Measuring, modelling and testing ozone exposure, flux and effects on vegetation in southern European conditions--what does not work? A review from Italy

A Machine Learning Approach to Investigate the Surface Ozone Behavior

The concentration of surface ozone (O3) strongly depends on environmental and meteorological variables through a series of complex and non-linear functions. This study aims to explore the performances of an advanced machine learning (ML) method, the boosted regression trees (BRT) technique, in exploring the relationships between surface O3 and its driving factors, and in predicting the levels of O3 concentrations. To this end, a BRT model was trained on hourly data of air pollutants and meteorological parameters, acquired, over the 2016–2018 period, in a rural area affected by an anthropic source of air pollutants. The abilities of the BRT model in ranking, visualizing, and predicting the relationship between ground-level O3 concentrations and its driving factors were analyzed and illustrated. A comparison with a multiple linear regression (MLR) model was performed based on several statistical indicators. The results obtained indicated that the BRT model was able to account for 81% of changes in O3 concentrations; it slightly outperforms the MLR model in terms of the predictions accuracy and allows a better identification of the main factors influencing O3 variability on a local scale. This knowledge is expected to be useful in defining effective measures to prevent and/or mitigate the health damages associated with O3 exposure.

On the interactions among tropospheric ozone levels and typical environmental stresses challenging Mediterranean crops

The main environmental stresses of Italian croplands are discussed in relation to their interactions with ozone effects on crops. Water deficit and salinization are frequent in Mediterranean environments during spring-summer causing a decrease of soil water potential and water uptake by roots and consequently stomatal closure. These stresses also stimulate secondary metabolism and antioxidant accumulation, which also serves as a stress protection mechanism. High concentrations of tropospheric ozone are common all over Italy during the spring-summer season. Ozone injuries to vegetation are related to its penetration into plant tissues, mostly via stomatal uptake, rather than to tropospheric concentrations per se. In several crops, closure of stomata due to drought/salinization reduces ozone entering into leaf tissues and counteracts possible ozone damages. Furthermore, the stimulation of antioxidant synthesis as a response to environmental stresses can represent a further protection factor from ozone injuries for Mediterranean crops.The co-existence of stress-induced stomatal closure and high ozone levels during spring-summer in Mediterranean environments implies that models that do not take into account physiological responses of crops to drought and salinity stress may overestimate ozone damages when stress responses overlap with seasonal ozone peaks. The shift from concentration-based to flux-based approaches has improved the accuracy of models to assess ozone effects on agricultural crops. It is, however, necessary to further refine the flux concept with respect to the plant abiotic stress defense capacity that can differ among genotypes, climatic conditions, and physiological states.

Scarce evidence of ozone effect on recent health and productivity of alpine forests-a case study in Trentino, N. Italy

We investigated the significance of tropospheric ozone as a factor explaining recent tree health (in terms of defoliation) and productivity (in terms of basal area increment, BAI) in 15 ICP Forests level I and one level II plots in alpine forests in Trentino (N. Italy). Mean daily ozone summer concentrations varied between 30 and 72 parts per billion (ppb) leading to large exceedance of concentration-based critical levels set to protect forest trees. Phytoxic ozone dose (POD₀) estimated at the level II plot over the period 1996-2009 was 31-61 mmol m^-2 projected leaf area (PLA). The role of ozone was investigated taking into account other site and environmental factors. Simple linear regression, multiple linear regression (MLR, to study mean periodical defoliation and mean periodical BAI), and linear mixed models (LMM, to study annual defoliation data) were used. Our findings suggest that-regardless of the metric adopted-tropospheric ozone is not a significant factor in explaining recent status and trends of defoliation and BAI in the alpine region examined. Both defoliation and BAI are in turn driven by biotic/abiotic damage, nutritional status, DBH (assumed as a proxy for age), and site characteristics. These results contrast with available ozone-growth dose response relationships (DRRs) and other observational studies. This may be due to a variety of concurrent reasons: (i) DRRs developed for individual saplings under controlled condition are not necessarily valid for population of mature trees into real forest ecosystems; (ii) some observational studies may have suffered from biased design; and (iii) since alpine forests have been exposed to high ozone levels (and other oxidative stress) over decades, possible acclimation mechanisms cannot be excluded.

Ozone exposure- and flux-based response relationships with photosynthesis, leaf morphology and biomass in two poplar clones

Poplar clones 546 (P. deltoides cv. '55/56'×P. deltoides cv. 'Imperial') and 107 (P. euramericana cv. '74/76') were exposed to five ozone concentrations in 15 open-top chambers (OTCs). Both ozone exposure (AOT40, Accumulation Over a Threshold hourly ozone concentration of 40ppb) and flux-based (POD₇, Phytotoxic Ozone Dose above an hourly flux threshold of 7nmol O₃ m^-2 PLA (projected leaf area) s^-1) response relationships were established with photosynthesis, leaf morphology and biomass variables. Increases in both metrics showed significant negative relationships with light-saturated photosynthesis rate, chlorophyll content, leaf mass per area, actual photochemical efficiency of PSII in the light and root biomass but not with stomatal conductance (g_s), leaf and stem biomass. Ozone had a greater impact on belowground than on aboveground biomass. The ranking of these indicators from higher to lower sensitivity to ozone was: photosynthetic parameters, morphological index, and biomass. Clone 546 had a higher sensitivity to ozone than clone 107. The coefficients of determination (R²) were similar between exposure- and flux-based dose-response relationships for each variable. The critical levels (CLs) for a 5% reduction in total biomass for the two poplar clones were 14.8ppmh for AOT40 and 9.8mmol O₃ m^-2 PLA for POD₇. In comparison, equivalent reduction occurred at much lower values in photosynthetic parameters (4ppmh for AOT40 and 3mmol O₃ m^-2 PLA for POD₇) and LMA (5.8ppmh for AOT40 and 4mmol O₃ m^-2 PLA for POD₇). While in recent decades different CLs have been proposed for several plant receptors especially in Europe, studies focusing on both flux-based dose-response relationships and CLs are still scarce in Asia. This study is therefore valuable for regional O₃ risk assessment in Asia.

Effects of the Antiozonant Ethylenediurea (EDU) on Fraxinus ornus L.: The Role of Drought

Ethylenediurea (EDU) is a synthetic chemical known to protect plants from the phytotoxic effects of tropospheric ozone (O3). Although many studies have proposed the use of EDU for studying the O3 effects under field conditions, its mechanism of action is not fully understood, and it is unclear whether it exerts a specific antiozonant action, or if it may also interact with other oxidative stresses. The aim of this work was to evaluate the effect of EDU on forest species in a Mediterranean environment where, during summer, vegetation is exposed to multiple oxidative stresses, such as O3 and drought. The experiment was conducted on Fraxinus ornus L. (Manna ash) plants growing in six mesocosms, three maintained under full irrigation, while the other three were subjected to drought for 84 days. In each mesocosm, three plants were sprayed every 15 days with 450 ppm EDU. Gas exchange and chlorophyll “a” fluorescence measurements carried out through the experimental period highlighted that EDU did not affect stomatal conductance and had an ameliorative effect on the functionality of drought-stressed plants, thus suggesting that it may act as a generic antioxidant. The implications of these findings for the applicability of EDU in field studies are discussed.

Ozone risk and foliar injury on Viburnum lantana L.: a meso-scale epidemiological study

A stratified random sampling design was adopted to contrast sites with different ozone exposure levels (≤ 18,000 and >18,000 μg m(-3) h) in order to define whether and to what extent a relationship exists between potential risk (estimated by exposure to ozone) and the response of Viburnum lantana L. in terms of foliar symptoms. The study was designed over a meso-scale (6200 km(2)), carried out in 2010 and repeated in 2012 on a subset of sites. No difference was found between the occurrences of symptoms in relation to soil moisture or plant size. Although no direct significant exposure-response function could be identified, when data were aggregated according to ozone exposure levels the symptoms (in terms of number of symptomatic plants and symptomatic leaves per plant) were found to be significantly more frequent at sites with higher exposure (AOT40>18,000 μg m(-3) h), especially at high elevations (>700 ma.s.l.). The 2012 results confirmed the 2010 findings. Although ozone levels in the region were almost similar between 2010 and 2012, symptoms were significantly less frequent in 2012. This was likely due to drier conditions in 2012 (+1.1 °C; -23% precipitation), a situation that may have prevented in part ozone uptake and therefore the expression of symptoms. These results are useful in several respects: (i) for identifying areas where ozone is likely to impact vegetation; (ii) for testing the appropriateness of EU standards to protect vegetation from ozone; and (iii) for designing biomonitoring surveys. We suggest that V. lantana is a suitable indicator for assessing qualitatively (but not quantitatively) the potential risk of ozone damage to vegetation over remote, large areas.

Assessing ozone and nitrogen impact on net primary productivity with a Generalised non-Linear Model

Some studies suggest that in Europe the majority of forest growth increment can be accounted for N deposition and very little by elevated CO(2). High ozone (O(3)) concentrations cause reductions in carbon fixation in native plants by offsetting the effects of elevated CO(2) or N deposition. The cause-effect relationships between primary productivity (NPP) of Quercus cerris, Q. ilex and Fagus sylvatica plant species and climate and pollutants (O(3) and N deposition) in Italy have been investigated by application of Generalised Linear/non-Linear regression model (GLZ model). The GLZ model highlighted: i) cumulative O(3) concentration-based indicator (AOT40F) did not significantly affect NPP; ii) a differential action of oxidised and reduced nitrogen depositions to NPP was linked to the geographical location; iii) the species-specific variation of NPP caused by combination of pollutants and climatic variables could be a potentially important drive-factor for the plant species' shift as response to the future climate change.

Method for automatic determination of soybean actual evapotranspiration under open top chambers (OTC) subjected to effects of water stress and air ozone concentration

The present study describes an operational method, based on the Katerji et al. (Eur J Agron 33:218-230, 2010) model, for determining the daily evapotranspiration (ET) for soybean inside open top chambers (OTCs). It includes two functions, calculated day par day, making it possible to separately take into account the effects of concentrations of air ozone and plant water stress. This last function was calibrated in function of the daily values of actual water reserve in the soil. The input variables of the method are (a) the diurnal values of global radiation and temperature, usually measured routinely in a standard weather station; (b) the daily values of the AOT40 index accumulated (accumulated ozone over a threshold of 40 ppb during daylight hours, when global radiation exceeds 50 Wm(-2)) determined inside the OTC; and (c) the actual water reserve in the soil, at the beginning of the trial. The ensemble of these input variables can be automatable; thus, the proposed method could be applied in routine. The ability of the method to take into account contrasting conditions of ozone air concentration and water stress was evaluated over three successive years, for 513 days, in ten crop growth cycles, excluding the days employed to calibrate the method. Tests were carried out in several chambers for each year and take into account the intra- and inter-year variability of ET measured inside the OTCs. On the daily scale, the slope of the linear regression between the ET measured by the soil water balance and that calculated by the proposed method, under different water conditions, are 0.98 and 1.05 for the filtered and unfiltered (or enriched) OTCs with root mean square error (RMSE) equal to 0.77 and 1.07 mm, respectively. On the seasonal scale, the mean difference between measured and calculated ET is equal to +5% and +11% for the filtered and unfiltered OTCs, respectively. The ability of the proposed method to estimate the daily and seasonal ET inside the OTCs is therefore satisfactory following inter- and intra-annual tests. Finally, suggestions about the applications of the proposed method for other species, different from soybean, were also discussed.

Ozone uptake by adult urban trees based on sap flow measurement

The O(3) uptake in 17 adult trees of six urban species was evaluated by the sap flow-based approach under free atmospheric conditions. The results showed very large species differences in ground area scaled whole-tree ozone uptake (F(o)₃), with estimates ranging from 0.61 ± 0.07 nmol m(-2) s(-1) in Robinia pseudoacacia to 4.80 ± 1.04 nmol m(-2) s(-1) in Magnolia liliiflora. However, average F(o)₃by deciduous foliages was not significantly higher than that by evergreen ones (3.13 vs 2.21 nmol m(-2) s(-1), p = 0.160). Species of high canopy conductance for O(3) (G(o)₃) took up more O(3) than those of low G(o)₃, but that their sensitivity to vapour pressure deficit (D) were also higher, and their F(o)₃decreased faster with increasing D, regardless of species. The responses of F(o)₃to D and total radiation led to the relative high flux of O(3) uptake, indicating high ozone risk for urban tree species.

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 CO(2) 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.

Crops' responses to ozone in Mediterranean environments

The Mediterranean environment, and most of the Italian peninsula, presents some peculiarities in terms of crop response to O(3) since most physiological mechanisms activated upon O(3) exposure, such as stomatal closure, often overlap and interact with those that underlie plant adaptation to drought and hyperosmotic stress, which are typical of these environments. OTC and EDU experiments have demonstrated that O(3) causes strong yield losses when crops are grown without water limitations. However, exposure to water or saline stress significantly reduced O(3) effects on crop yield. In this review, we present the methodological approaches that have been used to study plant-ozone interactions in Italy as well as biochemical, physiological and agronomic responses for representative cropping systems of the Mediterranean climate.

Assessment of present and future risk to Italian forests and human health: modelling and mapping

A review of ozone pollution in Italy shows levels largely above the thresholds established by EU regulation for vegetation and human health protection. The Italian air quality monitoring network appears quantitatively inadequate to cover all the territorial surface, because of scarcity and unequal distribution of monitoring sites. By applying the integrated assessment model RAINS-Italy to the year 2000, the whole of Italy exceeds the AOT40 critical level for forest, while Northern and central areas show strong potential of O(3) impact on human health with approximately 11% of territory >10 O(3)-induced premature deaths. Two scenarios for the year 2020, the Current Legislation and the Maximum Technical Feasible Reduction, show a reduction of AOT40Forest by 29% and 44%, SOMO35 by 31% and 47%, and O(3)-induced premature deaths by 32% and 48%, compared to 2000. RAINS-Italy can be used to improve the map quality and cover areas not reached by the national monitoring network.

Responses to ozone pollution of alfalfa exposed to increasing salinity levels

Stomatal closure and biosynthesis of antioxidant molecules are two fundamental components of the physiological machinery that lead to stress adaptation during plant's exposure to salinity. Since high stomatal resistance may also contribute in counteracting O(3) damages, we hypothesized that soil salinization may increase O(3) tolerance of crops. An experiment was performed with alfalfa grown in filtered (AOT40=0 in both years) and non-filtered (AOT40=9.7 in 2005 and 6.9 ppm h in 2006) open-top chambers. Alfalfa yield was reduced by O(3) (-33%) only in plants irrigated with salt-free water, while the increasing levels of soil salinity until 1.06 dS m(-1) reduced both stomatal conductance and plant O(3) uptake, thus linearly reducing O(3) effects on yield. Therefore a reliable flux-based model for assessing the effects of O(3) on crop yield should take into account soil salinity.

Ozone fluxes over various plant ecosystems in Italy: a review

Among air pollutants, ozone is the most important stressor to vegetation, which undergoes damage and biomass reduction after penetration of ozone molecules into the leaf tissues through the stomata. Stomatal ozone fluxes are considered the governing factor needed to assess risk to plant health due to ozone. Although this parameter may be calculated by modeling, direct measurements are scarce. Moreover, southern European situations, especially regarding Italy, require special attention due to the decoupling between ozone concentrations and fluxes. This work reviews ozone flux measurements made during the last 15 years through Italy.

Plant cell death and cellular alterations induced by ozone: key studies in Mediterranean conditions

An account of histo-cytological and ultrastructural studies on ozone effect on crop and forest species in Italy is given, with emphasis on induced cell death and the underlying mechanisms. Cell death phenomena possibly due to ambient O(3) were recorded in crop and forest species. In contrast, visible O(3) effects on Mediterranean vegetation are often unclear. Microscopy is thus suggested as an effective tool to validate and evaluate O(3) injury to Mediterranean vegetation. A DAB-Evans blue staining was proposed to validate O(3) symptoms at the microscopic level and for a pre-visual diagnosis of O(3) injury. The method has been positively tested in some of the most important crop species, such as wheat, tomato, bean and onion and, with some restriction, in forest species, and it also allows one to gain some very useful insights into the mechanisms at the base of O(3) sensitivity or tolerance.

Visible injury, crown condition, and growth responses of selected Italian forests in relation to ozone exposure

The impact of ozone on forest ecosystems in Italy is monitored within the CONECOFOR programme. Ozone levels are measured in 30 plots using passive samplers. Response parameters used are: crown condition (transparency), BAI (basal area increment), and visible symptoms on spontaneous vegetation. Levels of AOT40 are above the concentration-based critical level of 5 ppm h in all sites, but the evidence of impact on forest vegetation remains limited. Ozone is a predictor of crown transparency residuals in beech sites over two consecutive years, but the variance explained amounts to less than 10%. The relation between BAI reduction and ozone is even less certain. Transparency and BAI are more readily explainable in terms of ecological conditions of the site and climate fluctuations. The interpretation of visible symptoms is doubtful, and is conditioned by the prevailing ecological factors in the areas.

White clover clones as a cost-effective indicator of phytotoxic ozone: 10 years of experience from central Italy

Data collected at one site in central Italy using the NC-S/NC-R clover (Trifolium repens) biotype system during 1997-2007 were analysed in order to assess: (a) its performance under Mediterranean conditions; (b) variations of ozone damage linked with meteorological conditions; (c) if critical level approach is a good predictor of ozone risk on vegetation. NC-S dry biomasses were systematically lower than those of NC-R, the mean ratio being 0.7. Relevant relationship between ozone visible injury and cumulated values of AOT40 were also reported. Temperature and number of rainy days were the most important factors associated with ozone presence and, as a consequence, with leaf injury index. Photosynthetic gas exchange properties indicate that NC-S has higher values of stomatal conductance.

Toward a biologically significant and usable standard for ozone that will also protect plants

Ozone remains an important phytotoxic air pollutant and is also recognized as a significant greenhouse gas. In North America, Europe, and Asia, incidence of high concentrations is decreasing, but background levels are steadily rising. There is a need to develop a biologically significant and usable standard for ozone. We compare the strengths and weaknesses of concentration-based, exposure-based and threshold-based indices, such as SUM60 and AOT40, and examine the O(3) flux concept. We also present major challenges to the development of an air quality standard for ozone that has both biological significance and practicality in usage.

Integrated effects of air pollution and climate change on forests: a northern hemisphere perspective

Many air pollutants and greenhouse gases have common sources, contribute to radiative balance, interact in the atmosphere, and affect ecosystems. The impacts on forest ecosystems have been traditionally treated separately for air pollution and climate change. However, the combined effects may significantly differ from a sum of separate effects. We review the links between air pollution and climate change and their interactive effects on northern hemisphere forests. A simultaneous addressing of the air pollution and climate change effects on forests may result in more effective research, management and monitoring as well as better integration of local, national and global environmental policies.

Why should we calculate complex indices of ozone exposure? Results from Mediterranean background sites

While moving towards a flux-based approach, exposure-based ozone metrics are still a practical measure for summarising ambient air quality. Ozone hourly concentrations for the period 2000-2004 from sites in the Mediterranean Italy (< or =600 m a.s.l.) were examined to define the O3 summary statistic in the area, and to determine how O3 exposure indices correlate to each other. Thirty-four of the most common O3 exposure metrics were calculated. The results show that background O3 pollution in Italy exceeds the European and North American standards. The exceedances of the target value, information and alert thresholds set by the 2002/3/CE Directive should encourage Italy to take the appropriate measures to reduce the risk. All the O3 exposure indices, except the maximum permissible ozone concentration (MPOC) for forests, point to the potential for negative effects on vegetation and human health across Italy. As indices evaluated significantly correlated with each other, we suggest use of the most biologically meaningful metric when summarizing air quality information.

Promoting the O3 flux concept for European forest trees

Tropospheric ozone (O3) levels are predicted to stay high, being a factor within "global change" with potential effects on the carbon sink strength of forest trees. Hence, new approaches to O3 risk assessment and their validation are required, although appropriate databases for adult trees are scant. Approaches based on external O3 exposure are presently being evaluated against the ones on O3 flux into leaves, as the cumulative uptake has the capacity for deriving O3 risk from cause-effect relationships. The effective dose, however, needs to account for the trees' O3 defence and tolerance in addition to O3 uptake. The current status of promoting the preferable mechanistic O3 flux concept is highlighted for major regions of Europe, addressing refinements and simplifications needed for routine use. At the pan-European scale, however, the flux-based concept is ready for use in O3 risk assessment and has the potential of meso-scale application at the forest ecosystem level.

Ozone and forests in South-Western Europe--What have we learned?

Monitoring of forest condition and ozone (O3) at 83 sites in France, Italy, Luxenbourg, Spain and Switzerland resulted in a number of findings in relation to the knowledge of O3 exposure (concentration and cumulative AOT40), feasibility of the assessment of stomatal O3 flux and relationships between O3 and crown defoliation of beech and visible symptoms on native vegetation. However, the project provides evidence of issues to be addressed within the current monitoring system (data quality, validation sites and response indicators) and indications as to how the monitoring of O3 risk in the context of an effect-oriented monitoring program can be improved.