrounding additional sources of variation, examining its implications across natural forested landscapes represents a valuable way to advance current understanding of ozone effects, even while recognizing the limitations of its simplicity.

Summary and conclusions

It is widely accepted that tropospheric ozone concentrations are expected to increase in the near future. While ozone effects and the underlying mechanisms on plant tissue and single plants are reasonably well understood, there is still a wide gap of knowledge and understanding as to how air pollution in general and ozone in specific, affects forest ecosystems. Furthermore, considering the expected increase of the global average surface temperature and the simultaneous alteration in precipitation patterns, the air pollution effects, and in particular the effects ascribed to tropospheric ozone, may change as well. The application of mechanistic models combined with the respective future findings based on detection, monitoring and evaluation should allow us a more profound understanding to estimate a better risk assessment for forest ecosystems.

* Swiss Federal Institute for Forest, Snow and Landscape Research WSL, 8903, Birmensdorf, Switzerland - e-mail: marcus.schaub@wsl.ch

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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