**2.1 A cool island**

Urban greenness is essential for people's daily lives, while its contribution to air quality control is unclear. Green-roof is recently popular in big cities for moderating heat island phenomena through canopy transpiration, offering shade, and water management [44] even in cool climate regions. Some green-roof consist of vegetable garden for fun people and to inspire motivation of management. The mitigation effects of green spaces in urban areas are clearly observed as a role of the cool island [45, 46]. These imply the accumulation of a cold air mass in the park and its gravitational flow-out into the surrounding area. This practice of urban greenness is also found in Sapporo, northern Japan (**Figure 1**).

The expansion of an urban tree canopy is a commonly proposed nature-based solution to combat the excess urban heat-island problem. The influence of trees on urban climates via shading is driven by the morphological characteristics of trees, whereas tree transpiration is predominantly a physiological process dependent on environmental conditions and the built environment. The heterogeneous nature of urban landscapes, unique tree species (including vegetable crops) assemblages (**Figure 2**), and land management decisions make it difficult to predict the magnitude and direction of cooling by transpiration [48].

#### **Figure 1.**

*Thermal image in Sapporo city center with green areas and their vicinity via the Landsat data (12 July 2015), offered Dr. H. Tani and the courtesy of Kyoritsu Publisher,Tokyo [47].*

**Figure 2.**

*Roof green with perennial plants and shrubs in Sapporo where we have heavy snow (A), and vegetable garden with small shade trees near the railway station in Tokyo (B) taken by T. Koike.*

The Bowen ratios showed clearly the essential role of evaluation of transpiration capacity of urban tree canopy for moderating heat in a city. In this sense, we should keep the vigor and health of the green area in urban and suburban regions with high O3 tolerance and also with O3-sensitive plant species.

Ozone is a strong oxidative gas, and is not a substrate of plant physiological processes, but is an intensive stressor. We employ O3-sensitive plants, such as morning glory (*Ipomoea nil*), as a biological indicator for avoiding O3 stresses in a city [49]. As green area (trees and shrubs) has a high capacity for improvement of O3 that is detected by flux monitoring [12, 50, 51], the green area has higher removal capacity (3.4 g m<sup>2</sup> yr.<sup>1</sup> on average) than green roofs (2.9 g m<sup>2</sup> yr.<sup>1</sup> as average removal rate), with lower installation and maintenance costs [13]. To overcome present gaps and uncertainties, they proposed a novel species air quality index (S-AQI) of suitability to air quality improvement for tree/shrub species. We recommend city planners to select species with an S-AQI > 8, that is, with high O3 removal capacity, O3-tolerant (e.g., [34, 52, 53]), non-emitting species of BVOC as a precoucer of O3 [18], resistant to pests and diseases [38], tolerant to drought and non-allergenic plants. Consequently, green roofs can be used to supplement urban trees in improving air quality in cities.

#### **2.2 The decline of urban and suburb green resources**

With the effective promotion of national energy conservation and emission reduction measures, air pollution in China has been reduced [54].

Similar to most already-industrialized countries, China is now shifting away from SO2-dominated to NOx- and O3-dominated air pollution [55]. According to the bulletin on ecological and environmental conditions of China in 2019, the average O3 concentration in China in 2019 was the highest in the past 5 years (148 <sup>μ</sup><sup>g</sup><sup>m</sup><sup>3</sup> ), 20% higher than that in 2015, and the range of areas with high O3 concentration was also expanding. Measurements from the national network for monitoring air pollutants show severe and worsening O3 pollution in many areas, particularly the densely populated regions like Beijing-Tianjin-Hebei metropolitan area, YRD, PRD, etc. [56] (**Figure 3**). Beijing and its surrounding areas are typical composite pollution areas in North China, and the pollution degree has an obvious suburban gradient in somewhat [57–59], and plants have been threatened by high O3 concentration [23, 60].

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

**Figure 3.** *Representative cities and districts in east Asia YRD Yangtze River Delta PRD: Pearl River Delta economic zone.*

Although ground-level ozone (O3) concentrations are expected to increase over the twenty-first century, especially in east Asia, with increasing NO2 [4, 5].

#### *2.2.1 Forest stand scale*

In the O3-sensitive beech forest as suggested by screening data of 18 tree seedlingscale by OTCs [34] (**Table 1**), we revealed earlier autumn senescence caused by elevated O3 had already started, which was not found in the O3-tolerant oak forests [12]. Ozone flux-based risk assessment was performed with the use of flux towers of carbon sequestration capacity in old temperate forests in Japan; pure forests of Siebold's beech vs. mixed evergreen and deciduous forests dominated by Konara oak (*Quercus serrata*).

Higher phytotoxic O3-dose above a threshold of 0 uptake (POD-0) with higher canopy stomatal conductance (Gs) was observed in the beech forest than that in the oak forest. Light-saturated gross photosynthesis declined earlier in the late growing season with increasing POD-0.

The Tanzawa mountain range is located in the southwestern part of the region of Tokyo metropolis of Japan, and provides recreation activities to the residents of big cities. Dominate tree species are Siebold's beech forests are distributed in the highelevation areas but have declined significantly. Recent ozone monitoring data suggest


#### **Table 1.**

*Classification of Japanese forest tree seedlings to O3 sensitivity obtained from the OTC.*

that high ozone concentrations (<100 ppb) may be a possible chronic cause of the loss of beech vitality started in 1980 days to date [62, 63].

#### *2.2.2 Street landscape*

Recently, efficient use of internet data offered an outline of the health and vigor of urban green in relation to air quality data (Air Quality Index: PM2.5, PM10, SO2, NO2, CO, and O3) from 206 monitoring stations from 27 provincial capital cities in China [23–26]. Over 90% of air quality variation could be explained by socio-economics and geo-climates, suggesting that air quality control in China should first reduce efflux from social economics, while geo-climatic-oriented ventilation facilitation design is also critical. Pooled-data analysis at the national level showed that street-view greenness was responsible for 2.3% of the air quality variations in the summer and 3.6% in the winter; however, when separated into different regions, the explaining power increased to 16.2% at best. For different air quality components, greenness had the most significant associations with NO2, CO, and O3, and the street-view/bird-view ratio was the most powerful indicator of all greenness parameters. Large inter-city variations were observed in all the greenness parameters, and the weak associations between all street-view parameters and bird-eye greenspace percentage (21–73%) indicate their representatives of different aspects of green infrastructures.

Another assessment is shown in the traditional method of dendrochronology [64]. Urban tree growth is often affected by higher temperatures, reduced water availability, small and compacted planting pits like many avenue trees, as well as pollution inputs. Despite these reducing growth conditions, recent studies found a better growth of urban trees compared to trees at rural sites with high O3, and enhanced growth of trees in recent times as shown in poplar [65]. Sakhalin fir (*Abies sachalinensis*) trees growing in urban vs. rural (or suburb) sites in Sapporo, northern
