*2.2.3 Foliar O3 symptoms*

Symptoms of O3 injury had been observed at different levels in several plant species. The presence of foliar symptoms does not necessarily imply that there are significant effects on growth, yield or reproduction. Visible injury symptoms in bioindicator plants are more perceptible and comprehensible for laymen than air pollution concentrations, which may help to enhance the environmental awareness to the general public. Therefore, it is an effective way to monitor the O3 risk in urban and regional areas by investigating the typical characteristics of O3 injury on plants as well as the street view [23–26]. These studies have identified sensitive species that could contribute to further investigated under controlled conditions.

The growth and physiological response of plants exposed to high concentrations of O3 for a long time can reflect the degree of atmospheric O3 pollution. Ozone levels in Beijing area were high enough to induce foliar symptoms by the year of 2021 at least, injury has been observing in different species [8, 60, 66, 67]. Visible injury was investigated on July and August in the main parks, mountainous and plain areas. The typical symptoms of the O3 foliar injury are dark stipple, mottling and tip burn [66]. Some examples of typical symptom are illustrated as an atlas (**Figure 4**).

#### **Figure 4.**

*Examples of damaged leaves of trees in a big city polluted with ozone. Photos of poplar and plum are cited from Nouchi [49], and tree of heaven is cited from Feng et al. [52, 53], the handbook for diagnosing O3 damages of Chinese avenue and park trees.*

Sensitive plants exposed to O3 pollution usually show symptoms of visible leaf injury (ICP-Forest; [68]); most of them were surveyed on the basis of the Forest Health Expert Advisory System [69]. In recent years, foliar O3 symptoms on plants were studied in Beijing, Hebei, Nanchang, and the surrounding areas of field investigations following the procedures for O3 symptoms [8, 60, 66, 67, 70]; and [71].

Symptoms were more frequent in rural areas and mountains in northern Beijing, downwind from the city, and less frequent in city gardens [8]. *Ailanthus altissima* (Tree of heaven) is considered as the best bio-indicator tree for the Beijing area, because it is a native tree species with wide distribution, and symptoms of O3 injury are easy to characterize. Similar investigations of O3 damage symptoms on plant leaves were also carried out in Hebei Province. *Buxus megistophylla* was most frequently investigated and its injury symptoms of it were easy to be characterized, which will good to be used as an indicator species for ambient O3 pollution in this region [71]. The other investigation in Nanchang city in Jiangxi province, which can represent subtropical urban areas in China, find that the O3 concentration of suburban regions was significantly higher than in urban and exurban regions [70] (cf. **Figure 2**). The highest average O3 concentration occurred in June at 40 ppb (AOT40 = 35.5 mg m<sup>3</sup> h<sup>1</sup> ), well over the threshold considered (19.6 mg m<sup>3</sup> h<sup>1</sup> ) to exert negative influences on the growth of wild plants. *Cerasus yedoensis*, *Phoebe shearer*, *P. bournei,* and *Litsea cubeba* might work as bio-indicators of ozone pollution in Nanchang [70].

Ozone stress in plants is recognized as a visible symptom of leaves at various levels of O3 in the atmosphere [32, 52, 53, 72]. How can we obtain a bigger scale from individual tree scale to town scale? A typical example is recently reported in China with the use of big internet data [23–26]. They could identify the association between greenness and air pollution from a street view scale, which can favor urban greenness management and evaluation in other regions where street-view data are available. Urban forests play a vital role in terms of environmental quality related to particulate matter (PM including black carbon: BC) and studies on this area are increasing [73, 74]. The PM analyzes were divided into vertical and lateral directions at the stand scale [14]. The PM removal capacity of different vegetation types was usually in the following order: coniferous forest > evergreen forest > deciduous forest as related to foliar shape and longevity. Therefore, multi-scale analyzes on the effects of PM could help to better understand the roles of urban forests as a complex system.

## **3. Ozone and other environmental stress factors**

The singly or combined effects of elevated O3 on woody plants and surroundings were investigated using manipulative exposure experiment OTCs or O3-FACE. Feng et al. [52, 53] integrated the results of 46 current O3 fumigation experiments on 61 woody plants in OTCs in China. Net photosynthetic rate, growth index, and biomass of woody plants are affected by elevated O3, showed varying degrees of reduction (**Figure 5**). Drought, high nutrients, temperature stress, and tropospheric O3 pollution often co-occur in urban areas, adversely affecting urban plant health. Another experimental comprehensively analyzed the adsorption and absorption capacity of 537 species of plants in common urban forests in China to six kinds of air pollutants, namely, SO2, NOx, freon (F), chlorine (Cl), O3 and PM, six tree species with strong comprehensive tolerance to air pollutants were selected [75].

Identify possible indicators for O3 sensitivity in woody plant species that could be useful for improving risk assessment and selecting appropriate species for urban

*Vigor and Health of Urban Green Resources under Elevated O3 in Far East Asia DOI: http://dx.doi.org/10.5772/intechopen.106957*

#### **Figure 5.**

*Effects of elevated O3 concentrations on net photosynthetic rate, growth index, and biomass of woody plants in China (Adapted from a meta-analysis by [52, 53]).*

greening in areas with high O3 levels. It seems feasible to plant tree species that are less sensitive to O3 exposure, particularly in urban areas.

#### **3.1 Heat**

The combined impacts of air warming and O3 on phenology and its functional traits are still not well illustrated for urban trees and roof green, including root crops (radish) on mini-farmland on buildings (cf. **Figure 2**) are still poorly understood. The reduction in the growth of radish (*Raphanus sativus* var. *sativus*) caused by elevated O3 was accelerated by elevated temperature [72, 76]. This reduction discourages people's motivation for cultivation on a farm on rooftop (cf. **Figure 2**). Spring phenological and function in leaves of *Populus alba* and lian qiao (*Forsythia suspensa*) under ambient air (15.8°C, 35.7 ppb), increased air temperature (IT, ambient air temperature + 2°C) in a combination of two levels of O3 (EO3, ambient O3 + 40 ppb), and their combined treatments (17.7°C, 74.5 ppb) [23–26]. Compared to EO3, the combined treatment advanced the spring pheno-phase, increased growth, and induced a higher photosynthetic rate and antioxidative enzyme activities, which indicated that the positive effects of increased temperature alleviated the inhibition of growth induced by O3. We expect that high temperatures open many stomata and increase the transpiration rate to a certain threshold. Therefore, it was expected that the amount of O3 absorbed would also increase because the stomatal conductance would increase. In Italy, among three species with different water requirements (poplar>beech> oak), fast-growing plant species with high water requirements show more susceptible to O3 and drought stress via the use of 13C in leaves [77].

#### **3.2 Nitrogen deposition**

With the rapid industrial development and modern agricultural practices, increasing N deposition can cause nutrient imbalance. With remarkably high levels of NO2 in east Asia [4, 5], O3 could adversely affect the productivity of forest tree species, but risk assessments of O3 impact were still limited [78]. We analyzed risk assessment of

O3 on suburb forest tree species based on two previous studies via OTC [79], that is, the growth data in potted seedlings of Siebold's beech (*Fagus crenata*) and of three representative conifers, cypress (*Cryptomeria japonica*)*,* pine (*Pinus densiflora*) and Japanese larch (*Larix kaempferi*). From deciduous leaf habit, N deposition increased in the O3 sensitivity of beech while unchanged that of larch ([79], **Figure 6**). Based on the results, we conclude that the area with a high risk of O3 impact does not necessarily correspond to the area with high O3 exposure under different N levels.

Japanese larch and its hybrid larch (F1; *Larix gmelinii var. japonica Larix kaempferi*) are an important afforestation species in northeast Asia. We investigated whether N loading mitigates the negative O3 impacts on two larch species [80]. Although N loading mitigated the negative effects of O3 on Japanese larch, N loading did not mitigate O3-induced inhibition of growth and photosynthetic capacity in the F1. Elevated O3 also reduced leaf nitrogen/phosphorus (N/P) ratio by elevated O3, with significant effects in F1, particularly under N loading. To avoid the negative effect of increasing O3, we will plant Japanese larch but not F1 at N rich site and/or accumulation of the N deposition site.

Larch species are associated with ectomycorrhizal (ECM) fungi, which play a critical role in nutrient acquisition for their hosts [4, 81, 82]). In this study, we investigated species richness and diversity of ECM fungi associated with a hybrid larch (F1) and its parents (Dahurian larch; *L. gmelinii var. japonica,* Japanese larch; *L. kaempferi*), under simulated N deposition (0 and 100 kg ha<sup>1</sup> yr.<sup>1</sup> ) with/without phosphorous (P) (0 and 50 kg ha<sup>1</sup> yr.<sup>1</sup> ) planted in immature volcanic ash (Vitric Andosols) with low nutrient availability. F1 showed heterosis in relative biomass, which was most apparent under high N treatments. Except for Dahurian larch, effects of the nutrient addition to ECM fungal community in F1 were intermediate. F1 was tolerant to high N loading, which was due to a consistent, relatively high association with *Suillus* sp. and *Hebeloma* sp. [81].

A recent trial was carried out for analyzing the combined effects of O3 (low and two times ambient O3), elevated CO2 (ambient vs. 700 ppm), and 3 levels of soil N supply on Siebold's beech seedlings grown in climatized chambers [83]. They found that elevated CO2 ameliorated O3-induced reductions in photosynthetic activity, whereas the negative effects of O3 on photosynthetic traits were enhanced by soil

#### **Figure 6.**

*Contrasting sensitivity of Siebold's beech vs. Japanese larch to elevated O3 (as shown in AOT40) with increasing nitrogen loading to the soil (Re-illustrated from [79]).*

*Vigor and Health of Urban Green Resources under Elevated O3 in Far East Asia DOI: http://dx.doi.org/10.5772/intechopen.106957*

**Figure 7.**

*Ratio of aboveground mass (S) to root mass (S/R) of Siebold's beech seedlings raised under different O3, CO2, and three levels of N roading (Adapted from [83]). \* p < 0.05, \*\* p < 0.01, \*\*\* p < 0.001, n.s. not significant, the actual p-values are shown when 0.05* ≤ *p < 0.10.*

nitrogen supply. Although we observed two- or three-factor interactions of gas and soil N in leaf photosynthetic traits, the shoot-to-root dry mass ratio (S/R) was the only parameter for which a significant interaction was detected (**Figure 7**) among seedlings. O3 caused a significant increase in S/R under ambient CO2, whereas no similar effects were observed under elevated CO2.

## **4. Biological stresses on urban green**

Elevated ground-level O3 induces adverse effects in plants. Vigor and health of plants under elevated O3 are discussed through biological stresses: (i) the composition and diversity of plant communities by affecting key physiological traits [38, 84, 85]; (ii) foliar chemistry and the emission of volatiles (mainly BVOC) [43], thereby affecting plant–plant communication, plant-insect interactions, and the composition of insect communities [38, 42, 43, 85]; and (iii) plant–soil-microbe interactions (e.g., [81]) and the composition of soil communities by disrupting plant litterfall [61] and altering root exudation, soil enzymatic activities, decomposition, and nutrient cycling [86, 87]. 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.
