Elena Paoletti

Ozone levels in European and USA cities are increasing more than at rural sites, while peak values are decreasing.

Ground-level ozone (O3) levels are usually lower in urban centers than nearby rural sites. To compare trends in O3 levels during the period 1990-2010, we obtained monitoring data from paired urban and rural sites from the European Environment Agency and the US Environmental Protection Agency. Ozone peaks decreased at both station types, with no significant differences between urban and rural stations. Ozone annual averages increased at both urban and rural sites, with a faster rate of increase for urban centers. The overall trend was for convergence between urban and rural O3 data. Ozone levels exceeded the criteria established for the protection of human and vegetation health at both urban and rural sites.

Advances of air pollution science: from forest decline to multiple-stress effects on forest ecosystem services.

Over the past 20 years, the focus of forest science on air pollution has moved from forest decline to a holistic framework of forest health, and from the effects on forest production to the ecosystem services provided by forest ecosystems. Hence, future research should focus on the interacting factorial impacts and resulting antagonistic and synergistic responses of forest trees and ecosystems. The synergistic effects of air pollution and climatic changes, in particular elevated ozone, altered nitrogen, carbon and water availability, must be key issues for research. Present evidence suggests air pollution will become increasingly harmful to forests under climate change, which requires integration amongst various stressors (abiotic and biotic factors, including competition, parasites and fire), effects on forest services (production, biodiversity protection, soil protection, sustained water balance, socio-economical relevance) and assessment approaches (research, monitoring, modeling) to be fostered.

Ozone and urban forests in Italy.

Ozone levels along urban-to-rural gradients in three Italian cities (Milan, Florence, Bari) showed that average AOT40 values at rural and suburban sites were 2.6 times higher than those determined at urban sites. However, O(3) also exceeded the European criteria to protect forest health at urban sites, even when the standards for human health protection were met. For protecting street trees in Mediterranean cities, the objectives of measurement at urban sites should extend from the protection of human health to the protection of vegetation as well. A review of forest effects on O(3) pollution and of O(3) pollution on forest conditions in Italian cities showed that it was not possible to distinguish the effect of O(3) in the complex mixture of urban pollutants and stressors. A preliminary list of tree species for urban planning in the Mediterranean area shows the average tree capacity of O(3) removal and VOC emission.

Structural and physiological responses to ozone in manna ash (Fraxinus ornus L.) leaves of seedlings and mature trees under controlled and ambient conditions.

Leaf-level microscopical symptom structure and physiological responses were investigated in seedlings experimentally exposed to ozone (O3) in indoor chambers (150 ppb, 8 h d(-1)/7 weeks), and field trees of Manna ash (Fraxinus ornus) exposed to ambient O3 (max 93 ppb/one growing season). Ozone-induced leaf injury, including leaf reddening and stippling, was observed in both seedlings and mature trees, but the morphology of injury in the stipples differed, being hypersensitive-like (HR-like) in the chamber seedlings and accelerated cell senescence (ACS) in the field trees. In both exposure conditions, the main structural impact of O3 was on the mesophyll and especially the upper assimilating cell layers. The main physiological impact was on carbon assimilation and on stomatal sluggishness. These effects were not due to stomatal structural injury and were more severe in juvenile compared to mature trees because of environmental (water availability, light) and constitutional (gas exchange capacity) factors and differences in the cell physiology processes (HR-like vs. ACS) triggered by ozone stress. Given the plasticity of plant responses to ozone stress, dose/response relationships for tree seedlings in the indoor chambers cannot be extrapolated to mature trees unless ambient conditions are closely simulated.

Impacts of Air Pollution and Climate Change on Forest Ecosystems — Emerging Research Needs

Outcomes from the 22nd meeting for Specialists in Air Pollution Effects on Forest Ecosystems “Forests under Anthropogenic Pressure Effects of Air Pollution, Climate Change and Urban Development”, September 1016, 2006, Riverside, CA, are summarized. Tropospheric or ground-level ozone (O3) is still the phytotoxic air pollutant of major interest. Challenging issues are how to make O3 standards or critical levels more biologically based and at the same time practical for wide use; quantification of plant detoxification processes in flux modeling; inclusion of multiple environmental stresses in critical load determinations; new concept development for nitrogen saturation; interactions between air pollution, climate, and forest pests; effects of forest fire on air quality; the capacity of forests to sequester carbon under changing climatic conditions and coexposure to elevated levels of air pollutants; enhanced linkage between molecular biology, biochemistry, physiology, and morphological traits.

Ethylenediurea (EDU): A research tool for assessment and verification of the effects of ground level ozone on plants under natural conditions

Ethylenediurea (EDU) has been widely used to prevent ozone (O(3)) injury and crop losses in crop plants and growth reductions in forest trees. Successful use requires establishing a dose/response curve for EDU and the proposed plant in the absence of O(3) and in the presence of O(3) before initiating multiple applications to prevent O(3) injury. EDU can be used to verify foliar O(3) symptoms in the field, and to screen plants for sensitivity to O(3) under ambient conditions. Despite considerable research, the mode of action of EDU remains elusive. Additional research on the mode of action of EDU in suppressing O(3) injury in plants may also be helpful in understanding the mode of action of O(3) in causing injury in plants.

O3 and O3 + CO2 effects on a Mediterranean evergreen broadleaf tree, holm oak (Quercus ilex L.)

The aim of this work is to analyze the effects of a) short-term treatments with different concentrations of O 3 (daily 6-hour fumigation with 0, 65, 175 and 300 ppb for 3-4 days); and b) a medium term treatment with O 3 (150 ppb three times a week for thirty days on the whole) and CO 2 (700 ppm) on gas exchange, chlorophyll fluorescence, and peroxidase activity in holm oak (Quercus ilex L.) leaves. The results show that net photosynthesis, transpiration, F V /F M ratio and POD activity were not influenced until an O 3 concentration of 300 ppb was reached. At this threshold, significant alterations in these physiological and biochemical parameters were found. The treatment with +CO 2 +O 3 showed an increase of net photosynthesis suggesting thus an antagonistic effect of CO 2 with regard to O 3 -induced injuries, while CO 2 alone increased photosynthesis and decreased transpiration but induced no effects on F V /F M ratio or peroxidase activity. From out data, holm oak shows a resistance to episodes of photochemical O 3 stress, that in the Mediterranean region can reach peaks of > 100 ppb. This response is likely to be due to the inorpho-anatomic structure of holm oak leaves and their sclerophyllous adaptations. This might leads holm oak to adopt a stress tolerance strategy with regard to leaf response to O 3 , the effects of which cannot be ameliorated by an increase of atmospheric CO 2 .

The Abiotic Urban Environment: Impact of Urban Growing Conditions on Urban Vegetation

‘Planting of trees in towns should not be given in senseless and untrained hands, because one has to fight against unfavorable soil-, airand other conditions. From noticing them the safe and prospering development of expensive establishments highly depends’ (translated from Fintelmann 1877). Depending on the geographic situation and the urban structure, the environmental conditions in urbanized areas are more or less harsh and they differ from natural growing conditions. The impact of the urban environment on urban vegetation is neither constant in intensity nor periodical. There are numerous constraints that are crucial for the survival and vitality of urban vegetation. This chapter focuses on basic abiotic growing conditions in urbanized areas, on artificial deviations from the natural undisturbed habitat quality in forests that contribute to stresses and threats for urban vegetation. This includes both the qualitative and quantitative impact of different site factors and the time-scale as well: specifics of urban climate, air pollution, constraints and peculiarities of urban hydrological cycles, urban soil conditions in general and in particular unfavorable physical soil properties, unbalanced nutrient supply, soil pollution and fire problems. Possible remedies and precautions to improve growing conditions for urban trees are implicated. In short, the interactions between human activities and the urban environment are discussed to help minimize abiotic stresses that reduce the vigor and vitality especially of trees, and that increase their susceptibility to biotic stresses. Vice versa the impact of urban vegetation on urban ecology is summarized.

Use of the antiozonant ethylenediurea (EDU) in Italy: verification of the effects of ambient ozone on crop plants and trees and investigation of EDU's mode of action.

Twenty-four experiments where EDU was used to protect plants from ozone (O(3)) in Italy are reviewed. Doses of 150 and 450 ppm EDU at 2-3 week intervals were successfully applied to alleviate O(3)-caused visible injury and growth reductions in crop and forest species respectively. EDU was mainly applied as soil drench to crops and by stem injection or infusion into trees. Visible injury was delayed and reduced but not completely. In investigations on mode of action, EDU was quickly (<2h) uptaken and translocated to the leaf apoplast where it persisted long (>8 days), as it cannot move via phloem. EDU did not enter cells, suggesting it does not directly affect cell metabolism. EDU delayed senescence, did not affect photosynthesis and foliar nitrogen content, and stimulated antioxidant responses to O(3) exposure. Preliminary results suggest developing an effective soil application method for forest trees is warranted.

Protection of ash (Fraxinus excelsior) trees from ozone injury by ethylenediurea (EDU) : Roles of biochemical changes and decreased stomatal conductance in enhancement of growth

Treatments with ethylenediurea (EDU) protect plants from ozone foliar injury, but the processes underlying this protection are poorly understood. Adult ash trees (Fraxinus excelsior), with or without foliar ozone symptoms in previous years, were treated with EDU at 450 ppm by gravitational trunk infusion in May-September 2005 (32.5 ppm h AOT40). At 30-day intervals, shoot growth, gas exchange, chlorophyll a fluorescence, and water potential were determined. In September, several biochemical parameters were measured. The protective influence of EDU was supported by enhancement in the number of leaflets. EDU did not contribute its nitrogen to leaf tissue as a fertiliser, as determined from lack of difference in foliar N between treatments. Both biochemical (increase in ascorbate-peroxidase and ascorbic acid, and decrease in apoplastic hydrogen peroxide) and biophysical (decrease in stomatal conductance) processes regulated EDU action. As total ascorbic acid increased only in the asymptomatic trees, its role in alleviating O(3) effects on leaf growth and visible injury is controversial.