Toward stomatal-flux based forest protection against ozone: The MOTTLES approach

Long-term trajectory of ozone impact on maize and soybean yields in the United States: A 40-year spatial-temporal analysis

Understanding the long-term change trends of ozone-induced yield losses is crucial for formulating strategies to alleviate ozone damaging effects, aiming towards achieving the Zero Hunger Sustainable Development Goal. Despite a wealth of experimental research indicating that ozone's influence on agricultural production exhibits marked fluctuations and differs significantly across various geographical locations, previous studies using global statistical models often failed to capture this spatial-temporal variability, leading to uncertainties in ozone impact estimation. To address this issue, we conducted a comprehensive assessment of the spatial-temporal variability of ozone impacts on maize and soybean yields in the United States (1981-2021) using a geographically and temporally weighted regression (GTWR) model. Our results revealed that over the past four decades, ozone pollution has led to average yield losses of -3.5% for maize and -6.1% for soybean, translating into an annual economic loss of approximately $2.6 billion. Interestingly, despite an overall downward trend in ozone impacts on crop yields following the implementation of stringent ozone emission control measures in 1997, our study identified distinct peaks of abnormally high yield reduction rates in drought years. Significant spatial heterogeneity was detected in ozone impacts across the study area, with ozone damage hotspots located in the Southeast Region and the Mississippi River Basin for maize and soybean, respectively. Furthermore, we discovered that hydrothermal factors modulate crop responses to ozone, with maize showing an inverted U-shaped yield loss trend with temperature increases, while soybean demonstrated an upward trend. Both crops experienced amplified ozone-induced yield losses with rising precipitation. Overall, our study highlights the necessity of incorporating spatiotemporal variability into assessments of crop yield losses attributable to ozone pollution. The insights garnered from our findings can contribute to the formulation of region-specific pollutant emission policies, based on the distinct profiles of ozone-induced agricultural damage across different regions.

Surface ozone risk to human health and vegetation in tropical region: The case of Thailand

Tropospheric ozone (O3) is a threat to vegetation and human health over the world, in particular in Asia. Knowledge on O3 impacts on tropical ecosystems is still very limited. An O3 risk assessment to crops, forests, and people from 25 monitoring stations across the tropical and subtropical Thailand during 2005-2018 showed that 44% of sites exceeded the critical levels (CLs) of SOMO35 (i.e., the annual Sum Of daily maximum 8-h Means Over 35 ppb) for human health protection. The concentration-based AOT40 CL (i.e., sum of the hourly exceedances above 40 ppb for daylight hours during the assumed growing season) was exceeded at 52% and 48% of the sites where the main crops rice and maize are present, respectively, and at 88% and 12% of the sites where evergreen or deciduous forests are present, respectively. The flux-based metric PODY (i.e., Phytotoxic Ozone Dose above a threshold Y of uptake) was calculated and was found to exceed the CLs at 1.0%, 1.5%, 20.0%, 1.5%, 0% and 68.0% of the sites where early rice, late rice, early maize, late maize, evergreen forests, and deciduous forests can grow, respectively. Trend analysis indicated that AOT40 increased over the study period (+5.9% year-1), while POD1 decreased (- 5.3% year-1), suggesting that the role of climate change in affecting the environmental factors that control stomatal uptake cannot be neglected. These results contribute novel knowledge on O3 threat to human health, forest productivity, and food security in tropical and subtropical areas.

Low sensitivity of Pinus mugo to surface ozone pollution in the subalpine zone of continental Europe

High altitudes have been exposed to enhanced levels of surface ozone (O₃) concentrations over recent decades compared to the pre-industrial era. The responses of vegetation to this toxic pollutant are species-specific and depend on the climate conditions. In this paper, we explored the reaction of Pinus mugo (P. mugo) to O₃-induced stress in the continental climate of an ozone-rich mountain area in the High Tatra Mountains (Western Carpathians). The effects of O₃ doses modelled by a deposition model, O₃ concentrations and other factors on P. mugo were identified from (a) satellite-based data via NDVI (normalised differenced vegetation index) over 2000-2020 and (b) visible injury on needle samples gathered from P. mugo individuals at ground-truth sites in 2019 and 2020. Analysing the NDVI trend, we observed non-significant changes (p > 0.05) in the greenness of P. mugo despite growing in an environment with the average seasonal O₃ concentration around 51.6 ppbv, the maximum hourly concentrations more than 90 ppbv and increasing trend of O₃ doses by 0.1 mmol m^-2 PLA (plant leaf area) year^-1. The visible O₃ injury of samples collected at study sites was low (mean injury observed on 1-10% of needles' surface), and the symptoms of injury caused by other biotic and abiotic factors prevailed over those caused by O₃. In addition, the correlation analyses between NDVI and the climatic factors indicated a significant (p < 0.05) and positive relationship with photosynthetic active radiation (R = 0.45) in July, and with stomatal conductance (R = 0.52) and temperature factor (R = 0.43) in August. Therefore, we concluded that the positive effect of climate conditions, which support the growth processes of P. mugo, may suppress the negative effect of the mean O₃ doses of 17.8 mmol m^-2 PLA accumulated over the growing season.

Sensitivity of agricultural crops to tropospheric ozone: a review of Indian researches

Tropospheric ozone (O₃) is a long-range transboundary secondary air pollutant, causing significant damage to agricultural crops worldwide. There are substantial spatial variations in O₃ concentration in different areas of India due to seasonal and geographical variations. The Indo-Gangetic Plain (IGP) region is one of the most crop productive and air-polluted regions in India. The concentration of tropospheric O₃ over the IGP is increasing by 6-7.2% per decade. The annual trend of increase is 0.4 ± 0.25% year^-1 over the Northeastern IGP. High O₃ concentrations were reported during the summer, while they were at their minimum during the monsoon months. To explore future potential impacts of O₃ on major crop plants, the responses of different crops grown under ambient and elevated O₃ concentrations were compared. The studies clearly showed that O₃ is an important stress factor, negatively affecting the yield of crops. In this review, we have discussed yield losses in agricultural crops due to rising O₃ pollution and variations in O₃ sensitivity among cultivars and species. The use of ethylene diurea (EDU) as a research tool in assessing the losses in yield under ambient and elevated O₃ levels also discussed. Besides, an overview of interactive effects of O₃ and nitrogen on crop productivity has been included. Several recommendations are made for future research and policy development on rising concentration of O₃ in India.

Strategic roadmap to assess forest vulnerability under air pollution and climate change

Although it is an integral part of global change, most of the research addressing the effects of climate change on forests have overlooked the role of environmental pollution. Similarly, most studies investigating the effects of air pollutants on forests have generally neglected the impacts of climate change. We review the current knowledge on combined air pollution and climate change effects on global forest ecosystems and identify several key research priorities as a roadmap for the future. Specifically, we recommend (1) the establishment of much denser array of monitoring sites, particularly in the South Hemisphere; (2) further integration of ground and satellite monitoring; (3) generation of flux-based standards and critical levels taking into account the sensitivity of dominant forest tree species; (4) long-term monitoring of N, S, P cycles and base cations deposition together at global scale; (5) intensification of experimental studies, addressing the combined effects of different abiotic factors on forests by assuring a better representation of taxonomic and functional diversity across the ~73,000 tree species on Earth; (6) more experimental focus on phenomics and genomics; (7) improved knowledge on key processes regulating the dynamics of radionuclides in forest systems; and (8) development of models integrating air pollution and climate change data from long-term monitoring programs.

Spatiotemporal variations of ozone exposure and its risks to vegetation and human health in Cyprus: an analysis across a gradient of altitudes

Ground-level ozone (O₃) affects vegetation and threatens environmental health when levels exceed critical values, above which adverse effects are expected. Cyprus is expected to be a hotspot for O₃ concentrations due to its unique position in the eastern Mediterranean, receiving air masses from Europe, African, and Asian continents, and experiencing a warm Mediterranean climate. In Cyprus, the spatiotemporal features of O₃ are poorly understood and the potential risks for forest health have not been explored. We evaluated O₃ and nitrogen oxides (NO and NO₂) at four regional background stations at different altitudes over 2014-2016. O₃ risks to vegetation and human health were estimated by calculating accumulated O₃ exposure over a threshold of 40 nmol mol^-1 (AOT40) and cumulative exposure to mixing ratios above 35 nmol mol^-1 (SOMO35) indices. The data reveal that mean O₃ concentrations follow a seasonal pattern, with higher levels in spring (51.8 nmol mol^-1) and summer (53.2 nmol mol^-1) and lower levels in autumn (46.9 nmol mol^-1) and winter (43.3 nmol mol^-1). The highest mean O₃ exposure (59.5 nmol mol^-1) in summer occurred at the high elevation station Mt. Troodos (1819 m a.s.l.). Increasing (decreasing) altitudinal gradients were found for O₃ (NO_x), driven by summer-winter differences. The diurnal patterns of O₃ showed little variation. Only at the lowest altitude O₃ displayed a typical O₃ diurnal pattern, with hourly differences smaller than 15 nmol mol^-1. Accumulated O₃ exposures at all stations and in all years exceeded the European Union's limits for the protection of vegetation, with average values of 3-month (limit: 3000 nmol mol^-1 h) and 6-month (limit: 5000 nmol mol^-1 h) AOT40 for crops and forests of 16,564 and 31,836 nmol mol^-1 h, respectively. O₃ exposures were considerably high for human health, with an average SOMO35 value of 7270 nmol mol^-1 days across stations and years. The results indicate that O₃ is a major environmental and public health issue in Cyprus, and policies must be adopted to mitigate O₃ precursor emissions at local and regional scales.

Influence of Ozone and Drought on Tree Growth under Field Conditions in a 22 Year Time Series

Studying the effect of surface ozone (O3) and water stress on tree growth is important for planning sustainable forest management and forest ecology. In the present study, a 22-year long time series (1998–2019) on basal area increment (BAI) and fructification severity of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) H.Karst.) at five forest sites in Western Germany (Rhineland Palatinate) was investigated to evaluate how it correlates with drought and stomatal O3 fluxes (PODY) with an hourly threshold of uptake (Y) to represent the detoxification capacity of trees (POD1, with Y = 1 nmol O3 m−2 s−1). Between 1998 and 2019, POD1 declined over time by on average 0.31 mmol m−2 year−1. The BAI showed no significant trend at all sites, except in Leisel where a slight decline was observed over time (−0.37 cm2 per year, p < 0.05). A random forest analysis showed that the soil water content and daytime O3 mean concentration were the best predictors of BAI at all sites. The highest mean score of fructification was observed during the dry years, while low level or no fructification was observed in most humid years. Combined effects of drought and O3 pollution mostly influence tree growth decline for European beech and Norway spruce.

Addressing Gender Inequities in Forest Science and Research

Forest research and professional workforces continue to be dominated by men, particularly at senior and management levels. In this review, we identify some of the historical and ongoing barriers to improved gender inclusion and suggest some solutions. We showcase a selection of women in forestry from different disciplines and parts of the globe to highlight a range of research being conducted by women in forests. Boosting gender equity in forest disciplines requires a variety of approaches across local, regional and global scales. It is also important to include intersectional analyses when identifying barriers for women in forestry, but enhanced equity, diversity and inclusion will improve outcomes for forest ecosystems and social values of forests, with potential additional economic benefits.

Species-specific variation of photosynthesis and mesophyll conductance to ozone and drought in three Mediterranean oaks

Mesophyll conductance (g_mCO2 ) is one of the most important components in plant photosynthesis. Tropospheric ozone (O₃ ) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O₃ and drought on g_mCO2 are still largely unclear. We investigated leaf gas exchange during mid-summer in three Mediterranean oaks exposed to O₃ (ambient [35.2 nmol mol^-1 as daily mean]; 1.4 × ambient) and water treatments (WW [well-watered] and WD [water-deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO₂ inside a leaf. The combination of O₃ and WD significantly decreased net photosynthetic rate (P_N ) regardless of the species. The reduction of photosynthesis was associated with a decrease in g_mCO2 and stomatal conductance (g_sCO2 ) in evergreen Quercus ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (V_cmax ) and maximum electron transport rate (J_max ) with decreased diffusive conductance parameters. The reduction of g_mCO2 was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between g_mCO2 with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between g_sCO2 and ABA under high O₃ and WD due to impaired stomatal physiological behaviour, indicating that the reduction of P_N was related to g_mCO2 rather than g_sCO2 . The results suggest that g_mCO2 plays an important role in plant carbon gain under concurrent increases in the severity of drought and O₃ pollution.

Assessment of ozone risk to Central European forests: Time series indicates perennial exceedance of ozone critical levels

In this study, the stomatal ozone (O₃) fluxes were investigated at five low-elevation forest sites in Western Germany (Rhineland Palatinate) over the time period 1998-2019. The Phytotoxic Ozone Dose with an hourly threshold of uptake (Y), to represent the detoxification capacity of trees (POD1 in mmol m^-2 per leaf area, with Y = 1 nmol O₃ m^-2 s^-1), and the number of exceedances of the O₃ critical level of 5.2 mmol O₃ m^-2 per leaf area for European beech and 9.2 mmol O₃ m^-2 per leaf area for Norway spruce were calculated by using the DO₃SE model. A Principal Component Analysis revealed strong correlations between daily O₃ concentrations, daytime O₃ (for hours with global radiation exceeding 50 W m^-2), POD1, global radiation, vapor pressure deficit and air temperature. Moreover, a significant correlation was obtained between POD1 and soil water content (SWC) at all sites (r = 0.51-0.74). The Random Forests Analysis confirmed that the SWC is the most important predictor of stomatal O₃ fluxes. The soil water supply is very important for POD1 estimation, because drought decreases stomatal conductance, leading to a reduction of transpiration, as well as to lower O₃ uptake through stomata. Between 1998 and 2019, the drier and warmer climate induced a soil drought (on average, SWC - 0.15 % per year) leading to lower stomatal O₃ uptake by forests (- 0.36 mmol O₃ m^-2 per year). Hence, during growing seasons with sufficient water supply and often lower O₃ levels compared to hot and dry periods, forests are at higher O₃ risk than during hot and dry periods when the drought stress is more significant than O₃ stress despite relatively higher O₃ levels. Irrespective of these differences in O₃ uptake between relatively cool and humid as compared to relatively hot and dry years in the study region, the Critical Level for O₃ was exceeded in late spring/early summer (May/June) during all 22 years. Risk assessment for the protection of European forests, which is urgently needed due to the forests current critical state after several successive years of drought and exceedance of the O₃ critical level in large areas of Europe, should therefore become flux-based to account for the inter-twined effects of drought and O₃ on the physiology and health of forest trees in the region. For stomatal O₃ fluxes estimation, a better soil water and leaf parameterization is needed e.g., by taking into account both O₃- and drought-induced effects.

Economic and Life Cycle Analysis of Passive and Active Monitoring of Ozone for Forest Protection

At forest sites, phytotoxic tropospheric ozone (O3) can be monitored with continuously operating, active monitors (AM) or passive, cumulative samplers (PM). For the first time, we present evidence that the sustainability of active monitoring is better than that of passive sensors, as the environmental, economic, and social costs are usually lower in the former than in the latter. By using data collected in the field, environmental, social, and economic costs were analyzed. The study considered monitoring sites at three distances from a control station in Italy (30, 400, and 750 km), two forest types (deciduous and Mediterranean evergreen), and three time windows (5, 10, and 20 years of monitoring). AM resulted in more convenience than PM, even after 5 years, in terms of O3 depletion, global warming, and photochemical O3 creation potential, suggesting that passive monitoring of ozone is not environmentally sustainable, especially for long time periods. AM led to savings ranging from a minimum of EUR 9650 in 5 years up to EUR 94,796 in 20 years in evergreen forests. The resulting social cost of PM was always higher than that of AM. The present evaluation will help in the decision process for the set-up of long-term forest monitoring sites dedicated to the protection of forests from O3.

Impact of ground-level ozone on Mediterranean forest ecosystems health

Given the high ozone concentrations observed in the Mediterranean region during summer, it is crucial to extend our knowledge on the potential ozone impacts on forest health with in situ studies, especially to protect typical endemic forests of the Mediterranean basin. This study is focused on ozone measurements and exposures over the Eastern Adriatic coast and on the calculation of different O₃ metrics, i.e., accumulated exposure AOT40 (AOT40_dir, AOT40_ICP, AOT40_pheno) and stomatal O₃ fluxes with an hourly threshold of uptake (Y) to represent the detoxification capacity of trees (PODY, with Y = 0, 1, 2 nmol O₃ m^-2 s^-1) used for forest protection. Finally, we provide an assessment of the relationships between the forest response indicators and environmental variables. Passive ozone measurements and monitoring of forest health indicators, namely growth and crown defoliation, were performed for Quercus ilex, Quercus pubescens, Pinus halepensis, and Pinus nigra forests. Results showed that, for all the analysed species, ozone levels were close to reached the upper plausibility limits for passive monitoring of air quality at forest sites (100 ppb), with the highest values found on P. halepensis in the summer period. O₃ metrics based on exposure were found to be higher in pine plots than in oak plots, while the highest values of uptake-based metrics were found on P. nigra. Regarding relationships between environmental variables and forest-health response indicators, the crown defoliation was significantly correlated with the soil water content at various depth while the tree growth was correlated with the different O₃ metrics. The most important predictors affecting tree growth of Q. pubescens and Q. ilex were AOT40_pheno and AOT40_dir and POD0 for P. nigra.

Temporal Changes in Ozone Concentrations and Their Impact on Vegetation

Tropospheric concentrations of phytotoxic ozone (O3) have undergone a great increase from preindustrial 10–15 ppbv to a present-day concentration of 35–40 ppbv in large parts of the industrialised world due to increased emissions of O3 precursors including NOx, CO, CH4 and volatile organic compounds. The rate of increase in O3 concentration ranges between 1 ppbv per decade in remote locations of the Southern hemisphere and 5 ppbv per decade in the Northern hemisphere, where largest sources of O3 precursors are located. Molecules of O3 penetrating into the leaves through the stomatal apertures trigger the formation of reactive oxygen species, leading thus to the damage of the photosynthetic apparatus. Accordingly, it is assumed, that O3 increase reduces the terrestrial carbon uptake relative to the preindustrial era. Here we summarise the results of previous manipulative experiments in laboratory growth cabinets, field open-top chambers and free-air systems together with O3 flux measurements under natural growth conditions. In particular, we focus on leaf-level physiological responses in trees, variability in stomatal O3 flux and changes in carbon fluxes and biomass production in forest stands. As the results reported in the literature are highly variable, ranging from negligible to severe declines in photosynthetic carbon uptake, we also discuss the possible interactions of O3 with other environmental factors including solar radiation, drought, temperature and nitrogen deposition. Those factors were found to have great potential to modulate stomata openness and O3 fluxes.

On-site identification of ozone damage in fruiting plants using vapor-deposited conducting polymer tattoos

Climate change is leading to increased concentrations of ground-level ozone in farms and orchards. Persistent ozone exposure causes irreversible oxidative damage to plants and reduces crop yield, threatening food supply chains. Here, we show that vapor-deposited conducting polymer tattoos on plant leaves can be used to perform on-site impedance analysis, which accurately reveals ozone damage, even at low exposure levels. Oxidative damage produces a unique change in the high-frequency (>10⁴ Hz) impedance and phase signals of leaves, which is not replicated by other abiotic stressors, such as drought. The polymer tattoos are resilient against ozone-induced chemical degradation and persist on the leaves of fruiting plants, thus allowing for frequent and long-term monitoring of cellular ozone damage in economically important crops, such as grapes and apples.

Ozone affects plant, insect, and soil microbial communities: A threat to terrestrial ecosystems and biodiversity

Elevated tropospheric ozone concentrations induce adverse effects in plants. We reviewed how ozone affects (i) the composition and diversity of plant communities by affecting key physiological traits; (ii) foliar chemistry and the emission of volatiles, thereby affecting plant-plant competition, plant-insect interactions, and the composition of insect communities; and (iii) plant-soil-microbe interactions and the composition of soil communities by disrupting plant litterfall and altering root exudation, soil enzymatic activities, decomposition, and nutrient cycling. The community composition of soil microbes is consequently changed, and alpha diversity is often reduced. The effects depend on the environment and vary across space and time. We suggest that Atlantic islands in the Northern Hemisphere, the Mediterranean Basin, equatorial Africa, Ethiopia, the Indian coastline, the Himalayan region, southern Asia, and Japan have high endemic richness at high ozone risk by 2100.

Flux-Based Ozone Risk Assessment for a Plant Injury Index (PII) in Three European Cool-Temperate Deciduous Tree Species

This study investigated visible foliar ozone (O3) injury in three deciduous tree species with different growth patterns (indeterminate, Alnus glutinosa (L.) Gaertn.; intermediate, Sorbus aucuparia L.; and determinate, Vaccinium myrtillus L.) from May to August 2018. Ozone effects on the timing of injury onset and a plant injury index (PII) were investigated using two O3 indices, i.e., AOT40 (accumulative O3 exposure over 40 ppb during daylight hours) and PODY (phytotoxic O3 dose above a flux threshold of Y nmol m−2 s−1). A new parameterization for PODY estimation was developed for each species. Measurements were carried out in an O3 free-air controlled exposure (FACE) experiment with three levels of O3 treatment (ambient, AA; 1.5 × AA; and 2.0 × AA). Injury onset was found in May at 2.0 × AA in all three species and the timing of the onset was determined by the amount of stomatal O3 uptake. It required 4.0 mmol m−2 POD0 and 5.5 to 9.0 ppm·h AOT40. As a result, A. glutinosa with high stomatal conductance (gs) showed the earliest emergence of O3 visible injury among the three species. After the onset, O3 visible injury expanded to the plant level as confirmed by increased PII values. In A. glutinosa with indeterminate growth pattern, a new leaf formation alleviated the expansion of O3 visible injury at the plant level. V. myrtillus showed a dramatic increase of PII from June to July due to higher sensitivity to O3 in its flowering and fruiting stage. Ozone impacts on PII were better explained by the flux-based index, PODY, as compared with the exposure-based index, AOT40. The critical levels (CLs) corresponding to PII = 5 were 8.1 mmol m−2 POD7 in A. glutinosa, 22 mmol m−2 POD0 in S. aucuparia, and 5.8 mmol m−2 POD1 in V. myrtillus. The results highlight that the CLs for PII are species-specific. Establishing species-specific O3 flux-effect relationships should be key for a quantitative O3 risk assessment.

Assessing the Impact of Ozone on Forest Trees in An Integrative Perspective: Are Foliar Visible Symptoms Suitable Predictors for Growth Reduction? A Critical Review

Plant growth reduction (GR) is the most widely accepted damage parameter to assess the sensitivity of trees to tropospheric ozone (O3) pollution since it integrates different physiological processes leading to loss of photosynthetic activity and distraction of metabolic resources from growth to defense, repair, and recovery pathways. Because of the intrinsic difficulty to assess the actual O3 risk assessment for forests in field conditions, foliar visible symptoms (FVS) induced by O3 have been proposed as a proxy to estimate possible GR in forest trees. The rationale for this assumption is that the onset of FVS implies a reduction of the photosynthetic capacity of plants. In this review, we show that GR and FVS can be the consequences of independent physiological pathways involving different response mechanisms that can cause both FVS without GR and GR without FVS. The onset of FVS may not lead necessarily to significant GR at plant level for several reasons, including the rise of compensatory photosynthesis, the time lag between growth processes and the accumulation of critical O3 dose, and the negligible effect of a modest amount of injured leaves. Plant GR, on the other hand, may be induced by different physiological mechanisms not necessarily related to FVS, such as stomatal closure (i.e., carbon starvation) to avoid or reduce O3 uptake, and the increase of respiratory processes for the production of metabolic defense compounds. Growth reduction and FVS can be interpreted as different strategies for the acclimation of plants to a stressful environment, and do not mean necessarily damage. Growth reduction (without FVS) seems to prevail in species adapted to limiting environmental conditions, that avoid loss and replacement of injured leaves because of the high metabolic cost of their production; conversely, FVS manifestation (without GR) and the replacement of injured leaves is more common in species adapted to environments with low-stress levels, since they can benefit from a rapid foliar turnover to compensate for the decreased rate of photosynthesis of the whole plant.