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Northern, Southern and Equatorial Africa have been identified as among the regions most at risk from very high ozone concentrations. Whereas we know that many crop cultivars from Europe, north America and Asia are sensitive to ozone, almost nothing is known about the sensitivity of staple food crops in Africa to the pollutant. In this study cultivars of the African staple food crops, Triticum aestivum (wheat), Eleusine coracana (finger millet), Pennisetum glaucum (pearl millet) and Phaseolus vulgaris (bean) were exposed to an episodic ozone regime in solardomes in order to assess whether African crops are sensitive to ozone pollution. Extensive visible leaf injury due to ozone was shown for many cultivars, indicating high sensitivity to ozone. Reductions in total yield and 1,000‐grain weight were found for T. aestivum and P. vulgaris, whereas there was no effect on yield for E. coracana and P. glaucum. There were differences in sensitivity to ozone for different cultivars of an individual crop, indicating that there could be possibilities for either cultivar selection or selective crop breeding to reduce sensitivity of these crops to ozone.

The potential to capture additional air pollutants by introducing more vegetation or changing existing short vegetation to woodland on first sight provides an attractive route for lowering urban pollution. Here, an atmospheric chemistry and transport model was run with a range of landcover scenarios to quantify pollutant removal by the existing total UK vegetation as well as the UK urban vegetation and to quantify the effect of large-scale urban tree planting on urban air pollution. UK vegetation as a whole reduces area (population)-weighted concentrations significantly, by 10% (9%) for PM 2.5 , 30% (22%) for SO 2 , 24% (19%) for NH 3 and 15% (13%) for O 3 , compared with a desert scenario. By contrast, urban vegetation reduces average urban PM 2.5 by only approximately 1%. Even large-scale conversion of half of existing open urban greenspace to forest would lower urban PM 2.5 by only another 1%, suggesting that the effect on air quality needs to be considered in the context of the wider benefits of urban tree planting, e.g. on physical and mental health. The net benefits of UK vegetation for NO 2 are small, and urban tree planting is even forecast to increase urban NO 2 and NO x concentrations, due to the chemical interaction with changes in BVOC emissions and O 3 , but the details depend on tree species selection. By extrapolation, green infrastructure projects focusing on non-greenspace (roadside trees, green walls, roof-top gardens) would have to be implemented at very large scales to match this effect. Downscaling of the results to micro-interventions solely aimed at pollutant removal suggests that their impact is too limited for their cost–benefit analysis to compare favourably with emission abatement measures. Urban vegetation planting is less effective for lowering pollution than measures to reduce emissions at source. The results highlight interactions that cannot be captured if benefits are quantified via deposition models using prescribed concentrations, and emission damage costs. This article is part of a discussion meeting issue ‘Air quality, past present and future’.

New critical levels for ozone based on accumulated flux through stomata (phytotoxic ozone dose, POD), for temperate perennial grassland (semi-)natural vegetation, have been agreed for use within the Convention on Long-Range Transboundary Air Pollution. These were based on data from several experiments conducted under naturally fluctuating environmental conditions that were combined and analysed to give linear dose-response relationships. Dose-response functions and flux-based critical levels were derived based on biomass and flower number. These parameters showed a statistically significant decline with increasing accumulated stomatal ozone flux. The functions and critical levels derived are based on sensitive species and can be used for risk assessments of the damaging effect of ozone on temperate vegetation communities dominated by perennial grassland species. The critical level based on flower number was lower than that for biomass, representing the greater sensitivity of flower number to ozone pollution.

Tropospheric (ground-level) ozone is a harmful phytotoxic pollutant, and can have a negative impact on crop yield and quality in sensitive species. Ozone can also induce visible symptoms on leaves, appearing as tiny spots (stipples) between the veins on the upper leaf surface. There is little measured data on ozone concentrations in Africa and it can be labour-intensive and expensive to determine the direct impact of ozone on crop yield in the field. The identification of visible ozone symptoms is an easier, low cost method of determining if a crop species is being negatively affected by ozone pollution, potentially resulting in yield loss. In this study, thirteen staple African food crops (including wheat (Triticum aestivum), common bean (Phaseolus vulgaris), sorghum (Sorghum bicolor), pearl millet (Pennisetum glaucum) and finger millet (Eleusine coracana)) were exposed to an episodic ozone regime in a solardome system to monitor visible ozone symptoms. A more detailed examination of the progression of ozone symptoms with time was carried out for cultivars of P. vulgaris and T. aestivum, which showed early leaf loss (P. vulgaris) and an increased rate of senescence (T. aestivum) in response to ozone exposure. All of the crops tested showed visible ozone symptoms on their leaves in at least one cultivar, and ozone sensitivity varied between cultivars of the same crop. A guide to assist with identification of visible ozone symptoms (including photographs and a description of symptoms for each species) is presented.

Recent decades have seen a changing temporal profile of ground-level ozone (O3) in Europe. While peaks in O3 concentrations during summer months have been declining in amplitude, the background concentration has gradually increased as a result of the hemispheric transport of O3 precursors from other world regions. Ground-level O3 is known to adversely affect O3-sensitive vegetation, including reducing the yield of O3-sensitive crops such as common wheat (Triticum aestivum L.). The reduction in wheat yield has been shown to be linearly related to the phytotoxic O3 dose above a flux threshold of Y (PODY) accumulated over a specific period. In the current study, we tested whether the flux-effect relationships for wheat yield and 1,000-grain weight were affected by the temporal profile of O3 exposure. A modern wheat cultivar (Skyfall) was exposed to eight different realistic O3 profiles repeated weekly: four profiles with increasing background O3 concentrations (ca. 30–60 ppb) including small peaks and four profiles with increasing O3 peak concentrations (ca. 35–110 ppb). Both wheat yield and 1,000-grain weight declined linearly with increasingPODY. The slope of the flux-effect relationships was not affected significantly by the profile of O3 exposure. Hence, flux-effect relationships developed for wheat based on exposure to enhanced peak O3 concentrations are also valid for the changing European O3 profile with higher background and lower peak concentrations. The current study also shows that the modern wheat cultivar Skyfall is more sensitive to O3 than European wheat varieties tested for O3 sensitivity in the 1980s and 1990s.

A 'News & Views' paper. The 2016 Quadrennial Ozone Symposium (QOS-2016) was held on 4–9 September 2016 in Edinburgh, UK. The Symposium was organized by the International Ozone Commission (IO3C), the NERC Centre for Ecology & Hydrology and the University of Edinburgh, and was co-sponsored by the International Union of Geodesy and Geophysics, the International Association of Meteorology and Atmospheric Sciences, and the World Meteorological Organization. More than 300 participants from 39 countries attended. Key topics included: stratospheric and tropospheric ozone observations and modelling; interactions between ozone, atmospheric chemistry and climate; ozone measurement techniques; and effects on human health, ecosystems, and agriculture. Engagement with stakeholders and policymakers was also a key feature.

Nitrogen deposition and tropospheric ozone are important drivers of vegetation damage, but their interactive effects are poorly understood. This study assessed whether long-term nitrogen deposition altered sensitivity to ozone in a semi-natural vegetation community. Mesocosms were collected from sand dune grassland in the UK along a nitrogen gradient (5–25 kg N/ha/y, including two plots from a long-term experiment), and fumigated for 2.5 months to simulate medium and high ozone exposure. Ozone damage to leaves was quantified for 20 ozone-sensitive species. Soil solution dissolved organic carbon (DOC) and soil extracellular enzymes were measured to investigate secondary effects on soil processes. Mesocosms from sites receiving the highest N deposition showed the least ozone-related leaf damage, while those from the least N-polluted sites were the most damaged by ozone. This was due to differences in community-level sensitivity, rather than species-level impacts. The N-polluted sites contained fewer ozone-sensitive forbs and sedges, and a higher proportion of comparatively ozone-resistant grasses. This difference in the vegetation composition of mesocosms in relation to N deposition conveyed differential resilience to ozone. Mesocosms in the highest ozone treatment showed elevated soil solution DOC with increasing site N deposition. This suggests that, despite showing relatively little leaf damage, the ‘ozone resilient’ vegetation community may still sustain physiological damage through reduced capacity to assimilate photosynthate, with its subsequent loss as DOC through the roots into the soil. We conclude that for dune grassland habitats, the regions of highest risk to ozone exposure are those that have received the lowest level of long-term nitrogen deposition. This highlights the importance of considering community- and ecosystem-scale impacts of pollutants in addition to impacts on individual species. It also underscores the need for protection of ‘clean’ habitats from air pollution and other environmental stressors.

Air pollution presents a major risk to human health, resulting in premature deaths and reduced quality of life. Quantifying the role of vegetation in reducing air pollution concentrations is an important contribution to urban natural capital accounting. However, most current methods to calculate pollution removal are static, and do not represent atmospheric transport of pollutants, or interactions among pollutants and meteorology. An additional challenge is defining urban extent in a way that captures the green and blue infrastructure providing the service in a consistent way. We developed a refined urban morphology layer which incorporates urban green and blue space. We then applied an atmospheric chemistry transport model (EMEP4UK) to calculate pollutant removal by urban natural capital for pollutants including PM2.5, NO2, SO2, O3. We calculated health benefits directly from the change in pollutant concentrations (i.e. exposure) rather than from tonnes of pollutant removed. Urban natural capital across Britain removes 28,700 tonnes of PM2.5, NO2, SO2, O3. The economic value of the health benefits are substantial: £136 million in 2015, resulting from 900 fewer respiratory hospital admissions, 220 fewer cardiovascular hospital admissions, 240 fewer deaths and 3600 fewer Life Years Lost.

Ground-level ozone (O3) pollution is known to adversely affect the production of O3-sensitive crops such as wheat. The magnitude of impact is dependent on the accumulated stomatal flux of O3 into the leaves. In well-irrigated plants, the leaf pores (stomata) tend to be wide open, which stimulates the stomatal flux and therefore the adverse impact of O3 on yield. To test whether reduced irrigation might mitigate O3 impacts on flag leaf photosynthesis and yield parameters, we exposed an O3-sensitive Kenyan wheat variety to peak concentrations of 30 and 80 ppb O3 for four weeks in solardomes and applied three irrigation regimes (well-watered, frequent deficit, and infrequent deficit irrigation) during the flowering and grain filling stage. Reduced irrigation stimulated 1000-grain weight and harvest index by 33% and 13%, respectively (when O3 treatments were pooled), which compensated for the O3-induced reductions observed in well-watered plants. Whilst full irrigation accelerated the O3-induced reduction in photosynthesis by a week, such an effect was not observed for the chlorophyll content index of the flag leaf. Further studies under field conditions are required to test whether reduced irrigation can be applied as a management tool to mitigate adverse impacts of O3 on wheat yield.

Estimating food production under future air pollution and climate conditions in scenario analysis depends on accurately modelling ozone (O3) effects on yield. This study tests several assumptions that form part of published approaches for modelling O3 effects on photosynthesis and leaf duration against experimental data. In 2015 and 2016, two wheat cultivars were exposed in eight hemispherical glasshouses to O3 ranging from 22 to 57 ppb (24 h mean), with profiles ranging from raised background to high peak treatments. The stomatal O3 flux (Phytotoxic Ozone Dose, POD) to leaves was simulated using a multiplicative stomatal conductance model. Leaf senescence occurred earlier as average POD increased according to a linear relationship, and the two cultivars showed very different senescence responses. Negative effects of O3 on photosynthesis were only observed alongside O3-induced leaf senescence, suggesting that O3 does not impair photosynthesis in un-senesced flag leaves at the realistic O3 concentrations applied here. Accelerated senescence is therefore likely to be the dominant O3 effect influencing yield in most agricultural environments. POD was better than 24 h mean concentration and AOT40 (accumulated O3 exceeding 40 ppb, daylight hours) at predicting physiological response to O3, and flux also accounted for the difference in exposure resulting from peak and high background treatments.

To investigate whether nitrogen (N) load affects the ozone (O3) stomatal flux-effect relationship for birch biomass, three-year old birch saplings were exposed to seven different O3 profiles (24 h mean of 35–66 ppb) and four different N loads (10, 30, 50 and 70 kg ha−1 yr−1) in precision-controlled hemispherical glasshouses (solardomes) in 2012 and 2013. Stomatal conductance (gs) under optimal growth conditions was stimulated by enhanced N supply but was not significantly affected by enhanced O3 exposure. Birch root, woody (stem + branches) and total biomass (root + woody) were not affected by the Phytotoxic Ozone Dose (POD1SPEC) after two seasons of O3 exposure, and enhanced N supply stimulated biomass production independent of POD1SPEC (i.e. there were no POD1SPEC × N interactions). There was a strong linear relationship between the stem cross-sectional area and tree biomass at the end of the experiment, which was not affected by O3 exposure or N load. Enhanced N supply stimulated the stem cross-sectional area at the end of season 2, but not at the end of season 1, which suggests a time lag before tree biomass responded to enhanced N supply. There was no significant effect of POD1SPEC on stem cross-sectional area after either the first or second growing season of the experiment. Contrasting results reported in the literature on the interactive impacts of O3 and N load on tree physiology and growth are likely due to species-specific responses, different duration of the experiments and/or a limitation of the number of O3 and N levels tested.

Grasslands cover more than 70% of the world's agricultural land playing a pivotal role in global food security, economy, and ecology due to their flexibility and functionality. Climate change, characterized by changes in temperature and precipitation patterns, and by increased levels of greenhouse gases in the atmosphere, is anticipated to increase both the frequency and severity of extreme weather events, such as drought, heat waves, and flooding. Potentially, climate change could severely compromise future forage crop production and should be considered a direct threat to food security. This review aimed to summarize our current understanding of the physiological and metabolic responses of temperate grasses to those abiotic stresses associated with climate change. Primarily, substantial decreases in photosynthetic rates of cool‐season grasses occur as a result of high temperatures, water‐deficit or water‐excess, and elevated ozone, but not CO2 concentrations. Those decreases are usually attributed to stomatal and non‐stomatal limitations. Additionally, while membrane instability and reactive oxygen species production was a common feature of the abiotic stress response, total antioxidant capacity showed a stress‐specific response. Furthermore, climate change‐related stresses altered carbohydrate partitioning, with implications for biomass production. While water‐deficit stress, increased CO2, and ozone concentrations resulted in higher carbohydrate content, the opposite occurred under conditions of heat stress and flooding. The extent of damage is greatly dependent on location, as well as the type and intensity of stress. Fortunately, temperate forage grass species are highly heterogeneous. Consequently, through intra‐ and in particular inter‐specific plant hybridization (e.g., Festuca x Lolium hybrids) new opportunities are available to harness, within single genotypes, gene combinations capable of combating climate change.

In this study, we used eight sites from across Europe to investigate the implications of a future climate (2 °C warmer and 20% drier) and a changing ozone profile (increased background concentrations and reduced peaks) on stomatal ozone fluxes of three widely occurring plant species. A changing ozone profile with small increases in background ozone concentrations over the course of a growing season could have significant impacts on the annual accumulated stomatal ozone uptake, even if peak concentrations of ozone are reduced. Predicted increases in stomatal ozone uptake showed a strong relationship with latitude and were larger at sites from northern and mid-Europe than those from southern Europe. At the sites from central and northern regions of Europe, including the UK and Sweden, climatic conditions were highly conducive to stomatal ozone uptake by vegetation during the summer months and therefore an increase in daily mean ozone concentration of 3–16% during this time of year (from increased background concentrations, reduced peaks) would have a large impact on stomatal ozone uptake. In contrast, during spring and autumn, the climatic conditions can limit ozone uptake for many species. Although small increases in ozone concentration during these seasons could cause a modest increase in ozone uptake, for those species that are active at low temperatures, a 2 °C increase in temperature would increase stomatal ozone uptake even in the absence of further increases in ozone concentration. Predicted changes in climate could alter ozone uptake even with no change in ozone profile. For some southern regions of Europe, where temperatures are close to or above optimum for stomatal opening, an increase in temperature of 2 °C could limit stomatal ozone uptake by enhancing stomatal closure during the summer months, whereas during the spring, when many plants are actively growing, a small increase in temperature would increase stomatal ozone uptake.

Background This paper aims to investigate the correlations between the concentrations of nine heavy metals in moss and atmospheric deposition within ecological land classes covering Europe. Additionally, it is examined to what extent the statistical relations are affected by the land use around the moss sampling sites. Based on moss data collected in 2010/2011 throughout Europe and data on total atmospheric deposition modelled by two chemical transport models (EMEP MSC-E, LOTOS-EUROS), correlation coefficients between concentrations of heavy metals in moss and in modelled atmospheric deposition were specified for spatial subsamples defined by ecological land classes of Europe (ELCE) as a spatial reference system. Linear discriminant analysis (LDA) and logistic regression (LR) were then used to separate moss sampling sites regarding their contribution to the strength of correlation considering the areal percentage of urban, agricultural and forestry land use around the sampling location. After verification LDA models by LR, LDA models were used to transform spatial information on the land use to maps of potential correlation levels, applicable for future network planning in the European Moss Survey. Results Correlations between concentrations of heavy metals in moss and in modelled atmospheric deposition were found to be specific for elements and ELCE units. Land use around the sampling sites mainly influences the correlation level. Small radiuses around the sampling sites examined (5 km) are more relevant for Cd, Cu, Ni, and Zn, while the areal percentage of urban and agricultural land use within large radiuses (75–100 km) is more relevant for As, Cr, Hg, Pb, and V. Most valid LDA models pattern with error rates of

High sugar ryegrasses (HSG) have been developed to improve the uptake, digestion and nitrogen (N)-utilisation of grazing stock, with the potential to increase production yields and benefit climate by reducing methane (CH4) and nitrous oxide (N2O) emissions from livestock farming. In this study, the effects of tropospheric ozone pollution on the seasonal growth dynamics of HSG pasture mesocosms containing Lolium perenne cv. AberMagic and Trifolium repens cv. Crusader were investigated. Species-specific ozone (O3) dose-response relationships (seasonal means: 35, 41, 47, 51, 59 & 67 ppb) based on the Phytotoxic Ozone Dose (PODy) were constructed for above and below ground biomass, injury, N-fixation and forage quality. The dynamics of effects of ozone exposure on HSG pasture changed over the course of a season, with the strongest responses occurring in the first 4–8 weeks. Overall, strong negative responses to ozone flux were found for root biomass, root nodule mass and N-fixation rates, and ozone adversely impacted a range of forage quality parameters including total sugar content and relative and consumable food values. These results indicate that increasing ozone pollution could decrease the N-use efficiency and reduce the sugar content of managed pasture, and thereby partially detract from some of the suggested benefits of HSG.

In this study we have demonstrated that plants originating from upland peat bogs are sensitive to increasing background concentrations of ozone. Peatland mesocosms from an upland peat bog in North Wales, UK were exposed to eight levels of elevated background ozone in solardomes for 4 months from May to August, with 24 h mean ozone concentrations ranging from 16 to 94 ppb and cumulative AOT0 24hr ranging from 45.98 ppm h to 259.63 ppm h. Our results show that plant senescence increased with increasing exposure to ozone, although there was no significant effect of increasing ozone on plant biomass. Assessments of carbon dioxide and methane fluxes from the mesocosms suggests that there was no change in carbon dioxide fluxes over the 4 month exposure period but that methane fluxes increased as cumulative ozone exposure increased to a maximum AOT 0 24hr of approximately 120 ppm h and then decreased as cumulative ozone exposure increased further.

This paper provides a process-oriented perspective on the combined effects of ozone (O3), climate change and/or nitrogen (N) on vegetation. Whereas increasing CO2 in controlled environments or open-top chambers often ameliorates effects of O3 on leaf physiology, growth and C allocation, this is less likely in the field. Combined responses to elevated temperature and O3 have rarely been studied even though some critical growth stages such as seed initiation are sensitive to both. Under O3 exposure, many species have smaller roots, thereby enhancing drought sensitivity. Of the 68 species assessed for stomatal responses to ozone, 22.5% were unaffected, 33.5% had sluggish or increased opening and 44% stomatal closure. The beneficial effect of N on root development was lost at higher O3 treatments whilst the effects of increasing O3 on root biomass became more pronounced as N increased. Both responses to gradual changes in pollutants and climate and those under extreme weather events require further study.

Editorial.

Reduced belowground carbon (C) allocation by plants exposed to ozone may change properties and activities of the microbial community in soils. To investigate how soil microbial biomass and extracellular enzyme activities respond to elevated ozone, we collected soils from a temperate grassland after four years of ozone exposure under fully open-air field conditions. We measured soil microbial biomass, the metabolism of low molecular weight C substrates and hydrolytic extracellular enzyme activities in both bulk soil and isolated aggregates to assess changes in microbial activity and community function. After four years of elevated ozone treatment, soil total organic C was reduced by an average of 20% compared with ambient condition. Elevated ozone resulted in a small but insignificant reduction (4–10%) in microbial biomass in both bulk soil and isolated aggregates. Activities of extracellular enzymes were generally not affected by elevated ozone, except β-glucosidase, whose activity in bulk soil was significantly lower under elevated ozone than ambient condition. Activities of β-glucosidase, leucine aminopeptidase and acid phosphatase were higher in microaggregates (0.25 mm). Elevated ozone had no effects on mineralization rates of low molecular weight C substrates in both bulk soil and isolated aggregates. We therefore conclude that the size and activity rather than function of the soil microbial community in this semi-natural grassland are altered by elevated ozone.

This Tropospheric Ozone Assessment Report (TOAR) on the current state of knowledge of ozone metrics of relevance to vegetation (TOAR-Vegetation) reports on present-day global distribution of ozone at over 3300 vegetated sites and the long-term trends at nearly 1200 sites. TOAR-Vegetation focusses on three metrics over vegetation-relevant time-periods across major world climatic zones: M12, the mean ozone during 08:00–19:59; AOT40, the accumulation of hourly mean ozone values over 40 ppb during daylight hours, and W126 with stronger weighting to higher hourly mean values, accumulated during 08:00–19:59. Although the density of measurement stations is highly variable across regions, in general, the highest ozone values (mean, 2010–14) are in mid-latitudes of the northern hemisphere, including southern USA, the Mediterranean basin, northern India, north, north-west and east China, the Republic of Korea and Japan. The lowest metric values reported are in Australia, New Zealand, southern parts of South America and some northern parts of Europe, Canada and the USA. Regional-scale assessments showed, for example, significantly higher AOT40 and W126 values in East Asia (EAS) than Europe (EUR) in wheat growing areas (p < 0.05), but not in rice growing areas. In NAM, the dominant trend during 1995–2014 was a significant decrease in ozone, whilst in EUR it was no change and in EAS it was a significant increase. TOAR-Vegetation provides recommendations to facilitate a more complete global assessment of ozone impacts on vegetation in the future, including: an increase in monitoring of ozone and collation of field evidence of the damaging effects on vegetation; an investigation of the effects on peri-urban agriculture and in mountain/upland areas; inclusion of additional pollutant, meteorological and inlet height data in the TOAR dataset; where not already in existence, establishing new region-specific thresholds for vegetation damage and an innovative integration of observations and modelling including stomatal uptake of the pollutant.

Global Change Biology Published by John Wiley & Sons Ltd Increasing both crop productivity and the tolerance of crops to abiotic and biotic stresses is a major challenge for global food security in our rapidly changing climate. For the first time, we show how the spatial variation and severity of tropospheric ozone effects on yield compare with effects of other stresses on a global scale, and discuss mitigating actions against the negative effects of ozone. We show that the sensitivity to ozone declines in the order soybean > wheat > maize > rice, with genotypic variation in response being most pronounced for soybean and rice. Based on stomatal uptake, we estimate that ozone (mean of 2010–2012) reduces global yield annually by 12.4%, 7.1%, 4.4% and 6.1% for soybean, wheat, rice and maize, respectively (the “ozone yield gaps”), adding up to 227 Tg of lost yield. Our modelling shows that the highest ozone-induced production losses for soybean are in North and South America whilst for wheat they are in India and China, for rice in parts of India, Bangladesh, China and Indonesia, and for maize in China and the United States. Crucially, we also show that the same areas are often also at risk of high losses from pests and diseases, heat stress and to a lesser extent aridity and nutrient stress. In a solution-focussed analysis of these results, we provide a crop ideotype with tolerance of multiple stresses (including ozone) and describe how ozone effects could be included in crop breeding programmes. We also discuss altered crop management approaches that could be applied to reduce ozone impacts in the shorter term. Given the severity of ozone effects on staple food crops in areas of the world that are also challenged by other stresses, we recommend increased attention to the benefits that could be gained from addressing the ozone yield gap.

Introduction of high-performing crop cultivars and crop/soil water management practices that increase the stomatal uptake of carbon dioxide and photosynthesis will be instrumental in realizing the United Nations Sustainable Development Goal (SDG) of achieving food security. To date, however, global assessments of how to increase crop yield have failed to consider the negative effects of tropospheric ozone, a gaseous pollutant that enters the leaf stomatal pores of plants along with carbon dioxide, and is increasing in concentration globally, particularly in rapidly developing countries. Earlier studies have simply estimated that the largest effects are in the areas with the highest ozone concentrations. Using a modelling method that accounts for the effects of soil moisture deficit and meteorological factors on the stomatal uptake of ozone, we show for the first time that ozone impacts on wheat yield are particularly large in humid rain-fed and irrigated areas of major wheat-producing countries (e.g. United States, France, India, China and Russia). Averaged over 2010-2012, we estimate that ozone reduces wheat yields by a mean 9.9% in the northern hemisphere and 6.2% in the southern hemisphere, corresponding to some 85 Tg (million tonnes) of lost grain. Total production losses in developing countries receiving Official Development Assistance are 50% higher than those in developed countries, potentially reducing the possibility of achieving UN SDG2. Crucially, our analysis shows that ozone could reduce the potential yield benefits of increasing irrigation usage in response to climate change because added irrigation increases the uptake and subsequent negative effects of the pollutant. We show that mitigation of air pollution in a changing climate could play a vital role in achieving the above-mentioned UN SDG, while also contributing to other SDGs related to human health and well-being, ecosystems and climate change.

Supplementary data related to this article can be found at :http://dx.doi.org/10.1016/j.envpol.2015.06.033; To derive O3 dose-response relationships (DRR) for five European forest trees species and broadleaf deciduous and needleleaf tree plant functional types (PFTs), phytotoxic O3 doses (PODy) were related to biomass reductions.[br/]PODy was calculated using a stomatal flux model with a range of cut-off thresholds (y) indicative of varying detoxification capacities. Linear regression analysis showed that DRR for PFT and individual tree species differed in their robustness. A simplified parameterisation of the flux model was tested and showed that for most non-Mediterranean tree species, this simplified model led to similarly robust DRR as compared to a species- and climate region-specific parameterisation.[br/]Experimentally induced soil water stress was not found to substantially reduce PODy, mainly due to the short duration of soil water stress periods.[br/]This study validates the stomatal O3 flux concept and represents a step forward in predicting O3 damage to forests in a spatially and temporally varying climate.

In recent decades, naturally growing mosses have been used successfully as biomonitors of atmospheric deposition of heavy metals and nitrogen. Since 1990, the European moss survey has been repeated at five-yearly intervals. In 2010, the lowest concentrations of metals and nitrogen in mosses were generally found in northern Europe, whereas the highest concentrations were observed in (south-)eastern Europe for metals and the central belt for nitrogen. Averaged across Europe, since 1990, the median concentration in mosses has declined the most for lead (77%), followed by vanadium (55%), cadmium (51%), chromium (43%), zinc (34%), nickel (33%), iron (27%), arsenic (21%, since 1995), mercury (14%, since 1995) and copper (11%). Between 2005 and 2010, the decline ranged from 6% for copper to 36% for lead ; for nitrogen the decline was 5%. Despite the Europe-wide decline, no changes or increases have been observed between 2005 and 2010 in some (regions of) countries.

Tropospheric ozone is known to adversely affect crops and other vegetation. Most studies have focussed on the effects of elevated ozone levels vs. present ambient. We investigated the effect of present ambient surface ozone (O3) concentrations vs. preindustrial on a range of agronomically important response variables in field-grown wheat, using results from 33 experiments (representing 9 countries, 3 continents, 17 cultivars plus one set of 4 cultivars) having both charcoal filtered (CF) and non-filtered (NF) air treatments. Average filtration efficiency was 62%, reducing the O3 concentration from 35.6 ± 10.6 SD ppb in NF to 13.7 ± 8.8 SD ppb in CF. Average CF concentrations were in the range of levels believed to represent pre-industrial conditions, while NF concentrations were 7% lower than in the ambient air at plant height on the experimental sites. NF had significant (p

Inefficient use of fertilizers by crops increases the risk of nutrient leaching from agro-ecosystems, resulting in economic loss and environmental contamination. We investigated how ground-level ozone affects the efficiency by which wheat used applied nitrogen (N) fertilizer to produce grain protein (NEP, N efficiency with respect to protein yield) and grain yield (NEY, N efficiency with respect to grain yield) across a large number of open-top chamber field experiments. Our results show significant negative ozone effects on NEP and NEY, both for a larger data set obtained from data mining (21 experiments, 70 treatments), and a subset of data for which stomatal ozone flux estimates were available (7 experiments, 22 treatments). For one experiment, we report new data on N content of different above-ground plant fractions as well as grain K and P content. Our analysis of the combined dataset demonstrates that the grain yield return for a certain investment in N fertilizer is reduced by ozone. Results from the experiment with more detailed data further show that translocation of accumulated N from straw and leaves to grains is significantly and negatively affected by ozone, and that ozone decreases fertilizer efficiency also for K and P. As a result of lower N fertilization efficiency, ozone causes a risk of increased N losses from agroecosystems, e.g. through nitrate leaching and nitrous oxide emissions, a hitherto neglected negative effect of ozone. This impact of ozone on the N cycle implies that society is facing a dilemma where it either (i) accepts increased N pollution and counteracts ozone-induced yield reductions by increasing fertilization or (ii) counteracts N pollution under elevated ozone by reducing fertilization, accepting further yield loss adding to the direct effect of ozone on yield.

Ground-level ozone (O3) concentrations and atmospheric nitrogen (N) deposition rates have increased strongly since the 1950s. Rising ground-level O3 concentrations and atmospheric N deposition both affect plant physiology and growth, however, impacts have often been studied in isolation rather than in combination. In addition, studies are often limited to a control treatment and one or two elevated levels of ozone and/or nitrogen supply. In the current study, three-year old Betula pendula saplings were exposed to seven different O3 profiles (24 h mean O3 concentration of 36–68 ppb in 2013, with peaks up to an average of 105 ppb) in precision-controlled hemispherical glasshouses (solardomes) and four different N loads (10, 30, 50 or 70 kg N ha−1 y−1) in 2012 and 2013. Here we report on the effects of enhanced O3 concentrations and N load on leaf traits and gas exchange in leaves of varying age and developmental stage in 2013. The response of leaf traits to O3 (but not N) vary with leaf developmental stage. For example, elevated O3 did not affect the chlorophyll content of the youngest fully expanded leaf, but it reduced the chlorophyll content and photosynthetic parameters in aging leaves, relatively more so later than earlier in the growing season. Elevated O3 enhanced the N content of senesced leaves prior to leaf fall, potentially affecting subsequent N cycling in the soil. Enhanced N generally stimulated the chlorophyll content and photosynthetic capacity. Whilst elevated O3 reduced the light-saturated rate of photosynthesis (Asat) in aging leaves, it did not affect stomatal conductance (gs). This suggests that photosynthesis and gs are not closely coupled at elevated O3 under-light saturating conditions. We did not observe any interactions between O3 and N regarding photosynthetic parameters (Vc,max, Jmax, Asat), chlorophyll content, gs, N content in senesced leaves and leaf number. Hence, the sensitivity of these leaf traits to O3 in young silver birch trees is neither reduced nor enhanced by N load.

Globally, the legume-rhizobia symbiosis, contained within specialised organs called root nodules, is thought to add at least 30 Tg N annually to agricultural land. The growth and functioning of a moderncurrent white clover (Trifolium repens cv. Crusader) and red clover (T. pratense cv. Merviot) cultivar were investigated in current and future ozone scenarios in solardomes. Both cultivars developed leaf injury and had significant reductions in root biomass and root nodule number in response to ozone, with Crusader also displaying a reduced size and mass of nodules. In-situ measurements of N-fixation in Crusader by acetylene reduction assay revealed reduced N-fixation rates in a future scenario with an increased background and moderate peaks of ozone. The implications for the sustainability of temperate pasture are discussed.

Levels of reactive nitrogen (N) in the atmosphere have been elevated globally by man’s activities, with both positive and negative impacts. Since 1990, N emissions across Europe have declined by around 25% due to emissions controls. Ecosystem services provide a framework for valuing N impacts on the environment, and this study provides a synthesis of the evidence supporting impacts of atmospheric N deposition on ecosystem services. In an ex post assessment for the UK, for six services, we then estimate the marginal economic value of the decline in N deposition from 1987-2005. This decline resulted in a net benefit (Equivalent Annual Value) of £65m (£5m to £123m, 95% CI). The decline in N resulted in a cost (loss of value) for provisioning services: timber and livestock production of -£6.2m (-£3.5m to -£9.2m, 95% CI). There was a cost in terms of reduced CO2 sequestration and a benefit in terms of reduced N2O emissions which combined amounted to a cost for greenhouse gas regulation of -£15.7m (-£4.5m to -£30.6m). However, there were benefits for the cultural services of recreational fishing and appreciation of biodiversity, which amounted to £87.7m (£13.1m to £163.0m), outweighing costs to provisioning and regulating services. Knowledge gaps in both the under-pinning science and in the value-transfer evidence still prevent economic valuation of many services, particularly for cultural services, providing only a partial picture of N impacts which may underestimate the benefits of reducing N deposition.

Recent investigations proved that nitrogen (N) concentrations in mosses are primarily determined by atmospheric deposition. The correlations are country- and N compound-specific and agree well with spatial patterns and temporal trends across Europe as a whole and in single European countries. This study investigates whether correlations between the concentration of N in atmospheric deposition and mosses within the units of an ecological land classification of Europe can be established. To this end, N measurements from the 2005 European moss survey and modelled N atmospheric deposition in 2005 were intersected with a map of European landscapes. Then, considering minimum numbers of sampling sites required across Europe, in single European countries and within the landscapes of Europe and accounting for spatial auto-correlation, the correlations between the N concentration in mosses and corresponding deposition were calculated and mapped for each of those landscape units containing moss sampling sites. Using an example of one landscape with positive correlation and one landscape with no correlation between N concentrations in deposition and in mosses, influencing factors were ranked based on investigating the multivariate interactions between moss concentrations and, amongst others, atmospheric deposition, land use, elevation or moss species by classification and regression trees. From this study it could be concluded that the numbers of sampling sites within Europe and most participating countries as well as within most of the landscapes covering Europe are sufficient. Spatial patterns of correlations between the atmospheric N deposition and N concentration in mosses could be proven to vary across the landscapes of Europe. Where clear positive correlations between N concentrations in deposition and mosses exist in landscapes, multivariate ranking identifies the deposition as main influencing factor. In cases with no correlation between deposition and N concentrations in mosses, other factors such as e.g. moss species collected may be of importance. Therefore, mosses were proved to serve as biological indicators for atmospheric depositions and ecologically defined land classes could be identified as more complex indicators which allow relating exposure monitoring with effects assessment.

Risks associated with exposure of individual plant species to ozone (O[subcript 3]) are well documented, but implications for terrestrial biodiversity and ecosystem processes have received insufficient attention. This is an important gap because feedbacks to the atmosphere may change as future O[subcript 3] levels increase or decrease, depending on air quality and climate policies. Global simulation of O[subcript 3] using the Community Earth System Model (CESM) revealed that in 2000, about 40% of the Global 200 terrestrial ecoregions (ER) were exposed to O[subcript 3] above thresholds for ecological risks, with highest exposures in North America and Southern Europe, where there is field evidence of adverse effects of O[subcript 3], and in central Asia. Experimental studies show that O[subcript 3] can adversely affect the growth and flowering of plants and alter species composition and richness, although some communities can be resilient. Additional effects include changes in water flux regulation, pollination efficiency, and plant pathogen development. Recent research is unraveling a range of effects below-ground, including changes in soil invertebrates, plant litter quantity and quality, decomposition, and nutrient cycling and carbon pools. Changes are likely slow and may take decades to become detectable. CESM simulations for 2050 show that O[subcript 3] exposure under emission scenario RCP8.5 increases in all major biomes and that policies represented in scenario RCP4.5 do not lead to a general reduction in O[subcript 3] risks; rather, 50% of ERs still show an increase in exposure. Although a conceptual model is lacking to extrapolate documented effects to ERs with limited or no local information, and there is uncertainty about interactions with nitrogen input and climate change, the analysis suggests that in many ERs, O[subcript 3] risks will persist for biodiversity at different trophic levels, and for a range of ecosystem processes and feedbacks, which deserves more attention when assessing ecological implications of future atmospheric pollution and climate change., Great Britain. Department for Environment, Food & Rural Affairs (contract AQ0833), North American Electric Reliability Corporation, United States-Canada Research Consultation Group on the Long-Range Transport of Air Pollutants, United States. National Park Service e (grant H2370 094000/J2350103006), National Science Foundation (U.S.) (AGS- 1238109), United States. Department of Energy. Office of Science, National Center for Atmospheric Research (U.S.) Climate Simulation Laboratory (Computational and Information Systems Laboratory (CISL))

The Quadrennial Ozone Symposium 2016 Sophie GODIN-BEEKMANN*1, Irina PETROPAVLOVSKIKH2, Stefan REIS3,20, Paul NEWMAN4, Wolfgang STEINBRECHT5, Markus REX6, Michelle L. SANTEE7, Richard S. ECKMAN8, Xiangdong ZHENG9, Matthew B. TULLY10, David S. STEVENSON11, Paul YOUNG12, John PYLE13, Mark WEBER14, Johanna TAMMINEN15, Gina MILLS16, Alkiviadis F. BAIS17, Clare HEAVISIDE18, and Christos ZEREFOS19 1 Observatoire de Versailles Saint-Quentin en Yvelines, Universite de Versailles Saint-Quentin-en-Yvelines, CNRS, 78280 Guyancourt, France 2 CIRES, University of Colorado, Boulder, CO 80309, USA 3 NERC Centre for Ecology & Hydrology, Edinburgh EH26 0QB, UK 4 Goddard Space Flight Center, NASA, Greenbelt, MD 20771, USA 5 Hohenpeissenberg Meteorological Observatory, Deutscher Wetterdienst, 82383 Hohenpeissenberg, Germany 6 Alfred Wegener Institute, 14401 Potsdam, Germany 7 Jet Propulsion Laboratory, California Institute of Technology, CA 91109, USA 8 NASA Headquarters, Earth Science Division, Washington, DC, USA 9 Chinese Academy of Meteorological Sciences, Beijing, 100081, China 10 Bureau of Meteorology, Melbourne, Victoria 3001, Australia 11 University of Edinburgh, School of GeoSciences, Edinburgh EH9 3FE, UK 12 Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK 13 University of Cambridge, Department of Chemistry, Cambridge CB2 1EW, UK 14 University of Bremen, Institute of Environmental Physics, 28359 Bremen, Germany 15 Finnish Meteorological Institute, Earth Observation, FI-00101 Helsinki, Finland 16 NERC Centre for Ecology and Hydrology, Bangor, Gwynedd LL57 2UW, Wales, UK 17 Aristotle University of Thessaloniki, Thessaloniki, Greece 18 Public Health England, Centre for Radiation, Chemical and Environmental Hazards, London, UK 19 Research Center for Atmospheric Physics & Climatology, Academy of Athens, Athens 10680, Greece 20 University of Exeter Medical School, Truro TR1 3HD, UK

Referring to Europe as a whole and to single European countries, previous studies have shown that heavy metal concentrations in mosses (1) are primarily determined by atmospheric deposition of heavy metals; (2) are country and element–specific; and (3) agree well with respect to element–specific spatial patterns and temporal trends of atmospheric deposition of heavy metals. This paper investigates correlations between the concentrations of cadmium, lead and mercury in atmospheric deposition and mosses within the units of an ecological land classification of Europe. To this end, measurements from the 2005/2006 European moss survey and modeled atmospheric deposition in the previous three years were intersected with a map on ecologically defined land classes of Europe. Then, the minimum numbers of sampling sites required within the ecological land classes were computed. Considering spatial auto–correlations, subsequently the correlations between the concentrations of heavy metals in mosses and corresponding deposition were calculated and mapped for each of those ecological land classes containing moss sampling sites. It was concluded that the numbers of sampling sites within Europe and most participating countries as well as within most of the ecological land classes are sufficient for estimating the mean of measurements for the above mentioned three spatial levels within 20% of its true value with 95% confidence. Spatial patterns of correlations between the atmospheric deposition and bioaccumulation were shown to vary by element and ecologically defined land classes.

In this study we have demonstrated that rising background ozone has the potential to reduce grassland forage quality and explored the implications for livestock production. We analysed pasture samples from seven ozone exposure experiments comprising mesotrophic, calcareous, haymeadow and sanddune unimproved grasslands conducted in open-top chambers, solardomes and a field release system. Across all grassland types, there were significant increases in acid detergent fibre, crude fibre and lignin content with increasing ozone concentration, resulting in decreased pasture quality in terms of the metabolisable energy content of the vegetation. We derived a dose-response function for metabolisable energy of the grassland with ozone concentration, applicable to a range of grassland types, and used this to predict effects on pasture quality of UK vegetation at 1km resolution using modelled ozone data for 2007 and for predicted higher average ozone concentrations in 2020. This showed a potential total reduction in lamb production in the UK of approximately 4% in 2020 compared to 2007. The largest impacts were in geographical areas of modest ozone increases between the two years, but where large numbers of lambs were present. For an individual farmer working to a very small cost margin this could represent a large reduction in profit, both in regions where the impacts per lamb and those where the impacts per km2 of grazing land are largest. In the short term farmers could adapt their lamb management in response to changed forage quality by additional supplementary feed of high metabolisable energy content. Nationally this increase in annual additional feed in 2020 compared to 2007 would be 2,166 tonnes (an increase of 0.7%). Of added concern are the longer-term consequences of continual deterioration of pasture quality and the implications for changes in farming practices to compensate for potential reductions in livestock production capacity.

The growth, development and functioning of legumes are often significantly affected by exposure to tropospheric ozone (O3) pollution. However, surprisingly little is known about how leguminous Nitrogen (N) fixation responds to ozone, with a scarcity of studies addressing this question in detail. In the last decade, ozone impacts on N-fixation in soybean, cowpea, mung bean, peanut and clover have been shown for concentrations which are now commonly recorded in ambient air or are likely to occur in the near future. We provide a synthesis of the existing literature addressing this issue, and also explore the effects that may occur on an agroecosystem scale by predicting reductions in Trifolium (clovers) root nodule biomass in United Kingdom (UK) pasture based on ozone concentration data for a “high” (2006) and “average” ozone year (2008). Median 8% and 5% reductions in clover root nodule biomass in pasture across the UK were predicted for 2006 and 2008 respectively. Seasonal exposure to elevated ozone, or short-term acute concentrations >100 ppb, are sufficient to reduce N-fixation and/or impact nodulation, in a range of globally-important legumes. However, an increasing global burden of CO2, the use of artificial fertiliser, and reactive N-pollution may partially mitigate impacts of ozone on N-fixation.

Mesocosms representing the BAP Priority habitat ‘Calcareous Grassland’ were exposed to eight ozone profiles for twelve-weeks in two consecutive years. Half of the mesocosms received a reduced watering regime during the exposure periods. Numbers and timing of flowering in the second exposure period were related to ozone concentration and phytotoxic ozone dose (accumulated stomatal flux). For Lotus corniculatus, ozone accelerated the timing of the maximum number of flowers. An increase in mean ozone concentration from 30 ppb to 70 ppb corresponded with an advance in the timing of maximum flowering by six days. A significant reduction in flower numbers with increasing ozone was found for Campanula rotundifolia and Scabiosa columbaria and the relationship with ozone was stronger for those that were well-watered than for those with reduced watering. These changes in flowering timing and numbers could have large ecological impacts, affecting plant pollination and the food supply of nectar feeding insects.

Atmospheric pollution by reactive nitrogen (N) can have profound effects on ecosystem functioning and biodiversity. Numerous mechanisms are involved, and response times vary among habitats and species. This complex picture can make it difficult to convey the benefits of controlling N pollution to policy developers and the public. In this study we evaluate pressure, midpoint, and endpoint metrics for N pollution, considering those currently in use and proposing some improved metrics. Pressure metrics that use the concept of a critical load (CL) are useful, and we propose a new integrated measure of cumulative exposure above the CL that allows for different response times in different habitats. Biodiversity endpoint metrics depend greatly on societal values and priorities and so are inevitably somewhat subjective. Species richness is readily understood, but biodiversity metrics based on habitat suitability for particular taxa may better reflect the priorities of nature conservation specialists. Midpoint metrics indicate progress towards desired endpoints – the most promising are those based on empirical evidence. Moss tissue N enrichment is responsive to lower N deposition rates, and we propose a new Moss Enrichment Index (MEI) based on species-specific ranges of tissue N content. At higher N deposition rates, mineral N leaching is an appropriate midpoint indicator. Biogeochemical models can also be used to derive midpoint metrics which illustrate the large variation in potential response times among ecosystem components. Metrics have an important role in encouraging progress towards reducing pollution, and need to be chosen accordingly.

Ground-level ozone pollution is already decreasing global crop yields (from ∼2.2-5.5% for maize to 3.9-15% and 8.5-14% for wheat and soybean, respectively), to differing extents depending on genotype and environmental conditions, and this problem is predicted to escalate given climate change and increasing ozone precursor emissions in many areas. Here a summary is provided of how ozone pollution affects yield in a variety of crops, thus impacting global food security. Ozone causes visible injury symptoms to foliage; it induces early senescence and abscission of leaves; it can reduce stomatal aperture and thereby carbon uptake, and/or directly reduce photosynthetic carbon fixation; it can moderate biomass growth via carbon availability or more directly; it can decrease translocation of fixed carbon to edible plant parts (grains, fruits, pods, roots) due either to reduced availability at source, redirection to synthesis of chemical protectants, or reduced transport capabilities via phloem; decreased carbon transport to roots reduces nutrient and water uptake and affects anchorage; ozone can moderate or bring forward flowering and induce pollen sterility; it induces ovule and/or grain abortion; and finally it reduces the ability of some genotypes to withstand other stresses such as drought, high vapour pressure deficit, and high photon flux density via effects on stomatal control. This latter point is emphasized here, given predictions that atmospheric conditions conducive to drought formation that also give rise to intense precursor emission events will become more severe over the coming decades.

The combined impacts of simulated increased nitrogen (N) deposition (75 kg Nha(-1)yr (-1)) and increasing background ozone (O(3)) were studied using two mesotrophic grassland species (Dactylis glomerata and Ranunculus acris) in solardomes, by means of eight O(3) treatments ranging from 15.5 ppb to 92.7 ppb (24h average mean). A-C(i) curves were constructed for each species to gauge effects on photosynthetic efficiency and capacity, and effects on biomass partitioning were determined after 14 weeks. Increasing the background concentration of O(3) reduced the healthy above ground and root biomass of both species, and increased senesced biomass. N fertilisation increased biomass production in D. glomerata, and a significantly greater than additive effect of O(3) and N on root biomass was evident. In contrast, R. acris biomass was not affected by high N. The study shows the combined effects of these pollutants have differential implications for carbon allocation patterns in common grassland species.

Few studies have investigated effects of increased background ozone in the absence of episodic peaks, despite a predicted increase throughout the northern hemisphere over the coming decades. In this study Leontodon hispidus was grown with Anthoxanthum odoratum or Dactylis glomerata and exposed in the UK to one of eight background ozone concentrations for 20 weeks in solardomes. Seasonal mean ozone concentrations ranged from 21.4 to 102.5 ppb. Ozone-induced senescence of L. hispidus was enhanced when grown with the more open canopy of A. odoratum compared to the denser growing D. glomerata. There was increased cover with increasing ozone exposure for both A. odoratum and D. glomerata, which resulted in an increase in the grass:Leontodon cover ratio in both community types. Carry-over effects of the ozone exposure were observed, including delayed winter die-back of L. hispidus and acceleration in the progression from flowers to seed-heads in the year following ozone exposure.

The critical levels for ozone effects on vegetation have been reviewed and revised by the LRTAP Convention. Eight new or revised critical levels based on the accumulated stomatal flux of ozone (PODY, the Phytotoxic Ozone Dose above a threshold flux of Y nmol m−2 PLA s−1, where PLA is the projected leaf area) have been agreed. For each receptor, data were combined from experiments conducted under naturally fluctuating environmental conditions in 2–4 countries, resulting in linear dose–response relationships with response variables specific to each receptor (r2 = 0.49–0.87, p < 0.001 for all). For crops, critical levels were derived for effects on wheat (grain yield, grain mass, and protein yield), potato (tuber yield) and tomato (fruit yield). For forest trees, critical levels were derived for effects on changes in annual increment in whole tree biomass for beech and birch, and Norway spruce. For (semi-)natural vegetation, the critical level for effects on productive and high conservation value perennial grasslands was based on effects on important component species of the genus Trifolium (clover species). These critical levels can be used to assess protection against the damaging effects of ozone on food security, important ecosystem services provided by forest trees (roundwood production, C sequestration, soil stability and flood prevention) and the vitality of pasture.

Records of effects of ambient ozone pollution on vegetation have been compiled for Europe for the years 1990–2006. Sources include scientific papers, conference proceedings, reports to research funders, records of confirmed ozone injury symptoms and an international biomonitoring experiment coordinated by the ICP Vegetation. The latter involved ozone-sensitive (NC-S) and ozone-resistant (NC-R) biotypes of white clover (Trifolium repens L.) grown according to a common protocol and monitored for ozone injury and biomass differences in 17 European countries, from 1996 to 2006. Effects were separated into visible injury or growth/yield reduction. Of the 644 records of visible injury, 39% were for crops (27 species), 38.1% were for (semi-) natural vegetation (95 species) and 22.9% were for shrubs (49 species). Owing to inconsistencies in reporting effort from year to year it was not possible to determine geographical or temporal trends in the data. Nevertheless, this study has shown effects in ambient air in 18 European countries from Sweden in the north to Greece in the south. These effects data were superimposed on AOT40 (accumulated ozone concentrations over 40 ppb) and POD3gen (modelled accumulated stomatal flux over a threshold of 3 nmol m−2 s−1) maps generated by the EMEP Eulerian model (50 km × 50 km grid) that were parameterized for a generic crop based on wheat and NC-S/NC-R white clover. Many effects were found in areas where the AOT40 (crops) was below the critical level of 3 ppm h. In contrast, the majority of effects were detected in grid squares where POD3gen (crops) were in the mid-high range (>12 mmol m−2). Overall, maps based on POD3gen provided better fit to the effects data than those based on AOT40, with the POD3gen model for clover fitting the clover effects data better than that for a generic crop.

Tropospheric ozone concentrations are increasing, which may result in elevated background concentrations at rural high-altitude sites. In this study simulated upland grassland communities containing seven species were exposed to ozone treatments in solardomes for 12 weeks in each of two consecutive summers. Ozone profiles, based on future ozone predictions, were of elevated background concentrations, episodic peaks of ozone and a combination of the two. During the winter between the two exposures the communities were kept outdoors in ambient air. Whereas previous studies have demonstrated that peaks of ozone cause detrimental effects to vegetation, this study shows that for simulated grassland communities an increase in background ozone concentration in the absence of peaks of ozone also corresponded with increased senescence. In many cases senescence was further increased when peaks of ozone were also present. The species used showed no acclimation to ozone and the same relationship between senescence and ozone dose occurred in both years of the study. A decrease in cumulative biomass was demonstrated for Anthoxanthum odoratum, which contributed to a decrease in total community biomass and grass:forb ratio. These results indicate that current and future ozone concentrations could cause detrimental effects on growth and vitality of natural grassland communities and that for some species the consequences of increased background ozone concentration are as severe as that of increased peaks.

Two species found in temperate calcareous and mesotrophic grasslands (Dactylis glomerata and Leontodon hispidus) were exposed to eight ozone treatments spanning preindustrial to post-2100 regimes, and late-season effects on stomatal functioning were investigated. The plants were grown as a mixed community in 14 L containers and were exposed to ozone in ventilated solardomes (dome-shaped greenhouses) for 20 weeks from early May to late September 2007. Ozone exposures were based on O-3 concentrations from a nearby upland area, and provided the following seasonal 24 h means: 21.4, 39.9 (simulated ambient), 50.2, 59.4, 74.9, 83.3, 101.3 and 102.5 ppb. In both species, stomatal conductance of undamaged inner canopy leaves developing since a midseason cutback increased linearly with increasing background ozone concentration. Imposition of severe water stress by leaf excision indicated that increasing background ozone concentration decreased the ability of leaves to limit water loss, implying impaired stomatal control. The threshold ozone concentrations for these effects were 15-40 ppb above current ambient in upland UK, and were within the range of ozone concentrations anticipated for much of Europe by the latter part of this century. The potential mechanism behind the impaired stomatal functioning was investigated using a transpiration assay. Unlike for lower ozone treatments, apparently healthy green leaves of L. hispidus that had developed in the 101.3 ppb treatment did not close their stomata in response to 1.5 mu m abscisic acid (ABA); indeed stomatal opening initially occurred in this treatment. Thus, ozone appears to be disrupting the ABA-induced signal transduction pathway for stomatal control thereby reducing the ability of plants to respond to drought. These results have potentially wide-reaching implications for the functioning of communities under global warming where periods of soil drying and episodes of high vapour pressure deficit are likely to be more severe.