Stomatal Ozone Flux Research Articles

Substantial yield reduction in sweet potato due to tropospheric ozone, the dose-response function

Impacts of tropospheric ozone on sweet potato (Ipomoea batatas) are poorly understood despite being a staple food grown in locations deemed at risk from ozone pollution. Three varieties of sweet potato were exposed to ozone treatments (peaks of: 30 (Low), 80 (Medium), and 110 (High) ppb) using heated solardomes. Weekly measurements of stomatal conductance (gs) and chlorophyll content (CI) were used to determine physiological responses, along with final yield. gs and CI were reduced with increasing ozone exposure, but effects were partially masked due to elevated leaf senescence and turnover. Yield for the Erato orange and Murasaki varieties was reduced by ∼40% and ∼50% (Medium and High ozone treatments, respectively, vs Low) whereas Beauregard yield was reduced by 58% in both.The DO3SE (Deposition of Ozone for Stomatal Exchange) model was parameterized for gs in response to light, temperature, vapour pressure deficit and soil water potential. Clear responses of gs to the environmental parameters were found. Yield reductions were correlated with both concentration based AOT40 (accumulated ozone above a threshold of 40 ppb) and flux based POD6 (accumulated stomatal flux of ozone above a threshold of 6 nmol m− 2 s− 1) metrics (R2 0.66 p = 0.01; and R2 0.44 p = 0.05, respectively). A critical level estimate of a POD6 of 3 (mmol m−2 Projected Leaf Area−1) was obtained using the relationship. This study showed that sweet potato yield was reduced by ozone pollution, and that stomatal conductance and chlorophyll content were also affected. Results from this study can improve model predictions of ozone impacts on sweet potato together with associated ozone risk assessments for tropical countries.

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.

Exploring new strategies for ozone-risk assessment: A dynamic-threshold case study

Tropospheric ozone is a dangerous atmospheric pollutant for forest ecosystems when it penetrates stomata. Thresholds for ozone-risk assessment are based on accumulated stomatal ozone fluxes such as the Phytotoxic Ozone Dose (POD). In order to identify the effect of ozone on a Holm oak forest in central Italy, four flux-based ozone impact response functions were implemented and tested in a multi-layer canopy model AIRTREE and evaluated against Gross Primary Productivity (GPP) obtained from observations of Eddy Covariance fluxes of CO2. To evaluate if a clear phytotoxic threshold exists and if it changes during the year, six different detoxifying thresholds ranging between 0 and 5 nmol O3 m−2 s−1 were tested.The use of species-specific rather than more general response functions based on plant functional types (PFT) increased model accuracy (RMSE reduced by up to 8.5%). In the case of linear response functions, a threshold of 1 nmol m−2 s−2 produced the best results for simulations of the whole year, although the tolerance to ozone changed seasonally, with higher tolerance (5 nmol m−2 s−1 or no ozone impact) for Winter and Spring and lower thresholds in Summer and Fall (0–1 nmol m−2 s−1). A “dynamic threshold” obtained by extracting the best daily threshold values from a range of different simulations helped reduce model overestimation of GPP by 213 g C m−2 y−1 and reduce RMSE up to 7.7%. Finally, a nonlinear ozone correction based on manipulative experiments produced the best results when no detoxifying threshold was applied (0 nmol O3 m−2 s−1), suggesting that nonlinear functions fully account for ozone detoxification. The evidence of seasonal changes in ozone tolerance points to the need for seasonal thresholds to predict ozone damage and highlights the importance of performing more species-specific manipulative experiments to derive response functions for a broad range of plant species.

The response of Pinus species to ozone uptake in different climate regions of Europe

Abstract This study is focused on the research of selected Pinus species exposed to high ozone concentrations in the mountain environment. We noticed different values of modelled ozone doses (MOD) up-taken by Mountain pine (Pinus mugo Turra) in the High Tatra Mts (SK–HTMts) and Swiss stone pine (Pinus cembra L.) in the Alpes-Mercantour (FR–AlpMar) during the growing season 2019. The MOD values were obtained by multiplicative DO3SE model, while we also tested a new approach based on modification of input ozone data. The MOD values were obtained by multiplicative DO3SE model, while we also tested a new approach based on modification of input ozone data. Testing has shown that ozone input based on passive sampling may be used in MOD modelling for sites situated in the subalpine zone where the operation of active monitors is limited.. Presented results confirmed the assumption regarding stomatal ozone flux reduction due to the occurrence of soil drought in hot and dry summer weather typical for the Mediterranean climate region. Despite the limitation of stomatal flux, foliar ozone specific injury on two years needles of P. cembra was substantially higher in comparison to the incidence of ozone injury symptoms observed on two years needles of P. mugo in SK–HTMts. It may suggest low phytotoxicity of given MOD or efficient resistance of P. mugo against oxidative stress. In addition, the visible injury index (VINX) covering the broad effect of biotic and abiotic harmful agents was appraised on P. mugo. Percentage of affected surface indicated moderate deterioration of needle injury at the end of the growing season, particularly due to traces of mechanical damage.

Ozone critical levels for (semi-)natural vegetation dominated by perennial grassland species.

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.

Prediction of ozone effects on net ecosystem production of Norway spruce forest

Future ground-level concentrations of phytotoxic ozone are projected to grow in the Northern Hemisphere, at a rate depending on emission scenarios. We explored the likely changes in net ecosystem production (NEP) due to the increasing concentration of tropospheric ozone by applying a Generalized Additive Mixed Model based on measurements of ozone concentration ([O3]) and stomatal ozone flux (FsO3), at a mountainous Norway spruce forest in the Czech Republic, Central Europe. A dataset covering the growing period (May-August 2009) was examined in this case study. A predictive model based on FsO3 was found to be marginally more accurate than a model using [O3] alone for prediction of the course of NEP when compared to NEP measured by the eddy covariance technique. Both higher [O3] and FsO3 were found to reduce NEP. NEP simulated at low, pre-industrial FsO3 (0.5 nmol m-2 s-1) was higher by 24.8% as compared to NEP assessed at current rates of FsO3 (8.32 nmol m-2 s-1). However, NEP simulated at high FsO3 (17 nmol m-2 s-1), likely in the future, was reduced by 14.1% as compared to NEP values at current FsO3. The interaction between environmental factors and stomatal conductance is discussed in this paper.

A Site-Specific Analysis of the Implications of a Changing Ozone Profile and Climate for Stomatal Ozone Fluxes in Europe

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.

Long Term Variations of Ozone Concentration of in a Winter Wheat Field and Its Loss Estimate Based on Dry Matter and Yield

Surface ozone pollution and its negative effects on crops and food safety have attracted the attention of many people. In this study, ozone concentration and meteorological parameters in a winter wheat field were measured from 2014 to 2016. We analyzed the variations in ozone concentration and AOT40 during the growing season for winter wheat. According to the revised stomatal model, the leaf stomatal conductance of winter wheat was simulated and verified by comparing with measured results. Based on the flux model, the leaf stomatal fluxes of winter wheat were calculated. In addition, we estimated the effects of ozone concentration on winter wheat dry matter and yield by using exposure-response relationships and flux-response relationships. The results suggested that the concentration of ozone gradually increased during the growing season for winter wheat and showed a distinct unimodal pattern of diurnal variation. The mean concentrations of ozone were 36.2, 37.7, and 33.6 nL·L-1, respectively, and the ozone AOT40 values were 17.08, 17.90, and 11.84 μL·(L·h)-1, respectively, during the growing seasons for winter wheat from 2014 to 2016. The Jarvis-type factorial model of stomatal conductance could be used to simulate the stomatal conductance of winter wheat in the region, and approximately 81% of the variation of measured stomatal conductance could be accounted for by the model. The leaf stomatal ozone fluxes of winter wheat were 9.36, 9.32, and 8.65 mmol·m-2, respectively, from 2014 to 2016. Winter wheat yield decreased about 18.03%, and its dry matter decreased about 19.31% under ozone stress during these three years.

Ozone-induced stomatal sluggishness changes stomatal parameters of Jarvis-type model in white birch and deciduous oak.

Stomatal ozone flux is closely related to ozone injury to plants. Jarvis-type multiplicative model has been recommended for estimating stomatal ozone flux in forest trees. Ozone can change stomatal conductance by both stomatal closure and less efficient stomatal control (stomatal sluggishness). However, current Jarvis-type models do not account for these ozone effects on stomatal conductance in forest trees. We examined seasonal course of stomatal conductance in two common deciduous tree species native to northern Japan (white birch: Betula platyphylla var. japonica; deciduous oak: Quercus mongolica var. crispula) grown under free-air ozone exposure. We innovatively considered stomatal sluggishness in the Jarvis-type model using a simple parameter, s, relating to cumulative ozone uptake (defined as POD: phytotoxic ozone dose). We found that ozone decreased stomatal conductance of white birch leaves after full expansion (-28%). However, such a reduction of stomatal conductance by ozone fell in late summer (-10%). At the same time, ozone reduced stomatal sensitivity of white birch to VPD and increased stomatal conductance under low light conditions. In contrast, in deciduous oak, ozone did not clearly change the model parameters. The consideration of both ozone-induced stomatal closure and stomatal sluggishness improved the model performance to estimate stomatal conductance and to explain the dose-response relationship on ozone-induced decline of photosynthesis of white birch. Our results indicate that ozone effects on stomatal conductance (i.e. stomatal closure and stomatal sluggishness) are crucial for modelling studies to determine stomatal response in deciduous trees, especially in species sensitive to ozone.

Ozone Deposition and Risk Assessment for a Winter Wheat Field:Partitioning Between Stomatal and Non-stomatal Pathways

To better understand the ozone deposition and risk assessment over agroecosystems based on the ozone flux indices, an eddy-covariance system was used for measuring the ozone deposition continuously and dynamically in a winter wheat field. We analyzed the variations in ozone concentration, total ozone flux, and stomatal and non-stomatal flux. The relationships between stomatal/non-stomatal ozone deposition velocity and the main meteorological factors were investigated. Finally, the yield losses of winter wheat based on the ozone-dose index (AOT40) and ozone flux index (DFs06) were calculated. Results showed that average daily ozone concentration (cO3) was 32.9 nL·L-1. The daytime (08:00-18:00) and nighttime total ozone flux (FO3) were -7.6 nmol·(m2·s)-1 and -3.1 nmol·(m2·s)-1, respectively, and the mean diurnal FO3 was -5.1 nmol·(m2·s)-1. The mean daily stomatal ozone flux (Fs) and non-stomatal ozone flux (Fns) ranged from 0 to -5.1 nmol·(m2·s)-1 and from -1.43 to -10.31 nmol·(m2·s)-1, respectively. The mean diurnal Fs and Fns were -1.43 nmol·(m2·s)-1 and -3.66 nmol·(m2·s)-1. High solar radiation (SR), high temperature (T), and moderate humidity were used to analyze stomatal ozone deposition; high SR, moderate T, and high humidity were suitable to analyze non-stomatal ozone deposition. The cumulative total ozone flux (DFO3), cumulative stomatal ozone flux (DFs), and cumulative non-stomatal ozone flux (DFns) were 31.58, 9.99, and 21.59 mmol·m-2 during the entire experimental period, and DFs and DFns accounted for 32% and 68% of DFO3. The ranges of yield loss in winter wheat were estimated at 11.58%-20.37% and 20%-23.56% using different assessment models based on the ozone dose index AOT40 and ozone flux index DFs06, respectively.

The role of plant phenology in stomatal ozone flux modeling.

Plant phenology plays a pivotal role in the climate system as it regulates the gas exchange between the biosphere and the atmosphere. The uptake of ozone by forest is estimated through several meteorological variables and a specific function describing the beginning and the termination of plant growing season; actually, in many risk assessment studies, this function is based on a simple latitude and topography model. In this study, using two satellite datasets, we apply and compare six methods to estimate the start and the end dates of the growing season across a large region covering all Europe for the year 2011. Results show a large variability between the green-up and dormancy dates estimated using the six different methods, with differences greater than one month. However, interestingly, all the methods display a common spatial pattern in the uptake of ozone by forests with a marked change in the magnitude, up to 1.9 TgO3 /year, and corresponding to a difference of 25% in the amount of ozone that enters the leaves. Our results indicate that improved estimates of ozone fluxes require a better representation of plant phenology in the models used for O3 risk assessment.

Fertilizer efficiency in wheat is reduced by ozone pollution

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.

Testing a ratio of photosynthesis to O3 uptake as an index for assessing O3-induced foliar visible injury in poplar trees.

Visible foliar injury by ozone (ozone visible injury) is known as a biomarker to assess potential phytotoxicity of ozone. We investigated ozone visible injury in an ozone-sensitive poplar (Oxford clone) under a 2-year free-air controlled exposure (FACE) experiment and calculated three ozone indices (i.e., accumulative ozone exposure over 40ppb during daylight hours (AOT40), phytotoxic ozone dose above a flux threshold of 0nmolm-2s-1 (POD0), and the cumulative value of the ratio of hourly ozone uptake to net photosynthesis (ΣU/P n ) to assess the critical level (CL) at the time of the first symptom onset of ozone visible injury. We tested the hypothesis that ozone injury depends both on the amount of ozone entering a leaf and on the capacity for biochemical detoxification or repair with photosynthesis as a proxy. The CLs at the time of the first symptom onset of ozone visible injury were 19ppmh for AOT40, 26mmolm-2 for POD0, and 1.2molmol-1 for ΣU/P n in Oxford clone at the ozone FACE experiment. Our findings were then verified by 4-year observation-based data in central Italy on Oxford clone and white poplar (Populus alba L.). These observation-based data indicated that we found ozone visible injury in Oxford clone even though AOT40 was relatively low (11.7ppmh). On the other hand, when values of POD0 and ΣU/P n exceeded over the CLs, the occurrence of initial symptoms in Oxford clone was shown. White poplar did not show ozone visible injury. ΣU/P n of white poplar at the field sites reached ~1.0molmol-1 (less than the CL=1.2molmol-1, which was obtained from O3 FACE) during May-September, although the values of POD0 were relatively high in white poplar (44-47mmolm-2 during May-September). The result implies that ozone injury may have occurred in poplars when stomatal ozone flux exceeded the critical range of tolerance due to the assimilate shortage for repair and defense against ozone stress.

Five-year volume growth of European beech does not respond to ozone pollution in Italy.

A unique database of stand volume growth, estimated as periodic annual volume increment (in m3ha-1 per year over the period 2001-2005) from 728 European beech (Fagus sylvatica L.) sites distributed across Italy, was used to assess the effects of ambient ozone (O3), expressed as annual average (M24), accumulated exposure above a 40ppb hourly threshold (AOT40), and total stomatal ozone flux (POD0). Growth data were from the National forest inventory of Italy, while climate data and ozone concentrations were computed by the WRF and CHIMERE models, respectively. Results show that the growth increased with increasing solar radiation and air temperature and decreased with increasing number of cold days, while effects of soil water content and O3 were not significant. In contrast, the literature results suggest that European beech is sensitive to both drought and O3. Ozone levels resulted to be very high (48ppb M24, 51,200ppb h AOT40, 21.08mmolm-2 POD0, on average) and thus able to potentially affect European beech growth. We hypothesize that the high-frequency signals of soil water and O3 got lost when averaged over 5years and recommended finer time-resolution investigations and inclusion of other factors of variability, e.g., thinning, tree age, and size.

Stomatal conductance models for ozone risk assessment at canopy level in two Mediterranean evergreen forests

Forests in Mediterranean Europe are located in a hot spot for tropospheric ozone formation. We applied two canopy-level stomatal conductance models (Jarvis-type and Ball-Woodrow-Berry models) to two Mediterranean evergreen forests (Umbrella pine at San Rossore, and Holm oak at Castelporziano, in central Italy), which is essential for assessing ozone impact on forests via the stomatal flux-based approach. Parameterizations of the models was carried out by the Eddy Covariance technique. Both Jarvis-type and Ball-Woodrow-Berry models well explained the observed stomatal conductance and stomatal ozone flux in both forests. Maximum stomatal conductance was 72% higher in Umbrella pine than in Holm oak, leading to higher stomatal ozone flux. Inclusion of a soil water function improved the performance of the Jarvis-type model for the estimation of stomatal ozone flux. We found contrasting results concerning the coefficient m (the slope of the photosynthesis-stomatal conductance relationship) in the Ball-Woodrow-Berry model between the two forests: while m was constant for varying soil water status in the Holm oak forest, it declined with soil drying in the Umbrella pine forest. This may result from higher stomatal sensitivity to drought in Umbrella pine as a response to avoid drought stress. Overall, these results advance our understanding of ozone risk assessment in Mediterranean forests.

Assessing the role of soil water limitation in determining the Phytotoxic Ozone Dose (PODY) thresholds

Phytotoxic Ozone Dose (PODY), defined as the accumulated stomatal ozone flux over a threshold of Y, is considered an optimal metric to evaluate O3 effects on vegetation. PODY is often computed through the DO3SE model, which includes species-specific parameterizations for the environmental response of stomatal conductance. However, the effect of soil water content (SWC) on stomatal aperture is difficult to model on a regional scale and thus often ignored. In this study, we used environmental input data obtained from the WRF-CHIMERE model for 14,546 grid-based forest sites in Southern Europe. SWC was obtained for the upper 10 cm of soil, which resulted in a worst-case risk scenario. PODY was calculated either with or without water limitation for different Y thresholds. Exclusion of the SWC effect on stomatal fluxes caused a serious overestimation of PODY. The difference increased with increasing Y (78%, 128%, 237% and 565% with Y = 0, 1, 2 and 3 nmol O3 m−2 s−1, respectively). This behaviour was confirmed by applying the same approach to field data measured in a Mediterranean Quercus ilex forest. WRF-CHIMERE overestimated SWC at this field site, so under real-world conditions the SWC effect may be larger than modelled. The differences were lower for temperate species (Pinus cembra 50–340%, P. sylvestris 57–363%, Abies alba 57–371%) than for Mediterranean species (P. pinaster 87–356%, P. halepensis 96–429%, P. pinea 107–532%, Q. suber 104–1602%), although a high difference was recorded also for the temperate species Fagus sylvatica with POD3 (524%). We conclude that SWC should be considered in PODY simulations and a low Y threshold should be used for robustness.

Evaluating stomatal ozone fluxes in WRF-Chem: Comparing ozone uptake in Mediterranean ecosystems

The development of modelling tools for estimating stomatal uptake of surface ozone in vegetation is important for the assessment of potential damage induced due to both current and future near surface ozone concentrations. In this study, we investigate the skill in estimating ozone uptake in plants by the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) V3.6.1, with the Wesely dry deposition scheme. To validate the stomatal uptake of ozone, the model simulations were compared with field measurements of three types of Mediterranean vegetation, over seven different periods representing various meteorological conditions. Some systematic biases in modelled ozone fluxes are revealed; the lack of an explicit and time varying dependency on plants’ water availability results in overestimated daytime ozone stomatal fluxes particularly in dry periods. The optimal temperature in the temperature response function is likely too low for the woody species tested here. Also, too low nighttime stomatal conductance leads to underestimation of ozone uptake during night. We demonstrate that modelled stomatal ozone flux is improved by accounting for vapor pressure deficit in the ambient air. Based on the results of the overall comparison to measured fluxes, we propose that additional improvements to the stomatal conductance parameterization should be implemented before applying the modelling system for estimating ozone doses and potential damage to vegetation.

Testing approaches for calculating stomatal ozone fluxes from passive samplers

Current ozone (O3) levels are high enough to negatively affect vegetation and may become worse in the future. Ozone risk assessments have recently shifted from exposure-based to flux-based metrics. Modeling stomatal O3 fluxes requires hourly O3 and meteorological data, which are not always available. Large datasets of O3 concentrations measured with passive samplers exist worldwide, and usually provide weekly to monthly means. We tested the suitability of using aggregated data instead of hourly data for O3 flux calculations with 3-year time series of O3 data from 24 Spanish air quality stations. Five different approaches and three different parameterizations were tested. Ozone-averaged values in combination with hourly meteorological data provided the most robust estimates of accumulated O3 fluxes (Phytotoxic Ozone Dose with no threshold, POD0), and the median of the absolute percent error (MAPE) due to aggregation came close to 5%. Aggregations from 1week to 1month yielded similar errors, which is important in the cost-efficiency terms of the chosen passive sampler exposure periodicity. One major limitation of these approaches is that they are not suitable for high POD thresholds, and that accuracy of the measurements with passive samplers has to be strictly assured in order to finally obtain acceptable errors. A combination of meteorological data and O3 passive sampler measurements may be used to estimate O3 fluxes at remote forest sites as a valuable risk assessment tool.

Ozone stomatal flux and O3 concentration-based metrics for Astronium graveolens Jacq., a Brazilian native forest tree species

The current levels of surface ozone (O3) are high enough to negatively affect trees in large regions of São Paulo State, southeastern Brazil, where standards for the protection of vegetation against the adverse effects of O3 do not exist. We evaluated three O3 metrics – phytotoxic ozone dose (POD), accumulated ozone exposure over the threshold of 40 ppb h (AOT40), and the sum of all hourly average concentrations (SUM00) – for the Brazilian native tropical tree species Astronium graveolens Jacq. We used the DO3SE (Deposition of Ozone for Stomatal Exchange) model and calculated PODY for different thresholds (from 0 to 6 mmol O3 m−2 PLA s−1), evaluating the model's performance through the relationship between measured and modelled conductance. The response parameters were: visible foliar injury, considered as incidence (% injured plants), severity (% injured leaves in relation to the number of leaves on injured plants), and leaf abscission. The model performance was suitable and significant (R2 = 0.58; p < 0.001). POD0 was better correlated to incidence and leaf abscission, and SUM00 was better correlated to severity. The highest values of O3 concentration-based metrics (AOT40 and SUM00) did not coincide with those of POD0. Further investigation may improve the model and contribute to the proposition of a national standard for the protection of native species.

Accumulated phytotoxic ozone dose estimation for deciduous forest in Kanto, Japan in summer

Abstract With ozone concentrations simulated using a regional chemical transport model (ADMER-PRO) and high-spatial resolution meteorological data, we investigated the influence of ozone concentration on deciduous forests in the Kanto region of Japan in summer during 2003, 2004, and 2009: three years for which weather characteristics differed greatly. Ozone risk for plants was assessed by the accumulated phytotoxic ozone dose (POD), a flux-based index. The effects were analyzed by particularly addressing the relation between the stomatal ozone flux and meteorological elements. Results revealed high absorption areas not only where injury to forests had been visually detected in previous studies, but also where injury had not been observed to date. Regarding the relation between the stomatal ozone flux and meteorological elements, air temperature and vapor pressure deficit strongly affected POD in 2004, when high temperature and little rainfall were observed. Additionally, the ozone concentration and irradiance strongly affected POD in 2003 when low temperatures and heavy rainfall were observed. The meteorological elements affecting POD differed from year to year. Results demonstrate the importance of multi-year simulations and analyses in the field of ozone risk assessment.