Stomatal uptake of O3 in aspen and aspen-birch forests under free-air CO2 and O3 enrichment

Rising atmospheric carbon dioxide (CO₂) may alleviate the toxicological impacts of concurrently rising tropospheric ozone (O₃) during the present century if higher CO₂ is accompanied by lower stomatal conductance (g_s), as assumed by many models. We investigated how elevated concentrations of CO₂ and O₃, alone and in combination, affected the accumulated stomatal flux of O₃ (AFst) by canopies and sun leaves in closed aspen and aspen-birch forests in the free-air CO₂-O₃ enrichment experiment near Rhinelander, Wisconsin. Stomatal conductance for O₃ was derived from sap flux data and AFst was estimated either neglecting or accounting for the potential influence of non-stomatal leaf surface O₃ deposition. Leaf-level AFst (AFst_l) was not reduced by elevated CO₂. Instead, there was a significant CO₂ × O₃ interaction on AFst_l, as a consequence of lower values of g_s in control plots and the combination treatment than in the two single-gas treatments. In addition, aspen leaves had higher AFst_l than birch leaves, and estimates of AFst_l were not very sensitive to non-stomatal leaf surface O₃ deposition. Our results suggest that model projections of large CO₂-induced reductions in g_s alleviating the adverse effect of rising tropospheric O₃ may not be reasonable for northern hardwood forests.     

Simple model for estimating dry deposition velocity of ozone and its destruction in a polluted nocturnal boundary layer

Determining the destructions of both ozone and odd oxygen, O_x, in the nocturnal boundary layer (NBL) is important to evaluate the regional ozone budget and overnight ozone accumulation. This work develops a simple method to determine the dry deposition velocity of ozone and its destruction at a polluted nocturnal boundary layer. The destruction of O_x can also be determined simultaneously. The method is based on O₃ and NO₂ profiles and their surface measurements. Linkages between the dry deposition velocities of O₃ and NO₂ and between the dry deposition loss of O_x and its chemical loss are constructed and used. Field measurements are made at an agricultural site to demonstrate the application of the model. The model estimated nocturnal O₃ dry deposition velocities from 0.13 to 0.19 cm s^?1, very close to those previously obtained for similar land types. Additionally, dry deposition and chemical reactions account for 60 and 40% of the overall nocturnal ozone loss, respectively; ozone dry deposition accounts for 50% of the overall nocturnal loss of O_x, dry deposition of NO₂ accounts for another 20%, and chemical reactions account for the remaining 30%. The proposed method enables the use of measurements made in typical ozone field studies to evaluate various nocturnal destructions of O₃ and O_x in a polluted environment.     

Ozone flux over a Norway spruce forest and correlation with net ecosystem production

Daily ozone deposition flux to a Norway spruce forest in Czech Republic was measured using the gradient method in July and August 2008. Results were in good agreement with a deposition flux model. The mean daily stomatal uptake of ozone was around 47% of total deposition. Average deposition velocity was 0.39 cm s⁻¹ and 0.36 cm s⁻¹ by the gradient method and the deposition model, respectively. Measured and modelled non-stomatal uptake was around 0.2 cm s⁻¹. In addition, net ecosystem production (NEP) was measured by using Eddy Covariance and correlations with O₃ concentrations at 15 m a.g.l., total deposition and stomatal uptake were tested. Total deposition and stomatal uptake of ozone significantly decreased NEP, especially by high intensities of solar radiation.     

Belowground effects of enhanced tropospheric ozone and drought in a beech/spruce forest (Fagus sylvatica L./Picea abies [L.] Karst)

The effects of experimentally elevated O₃ on soil respiration rates, standing fine-root biomass, fine-root production and δ¹³C signature of newly produced fine roots were investigated in an adult European beech/Norway spruce forest in Germany during two subsequent years with contrasting rainfall patterns. During humid 2002, soil respiration rate was enhanced under elevated O₃ under beech and spruce, and was related to O₃-stimulated fine-root production only in beech. During dry 2003, the stimulating effect of O₃ on soil respiration rate vanished under spruce, which was correlated with decreased fine-root production in spruce under drought, irrespective of the O₃ regime. δ¹³C signature of newly formed fine-roots was consistent with the differing g_s of beech and spruce, and indicated stomatal limitation by O₃ in beech and by drought in spruce. Our study showed that drought can override the stimulating O₃ effects on fine-root dynamics and soil respiration in mature beech and spruce forests.     

A comparison of two stomatal conductance models for ozone flux modelling using data from two Brassica species

In this study we tested and compared a multiplicative stomatal model and a coupled semi-empirical stomatal-photosynthesis model in their ability to predict stomatal conductance to ozone (g_st) using leaf-level data from oilseed rape (Brassica napus L.) and broccoli (Brassica oleracea L. var. italica Plenck). For oilseed rape, the multiplicative model and the coupled model were able to explain 72% and 73% of the observed g_st variance, respectively. For broccoli, the models were able to explain 53% and 51% of the observed g_st variance, respectively. These results support the coupled semi-empirical stomatal-photosynthesis model as a valid alternative to the multiplicative stomatal model for O₃ flux modelling, in terms of predictive performance.     

Diurnal changes in photosynthetic parameters of Populus tremuloides, modulated by elevated concentrations of CO2 and/or O3 and daily climatic variation

The diurnal changes in light-saturated photosynthesis (Pn) under elevated CO₂ and/or O₃ in relation to stomatal conductance (g_s), water potential, intercellular [CO₂], leaf temperature and vapour-pressure difference between leaf and air (VPD_L) were studied at the Aspen FACE site. Two aspen (Populus tremuloides Michx.) clones differing in their sensitivity to ozone were measured. The depression in Pn was found after 10:00 h. The midday decline in Pn corresponded with both decreased g_s and decreased Rubisco carboxylation efficiency, Vc_max. As a result of increasing VPD_L, g_s decreased. Elevated [CO₂] resulted in more pronounced midday decline in Pn compared to ambient concentrations. Moreover, this decline was more pronounced under combined treatment compared to elevated CO₂ treatment.The positive impact of CO₂ on Pn was relatively more pronounced in days with environmental stress but relatively less pronounced during midday depression. The negative impact of ozone tended to decrease in both cases.     

Stomatal characteristics and infection biology of Pyrenopeziza betulicola in Betula pendula trees grown under elevated CO2 and O3

Two silver birch clones were exposed to ambient and elevated concentrations of CO₂ and O₃, and their combination for 3 years, using open-top chambers. We evaluated the effects of elevated CO₂ and O₃ on stomatal conductance (g_s), density (SD) and index (SI), length of the guard cells, and epidermal cell size and number, with respect to crown position and leaf type. The relationship between the infection biology of the fungus (Pyrenopeziza betulicola) causing leaf spot disease and stomatal characteristics was also studied. Leaf type was an important determinant of O₃ response in silver birch, while crown position and clone played only a minor role. Elevated CO₂ reduced the g_s, but had otherwise no significant effect on the parameters studied. No significant interactions between elevated CO₂ and O₃ were found. The infection biology of P. betulicola was not correlated with SD or g_s, but it did occasionally correlate positively with the length of the guard cells.     

A theoretical estimate of O3 and H2O2 dry deposition over the northeast United States

Estimates of short-term, regional-scale spatial distributions of ozone (O₃) and hydrogen peroxide (H₂O₂) dry deposition over the northeast U.S. are presented. Dry deposition fluxes to surfaces are computed using a regional tropospheric chemistry model with deposition velocities which vary with local meteorology, land type, insolation, seasonal factors and surface wetness. A compilation of O₃ surface resistances is presented based on a survey of O₃ dry deposition measurements. The surface resistance for H₂O₂ is assumed to be small under most conditions, causing H₂O₂ to dry deposit at a rate which is frequently limited by surface-layer turbulence. Regional patterns of dry deposition velocities for these oxidants over the northeast U.S. are computed using landuse data and meteorological information predicted using a mesoscale meteorology model. Domain-averaged O₃ deposition velocities during a spring period reach a mid-day peak of 0.7–0.8 cm s⁻¹ and drop to 0.1–0.2 cm s⁻¹ at night. Domain-averaged H₂O₂ deposition velocities at a height of approximately 80 m are predicted to reach a mid-day peak of 1.6–2.0cm s⁻¹, and fall to 0.6–0.9 cm s⁻¹ at night. Time-averaged surface-layer H₂O₂ concentrations show a latitude dependence, with higher concentrations in the south. H₂O₂ concentrations are significantly reduced due to efficient wet removal and chemical destruction during the passage of a cyclonic frontal system. In contrast, O₃ concentrations are predicted to rise during the passage of a frontal system due to efficient vertical exchange of midtropospheric air into the boundary layer during convective conditions, followed by synoptic-scale subsidence occurring in the high pressure airmass following a cyclone. Maximum O₃ deposition during this 3-day springtime period occurs in polluted agricultural areas. In contrast, H₂O₂ dry deposition exhibits a latitude dependence with maximum 3-day accumulations occurring in the south. Domain-averaged mid-day deposition rates for O₃ and H₂O₂ were 45–50 μmol m⁻² h⁻¹ and 4–5 μmol m⁻² h⁻¹. At night, deposition rates were approximately 5–10 μmol m⁻² h⁻¹ and 1.5–2.5 μmol m⁻² h⁻¹ for O₃ and H₂O₂. These model results show that regional patterns of oxidant dry deposition are strongly influenced by oxidant concentrations, atmospheric stability, surface roughness and numerous other surface and meteorological factors. Each of these factors must be well-characterized before regional patterns of biological damage associated with oxidant dry deposition can be quantified.     

Ozone uptake by citrus trees exposed to a range of ozone concentrations

The Citrus genus includes a large number of species and varieties widely cultivated in the Central Valley of California and in many other countries having similar Mediterranean climates. In the summer, orchards in California experience high levels of tropospheric ozone, formed by reactions of volatile organic compounds (VOC) with oxides of nitrogen (NO_x). Citrus trees may improve air quality in the orchard environment by taking up ozone through stomatal and non-stomatal mechanisms, but they may ultimately be detrimental to regional air quality by emitting biogenic VOC (BVOC) that oxidize to form ozone and secondary organic aerosol downwind of the site of emission. BVOC also play a key role in removing ozone through gas-phase chemical reactions in the intercellular spaces of the leaves and in ambient air outside the plants. Ozone is known to oxidize leaf tissues after entering stomata, resulting in decreased carbon assimilation and crop yield. To characterize ozone deposition and BVOC emissions for lemon (Citrus limon), mandarin (Citrus reticulata), and orange (Citrus sinensis), we designed branch enclosures that allowed direct measurement of fluxes under different physiological conditions in a controlled greenhouse environment. Average ozone uptake was up to 11 nmol s^?1 m^?2 of leaf. At low concentrations of ozone (40 ppb), measured ozone deposition was higher than expected ozone deposition modeled on the basis of stomatal aperture and ozone concentration. Our results were in better agreement with modeled values when we included non-stomatal ozone loss by reaction with gas-phase BVOC emitted from the citrus plants. At high ozone concentrations (160 ppb), the measured ozone deposition was lower than modeled, and we speculate that this indicates ozone accumulation in the leaf mesophyll.     

Anthropogenic NOx emissions alter the intrinsic water-use efficiency (WUEi) for Quercus cerris stands under Mediterranean climate conditions

We investigated the effect of N deposition (Ndep) on intrinsic water-use efficiency (WUE_i), the ratio of photosynthesis (A) to stomatal conductance (g_s), for two Quercus cerris stands at different distances to an oil refinery in Southern Italy. We used δ¹³C in tree rings for assessing changes in WUE_i; while the influence of climate and NO_x emission was explored through δ¹⁸O and δ¹⁵N, respectively. Differences in WUE_i between the two sites were significant, with trees exposed to different degrees of NO_x emissions showing an abrupt increase with the onset of pollution. Assuming similar g_s at the two sites, as inferred through δ¹⁸O, the higher N availability at the polluted site caused the shift of the A/g_s ratio in favour of A. Overall, our result suggests that an increase of Ndep may enhance tree WUE under a scenario of reduction of precipitation predicted for Mediterranean area.     

A canopy stomatal resistance model for gaseous deposition to vegetated surfaces

A gaseous deposition model, based on a realistic canopy stomatal resistance submodel, is described, analyzed and tested. This model is designed as one of a hierarchy of simulations, leading up to a “big-leaf” model of the processes contributing to the exchange of trace gases between the atmosphere and vegetated surfaces. Computations show that differences in plant species and environmental and physiological conditions can affect the canopy stomatal resistance by a factor of four. Canopy stomatal resistances to water vapor transfer computed with the present model are compared against values measured with a porometer and computed with the Penman-Monteith equation. Computed stomatal resistances from a soybean canopy in both well-watered and water-stressed conditions yield good agreement with test data. The stomatal resistance submodel responds well to changing environmental and physiological conditions. Model predictions of deposition velocities are evaluated for the case of ozone, transferred to maize. Calculated deposition velocities of O₃ overestimate measured values on the average by about 30%, probably largely as a consequence of uncertainties in leaf area index, soil and cuticle resistances, and other modeling parameters, but also partially due to imperfect measurement of O₃ deposition velocities.     

Contrasting ozone sensitivity in related evergreen and deciduous shrubs

Plant responses to enhanced ozone levels have been studied in two pairs of evergreen-deciduous species (Pistacia terebinthus vs. P. lentiscus; Viburnum lantana vs. V. tinus) in Open Top Chambers. Ozone induced widespread visible injury, significantly reduced CO₂ assimilation and stomatal conductance (g_s), impaired Rubisco efficiency and regeneration capacity (V_c,max,J_max) and altered fluorescence parameters only in the deciduous species. Differences in stomatal conductance could not explain the observed differences in sensitivity. In control plants, deciduous species showed higher superoxide dismutase (SOD) activity than their evergreen counterparts, suggesting metabolic differences that could make them more prone to redox imbalances. Ozone induced increases in SOD and/or peroxidase activities in all the species, but only evergreens were able to cope with the oxidative stress. The relevancy of these results for the effective ozone flux approach and for the current ozone Critical Levels is also discussed.     

Leaf age affects the responses of foliar injury and gas exchange to tropospheric ozone in Prunus serotina seedlings

We investigated the effect of leaf age on the response of net photosynthesis (A), stomatal conductance (g_wv), foliar injury, and leaf nitrogen concentration (N_L) to tropospheric ozone (O₃) on Prunus serotina seedlings grown in open-plots (AA) and open-top chambers, supplied with either carbon-filtered or non-filtered air. We found significant variation in A, g_wv, foliar injury, and N_L (P < 0.05) among O₃ treatments. Seedlings in AA showed the highest A and g_wv due to relatively low vapor pressure deficit (VPD). Older leaves showed significantly lower A, g_wv, N_L, and higher foliar injury (P < 0.001) than younger leaves. Leaf age affected the response of A, g_wv, and foliar injury to O₃. Both VPD and N_L had a strong influence on leaf gas exchange. Foliar O₃-induced injury appeared when cumulative O₃ uptake reached 8-12 mmol m⁻², depending on soil water availability. The mechanistic assessment of O₃-induced injury is a valuable approach for a biologically relevant O₃ risk assessment for forest trees.     

Combining sap flow and eddy covariance approaches to derive stomatal and non-stomatal O3 fluxes in a forest stand

Stomatal O₃ fluxes to a mixed beech/spruce stand (Fagus sylvatica/Picea abies) in Central Europe were determined using two different approaches. The sap flow technique yielded the tree-level transpiration, whereas the eddy covariance method provided the stand-level evapotranspiration. Both data were then converted into stomatal ozone fluxes, exemplifying this novel concept for July 2007. Sap flow-based stomatal O₃ flux was 33% of the total O₃ flux, whereas derivation from evapotranspiration rates in combination with the Penman-Monteith algorithm amounted to 47%. In addition to this proportional difference, the sap flow-based assessment yielded lower levels of stomatal O₃ flux and reflected stomatal regulation rather than O₃ exposure, paralleling the daily courses of canopy conductance for water vapor and eddy covariance-based total stand-level O₃ flux. The demonstrated combination of sap flow and eddy covariance approaches supports the development of O₃ risk assessment in forests from O₃ exposure towards flux-based concepts.     

Precision of atmospheric dry deposition data from the Clean Air Status and Trends Network

A collocated, dry deposition sampling program was begun in January 1987 by the US Environmental Protection Agency to provide ongoing estimates of the overall precision of dry deposition and supporting data entering the Clean Air Status and Trends Network (CASTNet) archive. Duplicate sets of dry deposition sampling instruments were installed adjacent to existing instruments and have been operated for various periods at 11 collocated field sites. All sampling and operations were performed using standard CASTNet procedures. The current study documents the bias-corrected precision of CASTNet data based on collocated measurements made at paired sampling sites representative of sites across the network. These precision estimates include the variability for all operations from sampling to data storage in the archive. Precision estimates are provided for hourly, instrumental ozone (O₃) concentration and meteorological measurements, hourly model estimates of deposition velocity (V_d) from collocated measurements of model inputs, hourly O₃ deposition estimates, weekly filter pack determinations of selected atmospheric chemical species, and weekly estimates of V_d and deposition for each monitored filter pack chemical species and O₃.Estimates of variability of weekly pollutant concentrations, expressed as coefficients of variation, depend on chemical species: NO₃⁻∼8.1%; HNO₃∼6.4%; SO₂∼4.3%; NH₄⁺∼3.7%; SO₄²⁻∼2.3%; and O₃∼1.3%. Precision of estimates of weekly V_d from collocated measurements of model inputs also depends on the chemical species: aerosols ∼2.8%; HNO₃∼2.6%; SO₂∼3.0%; and O₃∼2.0%. Corresponding precision of weekly deposition estimates are: NO₃⁻∼8.6%; HNO₃⁻∼5.2%; SO₂∼5.6%; NH₄⁺∼3.9%; SO₄²⁻∼3.5%; and O₃∼3.3%. Precision of weekly concentration, V_d estimates, and deposition estimates are comparable in magnitude and slightly smaller than the corresponding hourly values. Annual precision estimates, although uncertain due to their small sample size in the current study, are consistent with the corresponding weekly values.     

Mercury chemistry in the MBL: Mediterranean case and sensitivity studies using the AMCOTS (Atmospheric Mercury Chemistry over the Sea) model

The atmospheric oxidation of mercury in the Mediterranean marine boundary layer (MBL) has been studied using the Atmospheric Mercury Chemistry over the Sea (AMCOTS) model. The model results have been compared to measured data obtained during an oceanographic research campaign in 2000, with more success than previous modelling attempts. In light of the often high concentrations of ozone present in the Mediterranean boundary layer, seasonal case studies using typical meteorological conditions and average ozone concentrations have been performed to identify the main oxidants of elemental mercury. The sensitivity of the modelled reactive gaseous mercury (RGM) concentrations to the Hg+O₃ rate constant has been assessed using the two most recent rate determinations. The results using the higher of the two literature values gives results inconsistent with measured values of RGM when the reaction between Hg and O₃ is assumed to give a gas phase product. This does not necessarily indicate that the rate constant is incorrect but possibly that other rate constants in the model are overestimated or indeed that there may be reduction reactions occurring in the atmosphere which have yet to be identified. Alternatively, when the reaction product of Hg and O₃ is assumed to be a solid and therefore not contribute to RGM the modelled and measured results are comparable. The deposition rates calculated by the model when compared with calculated and measured sea surface emission fluxes available in the literature indicate that dry deposition flux of RGM is comparable to the sea surface emission flux. The calculated lifetime of Hg⁰ in the Mediterranean MBL is between one and two weeks.     

Description and evaluation of a model of deposition velocities for routine estimates of dry deposition over North America. Part II: review of past measurements and model results

Numerical sensitivity tests and four months of complete model runs have been conducted for the Routine Deposition Model (RDM). The influence of individual model inputs on dry deposition velocity as a function of land-use category (LUC) and pollutant (SO₂, O₃, SO²⁻₄ and HNO₃) were examined over a realistic range of values for solar radiation, stability and wind speed. Spatial and temporal variations in RDM deposition velocity (V_d) during June – September 1996 time period generated using meteorological input from a mesoscale model run at 35 km resolution over north-eastern North America were also examined. Comparison of RDM V_d values to a variety of measurements of dry deposition velocities of SO₂, O₃, SO²⁻₄ and NHO₃ that have been reported in the literature demonstrated that RDM produces realistic results. Over northeastern NA RDM monthly averaged dry deposition velocities for SO₂ vary from 0.2 to 3.0 cm s⁻¹ with the highest deposition velocities over water surfaces. For O₃, the monthly averaged dry deposition velocities are from 0.05 to 1.0 cm s⁻¹ with the lowest values over water surfaces and the highest over forested areas. For HNO₃, the monthly averaged dry deposition velocities have the range of 0.5 to 6 cm s⁻¹, with the highest values for forested areas. For SO²⁻₄, they range from 0.05–1.5 cm s⁻¹, with the lowest values over water and the highest over forest. The monthly averaged dry deposition velocities for SO₂ and O₃ are higher in the growing season compared to the fall, but this behaviour is not apparent for HNO₃ and sulphate. In the daytime, the hourly averaged dry deposition velocities for SO₂, O₃, SO²⁻₄ and HNO₃ are higher than that in the nighttime over most of the vegetated area. The diurnal variation is most evident for surfaces with large values for leaf area index (LAI), such as forests. Based on the results presented in this paper, it is concluded that RDM V_d values can be combined with measured air concentrations over hourly, daily or weekly periods to determine dry deposition amounts and with wet deposition measurements to provide seasonal estimates of total deposition and estimates of the relative importance of dry deposition.     

Photosynthesis, chloroplast pigments, and antioxidants in Pinus canariensis under free-air ozone fumigation

High O₃ levels, driving uptake and challenging defense, prevail on the Canary Islands, being associated with the hot and dry summers of the Mediterranean-type climate. Pinus canariensis is an endemic conifer species that forms forests across these islands. We investigated the effects of ozone on photosynthesis and biochemical parameters of P. canariensis seedlings exposed to free-air O₃ fumigation at Kranzberg Forest, Germany, where ambient O₃ levels were similar to those at forest sites in the Canary Islands. The twice-ambient O₃ regime (2xO₃) neither caused visible injury-like chlorotic or necrotic spots in the needles nor significantly affected violaxanthin, antheraxanthin and zeaxanthin levels and the de-epoxidation state of the xanthophyll cycle. In parallel, stomatal conductance for water vapour, net photosynthesis, intercellular CO₂ concentration, chlorophyll fluorescence parameters, as well as antioxidant levels were hardly affected. It is concluded that presently prevailing O₃ levels do not impose severe stress on P. canariensis seedlings.     

Contrasting effects of ozone under different water supplies in two Mediterranean tree species

The effects of ozone (O₃) exposure under different water availabilities were studied in two Mediterranean tree species: Quercus ilex and Ceratonia siliqua. Plants were exposed to different O₃ concentrations in open top chambers (charcoal-filtered air (CF), non-filtered air (NF)) and non-filtered air plus 40 ppb_v of O₃ ((7:00–17:00 solar time) (NF+)) during 2 years, and to different water regimes (IR, sample irrigation, and WS, reduced water dose to 50%) through the last of those 2 years. AOT40 in the NF+ treatment was 59265 ppb_v h (from March 1999 to August 1999) while in the NF treatment, the AOT40 was 6727 ppb_v h for the same period. AOT40 was always 0 in the CF treatment. WS plants presented lower stomatal conductances and net photosynthetic rates, and higher foliar N concentrations than IR plants in both species. The irrigation treatment did not change the response trends to ozone in Q. ilex, the most sensitive species to O₃ ambient concentrations, but it changed those of C. siliqua, the least sensitive species, since its ozone-fumigated WS plants did not decrease their net photosynthetic rates nor their biomass accumulation as it happened to its ozone-fumigated IR plants. These results show interspecific variations in O₃ sensitivity under different water availabilities.     

Establishing ozone flux–response relationships for winter wheat: Analysis of uncertainties based on data for UK and Polish genotypes

The work outlined in this paper had three objectives. The first was to explore the effects of ozone pollution on grain yield and quality of commercially-grown winter wheat cultivars. The second was to derive a stomatal ozone flux model for winter wheat and compare with those already developed for spring wheat. The third was to evaluate exposure- versus flux–response approaches from a risk assessment perspective, and explore the implications of genetic variation in modelled ozone flux.Fifteen winter wheat cultivars were grown in open-top chambers where they were exposed to four levels of ozone. During fumigation, stomatal conductance measurements were made over the lifespan of the flag leaf across a range of environmental conditions. Although significant intra-specific variation in ‘ozone sensitivity’ (in terms of impacts on yield) was identified, yield was inversely related (R² = 0.63, P < 0.001) to the accumulated hourly averaged ozone exposure above 40 ppb during daylight hours (AOT40) across the dataset. The adverse effect of ozone on yield was principally due to a decline in seed weight. Algorithms defining the influence of environmental variables on stomatal uptake were subtly different from those currently in use, based on data for spring wheat, to map ozone impacts on pan-European cereal yield. Considerable intra-specific variation in phenological effects was identified. This meant that an ‘average behaviour’ had to be derived which reduced the predictive capability of the derived stomatal flux model (R² = 0.49, P < 0.001, 15 cultivars included). Indeed, given the intra-specific variability encountered, the flux model that was derived from the full dataset was no better in predicting O₃ impacts on wheat yield than was the AOT40 index. The study highlights the need to use ozone risk assessment tools appropriate to specific vegetation types when modelling and mapping ozone impacts at the regional level.