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

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Studying weekend-weekday variation in ground-level ozone (O 3 ) allows one to better understand O 3 formation conditions, with a potential for developing effective strategies for O 3 control. Reducing inappropriately the O 3 precursors emissions can either produce no reduction or increase surface O 3 concentrations. This paper analyzes the weekend-weekday differences of O 3 at 300 rural and 808 urban background stations worldwide from 2005 to 2014, in order to investigate the O 3 weekend effect over time and assess the effectiveness of the precursors emissions control policies for reducing O 3 levels. Data were analyzed with the non-parametric Mann-Kendall test and Theil-Sen estimator. Rural sites typically did not experience a weekend-weekday effect. In all urban stations, the mean O 3 concentration on the weekend was 12% higher than on weekdays. Between 2005 and 2014, the annual mean of daily O 3 concentrations increased at 74% of urban sites worldwide (+ 0.41 ppb year −1 ) and decreased in the United Kingdom (- 0.18 ppb year −1 ). Over this time period, emissions of O 3 precursors declined significantly. However, a greater decline in nitrogen oxides (NO x ) emissions caused an increase in Volatile Organic Compounds (VOCs) to NO x ratios leading to O 3 formation. In France, South Korea and the United Kingdom, most urban stations showed a significant upward trend (+ 1.15% per year) for O 3 weekend effect. Conversely, in Canada, Germany, Japan, Italy and the United States, the O 3 weekend effect showed a significant downward trend (- 0.26% per year). Further or inappropriate control of anthropogenic emissions in Canada, Southern Europe, Japan, South Korea and the United States might result in increased daily O 3 levels in urban areas.

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