[show abstract][hide abstract] ABSTRACT: Between 2004 and 2005 a combined open plot and open-top chamber (OTC) experiment was carried out at Curno (Northern Italy) with cuttings of the poplar clone Oxford (Populus maximowiczii Henry x Populus berolinensis Dippel) grown in open plots (OPs, ambient air), charcoal-filtered OTCs (CF, ozone concentration reduced to 50% of ambient) or non-filtered OTCs (NF, ozone concentration reduced to 95% of ambient). Plants in half of the chambers were kept well-watered (WET), and plants in the remaining chambers were not watered (DRY). The onset and development of visible foliar injury and the stomatal conductance to water vapor (g(w)) were assessed during each growing season. A stomatal conductance model was parameterized by the Jarvis approach, allowing the calculation of ozone stomatal fluxes of plants in each treatment. The pattern of visible symptoms was analyzed in relation to ozone exposure (AOT40, accumulated ozone over a threshold of 40 ppb) and accumulated ozone stomatal fluxes (AF(ST)). Symptoms became visible at an AOT40 between 9584 and 13,110 ppb h and an AF(ST) between 27.85 and 30.40 mmol O(3) m(-2). The development of symptoms was more widespread and faster in plants in WET plots than in DRY plots. A slightly higher dose of ozone was required to cause visible symptoms in plants in DRY plots than in WET plots. By the end of each growing season, plants in the CF OTCs had absorbed a high dose of ozone (31.60 mmol O(3) m(-2) in 2004 and 32.83 mmol O(3) m(-2) in 2005, for WET plots), without developing any visible symptoms. A reliable dose-response relationship was defined by a sigmoidal curve model. The shape of this curve expresses the change in leaf sensitivity and physiologic state over a prolonged ozone exposure. After the appearance of the first symptoms, foliar injury increased more rapidly than the increases in ozone exposure and ozone absorbed dose; however, when the injury incidence reached 75%, the plant response declined.
Tree Physiology 02/2009; 29(1):67-76. · 2.85 Impact Factor
[show abstract][hide abstract] ABSTRACT: Ozone, water and energy fluxes were measured over a Mediterranean maquis ecosystem from 5 May until 31 July 2007 by means of the eddy covariance technique. Additional measurements of NOx fluxes were performed by the aerodynamic gradient technique. Stomatal ozone fluxes were obtained from water fluxes by a Dry Deposition Inferential Method based on a big leaf concept. The maquis ecosystem acted as a net sink for ozone. The different water availability between late spring and summer was the major cause of the changes observed in stomatal fluxes, which decreased, together with evapotranspiration, when the season became drier. NOx concentrations were significantly dependent on the local meteorology. NOx fluxes resulted less intense than the ozone fluxes. However an average upward flux of both NO and NO2 was measured. The non-stomatal pathways of ozone deposition were investigated. A correlation of non-stomatal deposition with air humidity and, in a minor way, with NO2 fluxes was found. Ozone risk assessment was performed by comparing the exposure and the dose metrics: AOT40 (Accumulated dose over a threshold of 40 ppb) and AFst1.6 (Accumulated stomatal flux of ozone over a threshold of 1.6 nmol m−2 s−1). AOT40, both at the measurement height and at canopy height was greater than the Critical Level for the protection of forests and semi-natural vegetation (5000 ppb h) adopted by UN-ECE. Also the AFst1.6 value (12.6 mmol m−2 PLA, Projected Leaf Area) was higher than the provisional critical dose of 4 mmol m−2 PLA for forests. The cumulated dose showed two different growth rates in the spring and in the summer periods, while the exposure showed a more irregular behavior in both periods.
[show abstract][hide abstract] ABSTRACT: This paper summarises some of the main results of a two-year experiment carried out in an Open-Top Chambers facility in Northern Italy. Seedlings of Populus nigra, Fagus sylvatica, Quercus robur and Fraxinus excelsior have been subjected to different ozone treatments (charcoal-filtered and non-filtered air) and soil moisture regimes (irrigated and non-irrigated plots). Stomatal conductance models were applied and parameterised under South Alpine environmental conditions and stomatal ozone fluxes have been calculated.The flux-based approach provided a better performance than AOT40 in predicting the onset of foliar visible injuries. Critical flux levels, related to visible leaf injury, are proposed for P. nigra and F. sylvatica (ranging between 30 and 33 mmol O3 m−2). Soil water stress delayed visible injury appearance and development by limiting ozone uptake. Data from charcoal-filtered treatments suggest the existence of an hourly flux threshold, below which may occur a complete ozone detoxification.
[show abstract][hide abstract] ABSTRACT: An Open-Top Chambers experiment on Fagus sylvatica and Quercus robur seedlings was conducted in order to compare the performance of an exposure-based (AOT40) and a flux-based approaches in predicting the appearance of ozone visible injuries on leaves. Three different ozone treatments (charcoal-filtered; non-filtered; and open plots) and two soil moisture treatments (watered and non-watered plots) were performed. A Jarvisian stomatal conductance model was drawn up and parameterised for both species and typical South Alpine environmental conditions, thus allowing the calculation of ozone stomatal fluxes for every treatment. A critical ozone flux level for the onset of leaf visible injury in beech was clearly identified between 32.6 and 33.6 mmolO3 m(-2). In contrast, it was not possible to identify an exposure critical level using the AOT40 index. Water stress delayed the onset of the leaf visible injuries, but the flux-based approach was able to take it into account accurately.
[show abstract][hide abstract] ABSTRACT: Ozone (O3) exposure at Italian background sites exceeds UN/ECE concentration-based critical levels (CLe(c)), if expressed in terms of AOT40. Yet the occurrence of adverse effects of O3 on forests and crops is controversial. Possible reasons include (i) ability of response indicators to provide an unbiased estimate of O3 effects, (ii) setting of current CLe(c) in terms of cut-off value and accumulation level, (iii) response functions adopted to infer a critical level, (iv) environmental limitation to O3 uptake and (v) inherent characteristics of Mediterranean vegetation. In particular, the two latter points suggest that critical levels based on accumulated stomatal flux (CLe(f)) can be a better predictor of O3 risk than CLe(c). While this concept is largely acknowledged, a number of factors may limit its applicability for routine monitoring. This paper reviews levels, uptake and vegetation response to O3 in Italy over recent years to discuss value, uncertainty and feasibility of different approaches to risk assessment.
[show abstract][hide abstract] ABSTRACT: Ozone (O3) induces deleterious effects on plants by its oxidising capacity. Efforts have aimed at defining O3 critical loads which are based on O3 penetrating into the plant through the stomata, being an important mechanism of O3 removal. Recently, papers have proposed the use of process-based models for estimating the O3 dose at the community level. These models should define the stomatal O3 flux (Fst) on vegetation, correlating it to O3-induced injury and hence to improve the data-base available for decision makers. A process-based model has been utilised for the quantification of O3 flux (F) toward a Holm oak forest and its repartition in Fst and non-stomatal flux (Fnost) during daylight. The model outputs have been compared with eddy covariance measurements. Simulation of O3 fluxes under the climatic limitations of summer 2003 showed that the mean values of Fst represented only 28.9% of F, which was similar to eddy covariance (31.5%). The use of LAIeffective to link the leaf Fst to the canopy scale O3 stomatal flux (Feffectjve; on ground unit) is shown to be useful not only for the model validation but also for upscaling purposes from leaf to canopy. Forest acts as a LAI=1, i.e. a “real” big leaf for canopy O3 uptake. Changes in O3 fluxes were more closely related to the factors that control O3 deposition than to the factors controlling O3 concentration. Simulations showed that Holm oak forest was most effective at taking up O3 under moderate to high irradiance and high physiological activity, but not in limiting environmental conditions. The model used here does not require a great number of input variables and it is based on simple assumptions with respect to other more recognised models. So, the use of this model can have useful applications for the risk assessment of level II for the forests.
[show abstract][hide abstract] ABSTRACT: Fluxes of ozone as well as of sensible and latent heat were measured over a barley field in Northern Italy from April to June 2002 with the eddy-correlation technique in order to determine the dose of ozone taken up by plants during the whole grain filling and maturation period. Stomatal ozone fluxes were then calculated by using the similarity between gaseous exchange processes occurring inside the stomata involving ozone and water vapour, whose flux was available from the measurements. The dose was then obtained by integrating the stomatal flux over time. On the average, the stomatal flux was found to be approximately 50% of the total flux. This approach was compared to the currently used ozone risk assessment procedure based on the use of the exposure index AOT40, calculated from routine ozone concentration records. Important differences between these two approaches appeared.In order to assess ozone risk to vegetation in a realistic way, a new concept of “effective exposure”, which combines both exposure and uptake approaches, is developed. A new risk assessment index is obtained by weighting exposure by a factor which includes stomatal conductance, obtained by a modelling procedure, which uses observational data available from routine monitoring stations. This new index, formally similar to AOT40, uses a lower ozone concentration threshold (<20 ppb instead of 40 ppb) and follows much more closely the stomatal ozone dose evolution than the original AOT40 index. This approach permits to calculate ozone exposure taking physiological aspects into account, for situations where only ozone concentrations and meteorological observations are available, as direct flux data are difficult to obtain on a routine basis.
[show abstract][hide abstract] ABSTRACT: Ozone deposition fluxes have been measured over a wheat field in Northern Italy in May–June 2001 with the eddy-correlation method in order to evaluate the amount of ozone taken up by plants during the whole grain filling period, from anthesis to harvest. Ozone uptake by plants is due to its penetration through the stomata; for this reason stomatal ozone fluxes were determined, using the analogy with water vapour fluxes (Penman–Monteith approach), which are easily measured. The total ozone dose was obtained by integrating the stomatal ozone fluxes over time. The observational results showed that ozone fluxes decrease with time, following the maturation and the senescence of plants. On the average, the stomatal flux was found to be 50–60% of the total flux, but this fraction decreased during the senescence, emphasizing the importance of non-stomatal ozone deposition pathways. The approach consisting on evaluating the total ozone dose by integrating stomatal fluxes was then compared to the currently used procedure based on the evaluation of the exposure index AOT40, which uses ozone concentrations. Important differences between these two methods are highlighted. The integration of stomatal ozone fluxes appears more suitable for the evaluation of physiological uptake than the use of the exposure index.
[show abstract][hide abstract] ABSTRACT: During the summer of 2001, 2-year-old Fraxinus excelsior and Fagus sylvatica plants were subjected to ozone-rich environmental conditions at the Regional Forest Nursery at Curno (Northern Italy). Atmospheric ozone concentrations and stomatal conductance were measured, in order to calculate the foliar fluxes by means of a one-dimensional model. The foliar structure of both species was examined (thickness of the lamina and of the individual tissues, leaf mass per area, leaf density) and chlorophyll a fluorescence was determined as a response parameter. Stomatal conductance was always greater in Fraxinus excelsior, as was ozone uptake, although the highest absorption peaks did not match the peaks of ozone concentration in the atmosphere. The foliar structure can help explain this phenomenon: Fraxinus excelsior has a thicker mesophyll than Fagus sylvatica (indicating a greater photosynthesis potential) and a reduced foliar density. This last parameter, related to the apoplastic fraction, suggests a greater ability to disseminate the gases within the leaf as well as a greater potential detoxifying capacity. As foliar symptoms spread, the parameters relating to chlorophyll a fluorescence also change. PI (Performance Index, Strasser, A., Srivastava, A., Tsimilli-Michael, M., 2000. The fluorescence transient as a tool to characterize and screen photosynthetic samples. In: Yunus, M., Pathre, U., Mohanty, P., (Eds.) Probing Photosynthesis: Mechanisms, Regulation and Adaptation. Taylor & Francis, London, UK, pp. 445-483.) has proved to be a more suitable index than Fv/Fm (Quantum Yield Efficiency) to record the onset of stress conditions.
[show abstract][hide abstract] ABSTRACT: Passive samplers (diffusion tubes with organic reagent, produced by Passam of Switzerland) were used in a sampling campaign for the detection of weekly mean ozone concentrations in 15 sites over a domain of 80 x 40 km on the southern side of the European Alps from May to August 1998. The area is characterized by vast natural terrain of complex topography, with conifer and broadleaf forests. It is difficult to access and monitor air quality there with continuous analysers. By applying geostatistical techniques (ordinary kriging), and correcting the interpolated ozone concentrations according to the altitude of each single grid cell (2 x 2 km), maps of weekly ozone concentrations were produced. The weekly ozone data were used to assess daily and hourly data by means of an iterative procedure based on a functional dependence of ozone concentrations both on altitude and on the time of day. This allowed the estimation of values with an exposure index such as AOT40 (accumulated exposure over the threshold of 40 ppb) in all 800 cells of the domain. This also allowed the mapping of risk assessment related to the effects of ozone on the regional forest vegetation. Results obtained show values that exceed the exposure standards adopted in the Kuopio protocol (1996). Excess exposure values also match values calculated over a wider territorial domain by using hourly data on ozone concentration derived from continuous automatic analysers.
[show abstract][hide abstract] ABSTRACT: Ozone is the most harmful air pollutant for plant ecosystems in the Mediterranean and Alpine areas due to its biological and economic damage to crops and forests. In order to evaluate the relation between ozone exposure and vegetation injury under on-field conditions, suitable ozone monitoring techniques were investigated. In the framework of a 5-year research project aimed at ozone risk assessment on forests, both continuous analysers and passive samplers were employed during the summer seasons (1994-1998) in different sites of a wide mountain region (80 x 40 km2) on the southern slope of the European Alps. Continuous analysers allowed the recording of ozone hourly concentration means necessary both to calculate specific exposure indexes (such as AOT, SUM, W126) and to record daily time-courses. Passive samplers, even though supplied only weekly mean concentration values, made it possible to estimate the altitude concentration gradient useful to correct the altitude dependence of ozone concentrations to be inserted into exposure indexes. In-canopy ozone profiles were also determined by placing passive samplers at different heights inside the forest canopy. Vertical ozone soundings by means of tethered balloons (kytoons) allowed the measurement of the vertical concentration gradient above the forest canopy. They also revealed ozone reservoirs aloft and were useful to explain the ozone advection dynamic in mountain slopes where ground measurement proved to be inadequate. An intercomparison between passive (PASSAM, CH) and continuous measurements highlighted the necessity to accurately standardize all the exposure operations, particularly the pre- and postexposure conservation at cold temperature to avoid dye (DPE) activity. Advantages and disadvantages from each mentioned technique are discussed.
[show abstract][hide abstract] ABSTRACT: The ozone exposure risk for vegetation in Lombardy (Northern Italy) has been assessed by the AOT40 exposure index, based on data taken from the existing local monitoring networks covering 5 growing seasons (1994 to 1998). One-square kilometer exposure maps were obtained by using geostatistic techniques (ordinary kriging) followed by an altitude detrendization of measurement's temporal series to account for the domain's large topographic heterogeneity. Risk areas (Level I maps) were identified using a GIS and overlaying the ozone-critical-level exceedance maps on the distribution maps of forests and sensitive species.The critical ozone exposure level of 10 000 ppb h, adopted by UN/ECE protocols, is exceeded over the whole Lombardy Territory over the 6-month growing season. The highest risk areas are the northwest pre-alpine and alpine belt, directly impacted by the photo-oxidant plume generated by the Milan urban area. Difficulties met in creating a proper Level II risk assessment for forests in mountain areas have been bypassed by comparing ozone exposures with summer climate features. Soil water availability was assumed not to be a significant modifying factor in the mountains of this region because of the frequent summer rains, whereas the opposite held true for wind ventilation which is generally weak. Field surveys have reported foliar injuries attributable to ozone in different species of forest trees and shrubs, which provide further evidence of potentially phytotoxic ozone levels.
[show abstract][hide abstract] ABSTRACT: In the latter years a flux-based approach for ozone risk assessment on crops was strongly supported by the scientific community. Nevertheless, the amount of ozone absorbed by plants through stomata is hardly measurable and very few dose-response relationships are available in the literature. Field experiments are usually performed in OTC (or Free Air Ozone Enrichment Systems, where available) and ozone dose is calculated (not measured) by stomatal conductance models fed with climatic parameters measured in the OTCs. At the moment OTCs are the only affordable system able to remove ozone molecules in field conditions and able to provide a ``control'' treatment for the validation of the ozone effects. At the same time the only way to measure ozone fluxes on vegetation is the Eddy Covariance (EC), a micrometeorological technique that, unfortunately, cannot be applied inside OTCs. These two techniques were combined and simultaneously used to study the effects of the ozone uptake by alfalfa fields in a two years pilot experiment in Italy. An EC tower mounted in the middle of the field and three OTC were placed randomly on a side of the field, two of them flushed with charcoal-filtered air (ozone-free) and one with ambient air only. The plant productivity within the OTCs and in the open field, as well as the forage quality, were compared at the end of each growing cycle. With the total ozone fluxes and water and heat ones obtained from the EC measurements, a resistive analysis was conducted in order to get the stomatal resistance of plants in the open field and thus their ozone uptake. Then, the ozone uptake by the plants inside the OTCs was calculated by assuming the same stomatal resistance obtained in the open field but applying an atmospheric Ra and sub-laminar Rb resistances fit on the OTC geometry and fan system flow. The results show that a considerably amount of the ozone deposited on the field was absorbed by plants through the stomata, with values between 66% and 94% in the different growing cycles. The plants exposed to ozone in the open field and in the ambient air flushed OTC showed on average a dry biomass production of 20% less than in the charcoal filtered chambers, but the forage quality resulted a little bit increased since the raw fibres content, as well as the ratio between acid and neutral deterged fibres fractions and the protein contents increased. Finally the results of five harvests were combined and related to both the ozone exposure (calculated as AOT40) and the cumulated ozone dose (flux) received by crop. The cumulated stomatal flux resulted a more reliable predictor (R2=0.67) of the biomass productivity than AOT40 (R2=0.03), and an average linear dose-biomass relationship of 0.0225 Kg of dry weight reduction per mmol m-2 of O3 taken up by crop was found. The same behaviour was confirmed more or less for all the parameters taken into account. This pilot study showed that the combined use of Eddy Covariance and OTC techniques could be promising for studies aimed to quantify dose-effect relationships.