Modelling ozone deposition fluxes: The relative roles of deposition and detoxification processes

A new Model of Ozone Deposition and Detoxification (MODD) is presented. This model describes stomatal ozone uptake and deposition on external plant surfaces and soil; it accounts for diurnal variability of detoxification processes and reactive ozone uptake on cuticular waxes and soil surface. The mechanistic modelling of plant defense reactions is based on the Plöchl et al. (2000) detoxification model in which the dynamics of apoplast chemistry are considered. To estimate ozone deposition fluxes on cuticular waxes and soil surface, we use a revised version of the Morrison and Nazaroff (2002) model developed to account for ozone uptake on material surfaces. This model which has been fully integrated with a soil-plant-atmosphere continuum model ensures a complete coupling between stomatal conductance and O₃ exchanges between leaves and the atmosphere. The observed diurnal variations in stomatal conductance which largely control the influx of O₃ into the leaf are well reproduced. Model simulations point out that the pool of ascorbate located in the mesophyll cell wall plays a significant role in the detoxification of O₃. Besides stomatal conductance, it is the key process involved in the control of ozone flux to the cell wall. A decrease in the pool of ascorbate lengthens the chemical lifetime of O₃ in the cell wall then the virtual apoplastic resistance is found to increase with decreasing ascorbate. Although the atmospheric ozone concentration increases as the weather becomes hot and dry, the virtual apoplastic resistance follows the same trend, indicating a decrease of the ascorbate pool in the mesophyll cell wall. Results also indicate that for the pre-senescence period 57% of the ozone is deposited onto the cuticular surfaces, 4% on soil and only 37% is absorbed by stomata. The comparison of modelled and measured data reported in this study indicates that the model is capable of predicting the major features of the patterns of total ozone flux.     

Measurements of ozone deposition to a potato canopy

Potatoes are an important staple crop, grown in many parts of the world. Although ozone deposition to many vegetation types has been measured in the field, no data have been reported for potatoes. Such measurements, including the latent-heat flux, were made over a fully grown potato field in central Scotland during the summer of 2006, covering a 4-week period just after rainfall and then dry, sunny weather. The magnitude of the flux was typical of many canopies showing the expected diurnal cycles. Although the bulk-canopy stomatal conductance declined as the field dried out (～300 mmol-O₃ m^?2 s^?1 to ～70 mmol-O₃ m^?2 s^?1), the total ozone flux did not follow the same trend, indicating that non-stomatal deposition was significant. Over a dry surface non-stomatal resistance (R_ns) was 270–450 s m^?1, while over a wet surface R_ns was ～50% smaller and both decreased with increasing surface temperature and friction velocity. From the variation with relative humidity (RH) it is suggested that three processes occur on leaf surfaces: on a very dry surface ozone is removed by thermal decomposition, possibly enhanced by photolytic reactions in the daytime and so R_ns decreases as temperature increases; at 50–70% RH a thin film of liquid blocks the “dry” process and resistance increases; above 60–70% RH sufficient surface water is present for aqueous reactions to remove ozone and resistance decreases.     

Ozone effects on Aleppo pine seedlings (Pinus halepensis Mill.) grown in open-top chambers

A three-year term experiment was performed in eastern Spain to assess the effects of ozone (O₃) on the physiological behaviour of Aleppo pine (Pinus halepensis). Seedlings of this species were enclosed in open-top chambers during the summer and exposed to either Charcoal Filtered Air (CFA) to exclude O₃, Non Filtered Air (NFA) or Non Filtered Air plus 40 ppb O₃, 9h/day, 5 days/week (NFA+40). Gas exchange, foliar chlorophyll and nutrient levels were measured throughout the year. Clear seasonal fluctuations were observed since chlorophyll levels, stomatal conductance and net photosynthesis rates were lower during the summer, and higher in winter. O₃-induced effects were apparent on previous year needles from the second year of exposure. A reduction in net photosynthesis, stomatal conductance, chlorophyll, N and P was found in the NFA+40 plots. The results showed the sensitivity of this species to ozone, since a three-month exposure to realistic O₃ concentrations over three consecutive years induced important alterations in Aleppo pine performance. The application of the results to the definition of a critical O₃ level for forest trees is discussed.     

Physiological responses of Quercus ilex Leaves to Water Stress and Acute Ozone Exposure Under Controlled Conditions

The combined effect of water stress and ozone (O₃) on stomatal O₃ flux, damage to photosynthesis, and detoxification by biogenic volatile organic compounds (BVOC) in Quercus ilex leaves was studied. A 4-weeks O₃ exposure (250 ppb, 4 h per day) caused a reduction of photosynthesis and stomatal conductance, which was fully recovered 1 week after the end of the treatment, in well-watered and water-stressed plants. Measurements of stomatal O₃ flux revealed a low stomatal flux of the pollutant, which became minimal after stomatal closure caused by water stress. An induction of volatile monoterpenes, important compounds in the O₃ scavenging system in Q. ilex, and a burst of lipoxygenase compounds (LOX), which are released as gaseous by-products of membrane peroxidation, was observed after 2–3 weeks of O₃ fumigation. However, these compounds were also released in control leaves that were exposed to ozone only briefly, to determine stomatal O₃ flux. The low stomatal flux that occurred in water stress conditions helped avoiding permanent damage to Q. ilex leaves, although during the O₃ treatment photosynthesis was severely limited by stomatal closure. In well-watered plants, O₃ fumigation caused a noticeable increase of nocturnal stomatal conductance. If confirmed on adult plants under field conditions, this effect can imply larger flux of O₃ at night and possible detrimental effects of O₃ on leaf functions in plants exposed to high nocturnal O₃ levels.     

Inter- and Intra-specific Differences in the Response of Chinese Leafy Vegetables to Ozone

The response to ozone (O₃) of a range of Chinese leafy vegetables was investigated with respect to both inter- and intra-specific differences in sensitivity. In the interspecific experiment local Chinese cultivars of pak choi, rape, leaf mustard, leaf lettuce and coriander were fumigated with 90?ppb O₃ for 9?h daily for 22?C30?days. A similar fumigation was carried out using four different cultivars of pak choi. Sequential measurements were made of leaf injury, photosynthetic rate, stomatal conductance and chlorophyll fluorescence, together with dry weights at a final harvest. O₃ injury appeared as white or yellow blemishes on the leaf surface of all species. The first signs of injury were seen following only 3-days?? O₃ exposure (pak choi); the extent of injured leaf area increased over time for all species and cultivars, with 44.6% of the leaf area visibly injured for leaf mustard, the species with the greatest extent of injury, following 30-days?? exposure. Significant reductions in photosynthetic rate (22.7?C40.7%) and stomatal conductance (19.1?C33.1%) were found for all species and cultivars following O₃ exposure. Plant productivity was also reduced in O₃ compared to filtered air, with significant yield reductions for all species (11.1?C50.8% above-ground dry weight) as well as for all cultivars of pak choi (15.8?C28.1% above-ground dry weight). The mechanisms for observed impacts are discussed, together with the implications for current and future production of vegetables in the southern China province of Guangdong.     

Modification of ozone effects on photosynthetic capacity of winter wheat (Triticum aestivum L. CV. perlo) at different levels of water availability

Attached leaves were used for the determination of the photochemical capacity by means of a portable fluorimeter. Repeated fluorescence measurements showed the negative effects of ozone on photochemical capacity and these negative effects increased with increasing ozone doses. But impairments of photochemical capacity were smallest if severe water stress co-occurred with ozone exposures. The upper leaf sides experienced more reduction of photochemical capacity in well-watered plants than the lower leaf sides, possibly by the additional effect of light stress on the upper leaf sides. In diurnal studies, a decline of F _v/F _m was observed at noontime and a recovery at evening in both control and ozone-fumigated leaves at two extreme water capacities (w.c.) (75% and 35% of w.c.). The extent of depression and recovery of F _v/F _m was not significantly varying. The oscillations of F _v/F _m could be due to short-term disturbances in the photosynthetic capacity, due to oxidative stress.     

Analysis of major photochemical pollutants with meteorological factors for high ozone days in Istanbul, Turkey

Hourly ozone, NO _x and VOC concentrations, measured during 2001–2003 summer periods, are analyzed in order to examine the interaction patterns between the major photochemical pollutants in ?stanbul. 34 high ozone days throughout the summer periods of the three years are determined and examined in the study together with the meteorological parameters like temperature, wind and vertical structure of the atmosphere. The results show that high levels of ozone are observed mostly under anticyclonic conditions with relatively low wind speeds. High ozone days generally experienced maximum concentrations at afternoon hours and minimum concentrations are reached at rush hours due to NO _x – titration by traffic emissions. High negative correlations with NO _x up to -0.84 are observed at the Saraçhane station while higher correlations for VOC species, up to ?0.75, are calculated for Kadiköy station. Some individual episodes experiencing high ozone concentrations up to 310 μg m^?3 in the early morning hours are also studied. It is found that decreasing inversion heights in the early hours of the day led to suppression of pollutants close to surface and thus, an increase in ozone concentrations was observed. Low wind speeds played a major role in the increase of pollution levels in the region. HYSPLIT model is applied to some particular episodes and the results show that the northeasterly transport to the region was dominant, especially in the early-morning maximums.     

Modeling of Stomatal Conductance for Estimating Ozone Uptake of Fagus crenata Under Experimentally Enhanced Free-air Ozone Exposure

We examined a performance of the multiplicative stomatal conductance model to estimate the stomatal ozone uptake for Fagus crenata. Parameterization of the model was carried out by in-situ measurements in a free-air ozone exposure experiment. The model performed fairly well under ambient conditions, with low ozone concentration. However, the model overestimated stomatal conductance under enhanced ozone condition due to ozone-induced stomatal closure. A revised model that included a parameter representing ozone-induced stomatal closure showed better estimation of ozone uptake. Neglecting ozone-induced stomatal closure induced a 20?% overestimation of the stomatal uptake of ozone. The ozone-induced stomatal closure was closely related to stomatal ozone uptake rather than accumulated concentrations of ozone exceeding 40?nmol mol^?1. Our results suggest that ozone-induced stomatal closure should be implemented to stomatal conductance model for estimating ozone uptake for F. crenata. The implementation will contribute to adequate risk assessments of ozone impacts on F. crenata forests in Japan.     

Monitoring stomatal conductance of Jatropha curcas seedlings under different levels of water shortage with infrared thermography

Model simulations and experimental measurements were used to investigate the applicability of infrared thermography for the estimation of stomatal conductance and drought stress under sub-optimal meteorological conditions. The study focused on the stomatal conductance index I_g, calculated from the leaf temperature and the temperature of a dry and wet reference leaf. The simulations revealed that I_g is influenced by leaf dimension, wind speed and air temperature and not or hardly by leaf angle, albedo, relative humidity or insolation. In addition, I_g was found to be very sensitive to differences in wind speed, air temperature, insolation, leaf dimension and leaf angle between the measured and the reference leaves. In the experimental part, we evaluated if infrared thermography can be used to improve the knowledge on the water use of Jatropha curcas L., a tropical biofuel crop. Thermal images from Jatropha seedlings grown under three different drought treatments were made on a day with very variable insolation and a day with very low insolation. Smaller newly formed leaves and the active control of the leaf angle proved efficient ways of Jatropha to protect leaves under drought stress from overheating. I_g, assessed in four different ways, and four simplified drought stress indices were derived and related to the measured stomatal conductance (g_s) of the seedlings. The strongest correlation with g_s and the highest discriminative power between the different water treatments were achieved when I_g was calculated by taking the average leaf temperature per plant and the temperature of the dry and wet reference leaves of this plant, rather than the average temperature of several reference leaves. Using the difference between the dry reference and the measured leaf (T_dry − T_l) as a simplified index gave similar results, although correlations were weaker. Other simplified thermal indices were not well correlated with leaf stomatal conductance or with water treatment. Recommendations were formulated for the measurement of I_g and (T_dry − T_l).     

The Antitranspirant Di-1-p-menthene, a Potential Chemical Protectant of Ozone Damage to Plants

A study was conducted to evaluate the effect of pinolene-based film-forming Vapor Gard (VG) emulsion (di-1-p-menthene), a commercial antitranspirant, on the response of the sensitive bean (Phaseolus vulgaris cv. Pinto) plants to realistic ozone fumigations. Plants treated with the chemical were significantly less damaged in comparison with untreated controls when exposed to as much as 150 ppb of ozone in the atmosphere for 4 h. In unozonated plants, photosynthesis as well as stomatal conductance was significantly depressed by the antitranspirant. In VG-treated individuals, (1) visible injury is strongly reduced; (2) membrane damage is counteracted; (3) photosynthetic activity is unchanged, as well as the stomatal conductance and the store of CO₂ in substomatal chamber; (4) F _v/F _m and the other parameters of chlorophyll fluorescence reveal a stability of the photochemical apparatus; and (v) antioxidant defence is not stimulated. Unexpectedly, our results highlighted a dramatic difference between the protective effects towards ozone damage induced by VG, depending on its mode of distribution. As reported above, when entire plants (??both leaves??) are treated or untreated with VG, those individuals wherein VG was not applied showed severe alterations in phenomenological, biochemical and ecophysiological parameters investigated due to ozone toxicity. This is not true in the cases where ??single primary leaves?? or selected regions (??half leaves??) are treated with VG. Even if visible injury is present after ozone fumigation, physiological parameters, such as A _max and G _w, in ozonated and VG-treated single leaves and half leaves are similar to unfumigated controls. Similar results were obtained for chlorophyll fluorescence parameters. A membrane-protective action is observed in half-leaves treatment. It appears that the presence of regions (single leaves or half leaves), which are treated with VG, modifies the behaviour to ozone of untreated regions. Possible explanations of the observed phenomenon are discussed. The antitranspirant di-1-p-menthene proved to be a low-cost, low-technology tool for assessing ozone injury in the field.     

Ozone deposition onto bare soil: A new parameterisation

The variables controlling ozone deposition onto bare soil are still unknown and it is necessary to understand this pathway well, as it represents a significant sink for ozone. Eddy-covariance measurements of ozone (O₃) fluxes were performed over bare soils in agricultural land. Three datasets with contrasted meteorological conditions and soil nitric oxide (NO) emissions were used to study the factors controlling soil deposition. It is considered that ozone deposition can be represented with an aerodynamic resistance (R_a), a quasi-laminar boundary layer resistance (Rb O₃), and an additional resistance, named soil resistance (R_soil). Although it is assumed in previous studies that soil resistance is a function of soil water content (SWC) and could be considered constant as variation of SWC at monthly scale are generally weak, the results of this study indicate that SWC is not the main factor controlling R_soil which shows daily and hourly variations. The main factor controlling soil resistance is the surface relative humidity which is positively correlated with R_soil, contrary to non stomatal resistance onto canopies which show a negative correlation with relative humidity. The relationship between R_soil and the surface relative humidity is probably due to a decrease in the surface available for ozone deposition, due to an increasing adsorption of water molecules onto the ground with relative humidity. A new parameterisation of R_soil was established, where R_soil is a function of the surface relative humidity only (R_soil = R_{soil min} × e^(k×RH_surf), and R_{soil min} = 21 ± 1.01 s m⁻¹ and k = 0.024 ± 0.001, mean ± SD). The measured and parameterised ozone deposition velocities agree well when soil NO emissions are negligible. However, when there are large soil NO emissions, the parameterised ozone deposition strongly underestimates the measured deposition velocity even if the chemical destruction of ozone by reaction with NO in the air column was evaluated to be negligible. This suggests that soil NO emissions enhance soil ozone deposition by chemical reaction at or near the soil surface. The new parameterisation allows a better estimation of soil deposition, especially during daytime when R_soil is overestimated using previously published parameterisations. It is an important step towards a better parameterisation of the non-stomatal uptake of ozone.     

Photosynthetic physiology of eucalypts along a sub-continental rainfall gradient in northern Australia

Leaf-level photosynthetic parameters of species in the closely related genera Eucalyptus and Corymbia were assessed along a strong rainfall gradient in northern Australia. Both instantaneous gas exchange measurements and leaf carbon isotope discrimination indicated little variation in intercellular CO₂ concentrations during photosynthesis (c_i) in response to a decrease in mean annual precipitation from ∼1700 mm to ∼300 mm. Correlation between stomatal conductance and photosynthetic capacity contributed toward the maintenance of relatively constant c_i among the sampled leaves, when assessed at ambient CO₂ concentration and photon irradiance similar to full sunlight. Leaf mass per area was the most plastic leaf trait along the rainfall gradient, showing a linear increase in response to decreasing mean annual precipitation. The maximum Rubisco carboxylation velocity, V_cmax, expressed on a leaf-area basis, showed a modest increase in response to decreasing rainfall. This modest increase in V_cmax was associated with the strongly expressed increase in leaf mass per area. These results suggest that variation in ecosystem-level gas exchange during the dry season in north-Australian savannas will likely be dominated by changes in leaf area index in response to increasing aridity, rather than by changes in photosynthetic performance per unit leaf area.     

Effects of Silicon on Photosynthesis of Young Cucumber Seedlings Under Osmotic Stress

Effects of silicon (Si) application on photosynthesis of solution-cultured cucumber seedlings were investigated under osmotic stress and unstressed conditions. In unstressed conditions, silicon application had no effect on growth and photosynthetic parameters. The responses of the photosynthetic parameters to abruptly imposed osmotic stress did not differ between silicon treatments. After 1 week exposure to osmotic stress, growth reduction was observed, but it was less severe in seedlings grown with silicon than in those without silicon. Although there were no differences between silicon treatments in stomatal conductance, transpiration rate, cuticular transpiration, or xylem sap exudation rate under osmotic stress, leaf intercellular carbon dioxide (CO₂) concentration was significantly lower and photosynthetic rate tended to be higher in seedlings supplied with silicon. These results suggested that the silicon-induced alleviation of growth reduction under osmotic stress in cucumber was due to amelioration of stress-induced damage of leaf tissues rather than to improvement of leaf water status.     

A model to simulate response of plant stomata to environmental conditions

In order to simulate plant transpiration under different field climatic conditions we have developed and verified a semi-empirical model for predicting the stomatal response to solar global radiation, leaf temperature, vapour pressure difference between the leaf and ambient air, ambient air CO₂ concentration and soil water potential. The transpiration and the stomatal relative conductance of a Nicotania Tabaccum var “samsun” plant leaves were measured in a laboratory apparatus and compared to those predicted by the model: good agreement was obtained between them for the different investigated cases. The model was incorporated in a numerical greenhouse microclimate model and its effects on the canopy microclimate are discussed here.     

Responses of transpiration and canopy conductance to partial defoliation of Eucalyptus globulus trees

Partial defoliation has been shown to affect the water relations and transpiration (gas exchange) of plants. Over one growing season, the water relations in response to partial (∼45%) defoliation were examined in four-year-old Eucalyptus globulus trees in southern Australia. Daily maximum transpiration rates (E_max), maximum canopy conductance (GCmax), and diurnal patterns of tree water-use were measured over a period of 215 days using the heat-pulse technique in adjacent control (non-defoliated) and defoliated trees. Sap-flux measurements were used to estimate canopy conductance and soil-to-leaf hydraulic conductance (K_P); leaf water potential (Ψ) and climate data were also collected. Following the removal of the upper canopy layer, defoliated trees exhibited compensatory responses in transpiration rate and canopy conductance of the remaining foliage. Defoliated E. globulus had similar predawn but higher midday Ψ_l, transpiration rates (E), canopy conductance (G_C) and K_P compared to the non-defoliated controls, possibly in response to increased water supply per unit leaf area demonstrated by higher midday Ψ_l. Higher E in defoliated E. globulus trees was the result of higher G_C in the morning and early afternoon. This paper also incorporates the cumulative effect of defoliation, in a phenomenological model of maximum canopy conductance of E. globulus. These results contribute to a mechanistic understanding of plant responses to defoliation, in particular the often observed up-regulation of photosynthesis that also occurs in response to defoliation.     

Evapotranspiration of a young irrigated olive orchard in southern Spain

This paper assesses the relationship between evapotranspiration (ET) of olive orchards and canopy size, under both dry and wet soil conditions. Measurements of ET of a 4 ha young, uniformly drip irrigated olive cv. ‘Arbequina’ orchard were performed using the eddy covariance and the water balance techniques, for 3 years, while leaf area index (LAI) varied from 0.01 to 1. The two techniques showed excellent agreement. Daily ET varied significantly depending on soil surface wetness. Data with dry soil were used to assess the energy partition as a function of LAI. An analysis for days with and without the irrigation drippers wet spots was used to establish the olive crop coefficient (K_c) under typical “summer” conditions and its relation with LAI and ground cover. The linear model proposed here predicts olive K_c of about 0.15 for 5% ground cover, increasing to 0.3 at 25%. The presence of wet spots from drip irrigation increased K_c by 37 and 8% for ground cover fractions of 5 and 25%, respectively. Further research is needed to parameterise more sophisticated models of canopy conductance in order to estimate tree transpiration and evaporation from the soil surface independently.     

Growth and Leaf Gas Exchange in Three Birch Species Exposed to Elevated Ozone and CO2 in Summer

We examined the effects of ozone and elevated CO₂ concentration in summer on the growth and photosynthetic traits of three representative birch species in Japan (mountain birch, Monarch birch, and white birch). Seedlings of the three birch species were grown in 16 open-top chambers and were exposed to two levels of ozone (6 and 60?nmol?mol^?1 for 7?h per day) in combination with two levels of CO₂ (370?C380 and 600???mol?mol^?1 for daytime) from July to October. No adverse effects of ozone were found in the Monarch birch or the white birch, but elevated ozone in summer reduced branch biomass and net photosynthesis, and accelerated leaf abscission, in the mountain birch. Elevated CO₂ promoted root development and thereby reduced the ratio of shoot dry mass (stem + branch) to root dry mass (S/R ratio) in the mountain birch and white birch. In contrast, there was no difference in dry mass between ambient and elevated CO₂ for the Monarch birch, due to downregulation of photosynthesis. Studies of the combined effect of CO₂ and ozone revealed that elevated CO₂ did not ameliorate the effect of ozone on mountain birch in late summer. In considering the ameliorating effect of CO₂ on ozone damage, it is necessary to take account of the species and the season.     

Bioindication of ozone in Estonia by using the tobacco variety Bel W3

The presence of tropospheric ozone was monitored by estimating necrotic flecks on the leaves of the ozone-sensitive tobacco variety Bel W3 in Estonia in 1990 and 1991. The indicator and control plants (variety Samsun) were planted in four localities (Käsmu, northern Estonia; Tartu, central Estonia, with two sites and Kooraste, southern Estonia). The percentage of the leaf area covered with necrotic flecks (necrotic index of the leaf) was determined individually for each leaf from mid-June until the end of September. The mean daily increment of the necrotic index for the period between two observations (typically 6–14 days) was calculated for each plant (NII_plant) and for the site (NII_site). The latter parameter indicates the variability of ozone episodes during the vegatation period. Ozone damage to leaves displayed an irregular pattern throughout the vegetation period, the values of NII_site ranging from 0 to 0.3 (Kooraste, 1990) and 0.9 to 2.8 (Käsmu, 1991). The average NII_site for the vegetation period was highest in northern Estonia (0.77 in 1990 and 1.47 in 1991) and lowest in southern Estonia (0.07 in 1990). High ozone damage in Käsmu can be explained by the close location (70–80 km) of the experimental site to major thermal power plants of Estonia and cities (Tallinn and Helsinki). Higher ozone damage in all the observed sites in 1991 as compared to 1990 can be accounted for different conditions for ozone formation.The number of sunny hours was 27% higher in 1991. In July the difference in sunny hours was almost twofold. It is concluded that ozone damage to plants is likely to occur in sunny summers in Estonia.     

开放式臭氧浓度升高对水稻叶片光合作用日变化的影响

运用开放式臭氧浓度升高系统（O3-FACE：Ozone-free air controlled enrichment）平台,以武运粳21（粳稻）和两优培九（杂交稻）两个耐性不同的品种为材料,研究了大气臭氧浓度升高对水稻叶片光合作用日变化的影响,旨在为高臭氧浓度条件下水稻生产和国家粮食安全的制定提供理论依据。结果表明,臭氧胁迫未改变光合作用日变化规律,处理和对照下净光合速率和气孔导度日变化相似,均呈现单峰曲线,高峰值出现在11：00—15：00之间;胞间CO2浓度日变化趋势与气孔导度和净光合速率日变化不一致;臭氧处理55 d对净光合速率和气孔导度影响较小,随着处理时间的延长,相关指标降低幅度变大,而胞间CO2浓度没有降低,说明臭氧对水稻的影响是一个累积过程,净光合速率降低的主要因素是由非气孔限制引起的;武运粳21的净光合速率和气孔导度在臭氧处理时的降低幅度小于两优培九,这种品种间的差异表明武运粳21比两优培九对臭氧耐受性强。     

Environmental Limitations to O3 Uptake - Some Key Results from Young Trees Growing at Elevated Co2 Concentrations

Elevated carbon dioxide concentrations and limited water supply have been shown to reduce the impact of ozone pollution on the growth and physiology of Quercus petraea in a long-term factorial experiment. These responses can be explained by observed reductions in stomatal conductance, and thus potential ozone exposure of 28% and 40% for CO₂ and drought treatments respectively. However, parameterisation of a stomatal conductance model for Quercus robur and Fagus sylvatica grown under ambient and elevated CO₂ concentrations in a separate experiment has demonstrated that elevated CO₂ also reduces the responsiveness of stomata to both saturation deficit (LAVPD) and soil moisture deficit (ψ) in beech, and to a lesser extent, in oak. Season-long model simulations of ozone fluxes suggest that LAVPD and ψ conductance parameters derived at ambient CO₂ concentrations will lead to these fluxes being underestimated by 24% and 2% for beech and oak respectively at 615 ppm CO₂.