**4.1 Plants' defense capacity under elevated ozone with elevated CO2**

Most carbon-based defense chemicals are synthesized from photosynthates [88]. As found in birch [89, 90] and cauliflower [20], we expect plant defense will increase at elevated (E) CO2 and decrease at elevated (E) O3. Aspen-FACE in the northcentral U.S.A. provided a combination of CO2 (560 ppm) and O3 (x ambient 1.5 times) [91] and showed performance of forest tent caterpillar (*Malacosoma disstria*) larvae. ECO2 reduced N and increased tremulacin (a glycoside in poplar plants) levels, whereas EO3 increased early season N and reduced tremulacin as compared to controls. With respect to insects, ECO2 had almost no effect on larval performance. Larval performance improved in EO3, but this response was negated by the addition of ECO2

(i.e., CO2 + O3 treatment). The tent caterpillars will have the greatest impact on aspen under current CO2 and EO3, due to increases in insect performance and reduced in tree growth, whereas the insect will have the least impact on aspen under high CO2 and low O3 levels, due to moderate changes in insect performance and enhances tree growth.

#### **4.2 Outbreak of insect and disease, a role of symbiosis**

Defense capacities of plants are mostly originated from photosynthates, as a result, defense traits (leaves, branches, bark, etc.) of plants under elevated O3 are lower. Consequently, trees and shrubs under elevated O3 are attacked by diseases, insects, and wild animals [38, 62]. Outbreaks of sawfly and repeated sawfly attacks are fatal for the weakened beech trees. Another indirect biotic factor is the increased population of sika deer (*Cervus nippon*), which eat up ground vegetation and destroys related community balance. Sika deer is not fond of eating plants containing high levels of C-based defense chemicals (such as phenolics and lignin) [92]. Therefore, individuals whose defense levels have decreased due to O3 are grazed.

The factors affecting beech decline in the Tanzawa mountains are complicated, however, further scientific research activities in various fields are required to understand the phenomena and to recover the beech forest vegetation [62, 63].

Colonization and species abundance of ECM fungi were revealed in 2-year-old hybrid larch (F1) under the combination of elevated CO2 and O3 in OTC (O3 < 6 nmol mol<sup>1</sup> , EO3: 60 nmol/mol; ambient and elevated CO2 [380 vs. 600 mmol mol<sup>1</sup> ]). After two growing seasons, ECM colonization and root biomass increased under elevated CO2 [81]. Additionally, O3 impaired ECM colonization and species richness, and reduced stem biomass. Concentrations of aluminum (Al), iron (Fe), molybdenum (Mo), and phosphorous (P) in needles were reduced by O3, while potassium (K) and magnesium (Mg) in the roots increased. No effects of combined fumigation were observed in any parameters except the P concentration in needles. The tolerance of F1 to O3 might potentially be related to a shift in ECM community structure. Special ECM to larch (*Suillus* sp.) could keep infection even under EO3, so we can expect *Suillus* sp. as a candidate for improving larch rhizosphere [81].

#### *4.2.1 Oak wilt disease*

Deciduous oak dieback in Japan has been known since the 1930s [93], but it has been spreading to the whole Japanese island, mainly facing to sea of Japan side where N deposition, as well as transboundary O3 levels, was high [94]. N deposition [83] and O3 bring an imbalance in S/R ratio [29] and physiological homeostasis. The symbiotic ambrosia fungus *Raffaelea quercivora* is the causal agent of oak dieback and is vectored by oak beetle (*Platypus quercivorus* [Murayama]). This is the first example of an ambrosia beetle fungus that kills vigorous trees. Future global warming will possibly accelerate the overlapping of the distributions of *P. quercivorus* and most *Quercus* sp. with the result that oak dieback will become more serious in Japan.

The Meiji Jingu (shrine) is an important green resource in a mega-city, Tokyo, Japan, and planned natural regeneration smoothly continued until around 1990, however, invasive palm seedlings started to prevent the regeneration of various

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

#### **Figure 8.**

*A: Inside view of tthe Meiji Shrine forest (courtesy of Ms. Harumi Ejiri-Noda). a: Insect trap with "continuous rotho" attached to the oak stem at holes for collecting adult beetle, b: Vinyl film inhibiting shatter-proof wrapped at damaged tree stem,c: Bark beetle (Platypus quercivorus), B: Dutch elm disease in the campus of Hokkaido University in Sapporo. Allow indicates declining individual elm tree infected with beetle. a: Trace under bark attacked by the beetle, b: Elm bark beetle (Scolytu esurient).*

broadleaved trees. Progress of "Heat Island phenomenon" may have enabled the overwintering capacity of invasive palm [95]. Since its seed dispersal depends on birds, seedlings of palm are found in many urban forests. Moreover, palm leaves are O3 tolerant, while O3 suppresses root growth [77], but the moist forest floor seems to allow them to continue to grow. However, mature oaks are declining with intensive infection of wilt disease (**Figure 8A**). How can we manage the serious damages? The Health and vigor of oaks seem to be declining even though most oaks have relative O3 tolerance (cf. **Table 1**) but their root growth is usually suppressed. We hardly regulate the progress of oak wilt disease.

#### *4.2.2 Die-back of elm tree*

A sudden increased mortality of *Ulmus davidiana* var. *japonica* (Japanese elm) trees occurred during 2014–2016 in Sapporo, northern Japan [96]. The estimated damaged tree age ranged between 36 and 186 years. Elm bark beetle (*Scolytus esuriens*) was regarded as a key insect for the spread of the disease. Fungi isolated from elm bark beetle and the wood of the galleries were identified as *Ophiostoma ulmi* and *Ophiostoma novo-ulmi*. Most declining trees with wilting branches were present around these beetle-attacked trees (**Figure 8B**). Therefore, Dutch elm disease caused this occurrence of Japanese elm dieback. This declining may be related to declining of tree vigor and health under degradation of the atmospheric environment with increasing nitrogen deposition and O3 [80].

Higher leaf N content and lower plant defense (low condensed tannin) content in N loading and lower condensed tannin content in elevated O3 were observed with O3- FACE, suggesting that both N loading and elevated O3 decreased the leaf defense and that N loading further enhanced the leaf quality as food resource of insect herbivores [19, 20, 89, 90]. Visible foliar injury caused by N loading might directly induce the reduction of the number of elm sawfly individuals.

Although elevated O3 suppressed the plant defense capacity in leaves as found in white birch [41, 42, 97] and poplar [43], a significantly lower number of elm leaf beetle was observed in elevated O3. Why did the number of leaf beetle in leaves in elevated O3 decrease with lowered chemical defense?

## *4.2.3 Insect-plant interaction: O3 as an environmental disturber*

Plant–insect interactions are basic components of biodiversity conservation. Ground-level O3 usually suppresses not only plant activities and disrupts interaction webs among plants-insect [41]. O3 mixing ratios in suburbs are usually higher than in the center of cities and may reduce photosynthetic productivity, as suggested by Gregg et al. [65] for poplar and Moser-Reischl et al. [64] for fir. As a result, carbonbased defense capacities of plants may be suppressed by elevated O3 more in the suburbs than in the urban region. Contrary to this expectation, grazing damage by leaf beetles (found in birch, alder, elm, and poplar: [43]) has been severe in some urban centers in comparison with the suburbs. To explain differences in grazing damages between urban areas and suburbs, the disruption of olfactory communication signals by elevated O3 via changes in plant-regulated biogenic volatile organic compounds (BVOC) and long-chain fatty acids with double bonds are considered (e.g., [41, 43]). Ozone-disrupted BVOCs of plants should be considered to explain insect herbivory activities in urban and suburban systems.

## **5. Construction and maintenance of urban greening**

The green space in the city not only serves as a cool island but also provides relaxation. However, even with management, there is always a risk of decline due to air pollution, high temperatures, high vapor deficit, nutrient imbalance, etc. due to narrow planting holes and restriction of the root system. "Meiji Jingu" forest was designed to be maintained by natural regeneration created in the center of Tokyo metropolis [98], but the invasion of palms by the heat island and the decline of oak wilt disease are imminent (**Figure 8A**, **B**). Even if the roadside trees and rooftop greening can be managed sufficiently, the viewpoint of nutrient cycling is important for park trees [99]. Additionally, we should introduce a moderating method of application of ethylene-di-urea (EDU). This is considered a chemical that offers protection to the treated plants against O3 [100–102].

#### **5.1 Nutrient cycling**

Effects of tree species on mineral soil, litter, and root properties are found to be inconsistent, and understanding general cross-site patterns and the possible mechanism is important for enhancing the forest ecological service through proper tree species selection. Leaf litterfall nutrient concentrations and their ratios are a common indicator of site nutrient status and a critical component of many ecosystems [87]. Concentrations of N and P in the leaf litter are related to foliar concentrations, but they are reduced by nutrient resorption during senescence as affected by O3 [61, 84]. However, few studies have assessed how the timing of litter collection affects estimates of nutrient concentration [61, 84]. The emphasis on sampling senesced leaf tissue at a single point in time leads to biased estimates of nutrient concentrations, stoichiometry, and litterfall and resorption fluxes, especially for P than for N in birch, beech and maple at the Hubbard Brook Forest. In northeast China, mineral conditions of four tree species; larch (*Larix gmelinii*), pine (*Pinus sylvestris* var. *mongolica*), poplar (*Populus* spp.), and elm (*Ulmus pumila*) were compared in several minerals, the component of SOC and soil N concentrations. Elm could capture more mineral SOC

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

and nutrients, poplar induced mineral soil P depletion, and pine litter was of more recalcitrance for decomposition [103].

The effects of elevated O3 in different soil conditions (brown forest, volcanic ash, and serpentine soil) on foliar elements stoichiometry were investigated in *Betula platyphylla* var. *japonica* (white birch), (*Quercus mongolica* var. *crispula* (oak) and Siebold's beech (*Fagus crenata*) with a O3-FACE [61]. Soil nutrients have distinct impacts on retranslocation rate of K, Fe, and P. A negative correlation between foliar N and the metal elements was found in white birch. From the differences of foliar contents as well as their retranslocation rate, Siebold's beech with determinate shoot growth pattern was rather more sensitive to O3 stress on foliar contents, meanwhile oak was possibly susceptible to O3 on dynamics of immobile elements. Mn and K can become indices in assessing the O3 and soil effects.

Decomposition is directly related to nutrient cycling [104]. Litter decay dynamics of paper birch (*Betula papyrifera*) were assessed at the Aspen-FACE in the northcentral USA. Leaf litter was decomposed for 12 months under factorial combinations of ambient (360 ppm) vs. elevated CO2 (560 ppm), crossed with 36 vs. 55 nLO3 L<sup>1</sup> . *In situ*, litterbags methods revealed that CO2 enrichment regardless of O3 produced poorer quality litter (high C/N, lignin/N, and condensed tannins) than did ambient CO2 (low C/N, lignin/N, and condensed tannins). Substrate quality differences were reflected in the mass loss rates of litter (k-values), which were high for litter generated under ambient CO2 (0.89 yr.<sup>1</sup> ) and low for litter generated under elevated CO2 (0.67 yr.<sup>1</sup> ), which suggests regulating the storage of fixed C and the release of CO2 from northern forest ecosystems. Additionally, revegetation should be needed in NE China where we have a limited amount of rainfall (<500 mm yr.<sup>1</sup> ) after the thinning or harvest of the "restoring agricultural land forest" [7]. We should carefully select larch species, such as *Larix gmelnii* under salt stress and EO3 [105].

#### **5.2 EDU application**

Increasing evidence on the antiozonate efficacy of EDU against the phytotoxic action of O3 is becoming more readily available [100, 101]. EDU is a very promising antiozonant with its antiozonate action being observed when applied to roots in concentrations of 275.7 to 374.3 mg L<sup>1</sup> . The effect of ambient O3 on visible foliar injury, growth, and biomass in field-grown poplar cuttings of an Oxford clone sensitive to O3 irrigated with EDU or water for 3 years. Protective effects of EDU on O3 visible injury were found but no increase in stem height and diameter was found [102]. EDU was more effective in some cultivars compared with others although this remains inexplicable [33]. Additionally, the biochemical mechanism of its antiozonate activity is still unclear [78].

## **6. Concluding remarks and future perspective**

Urban vegetation or green infrastructure, as a cost-effective and nature-based approach, aids in meeting clean air standards, which should be taken into account by policy-makers. Nowadays, complex atmospheric pollution seriously threatens the vitality of suburb trees as well as urban trees (e.g., [23]). Continuous attention should be paid to the long-term changes of O3 and related impacts on urban greening. On the basis of "matching site with trees," more attention should be paid to the relationship between plants and urban planting small environment [33]. Highly ranked tree

species that can inclusively resist O3 and other pollutants which will be the priority target of urban greening.

Recent high air pollution in many cities indicates the urgent need for policy action and for urban development based on local air quality management; the prospects of improving urban air quality through proper design and protection of vegetation within local planning strategies [106], such as selecting of non-emission species of BVOC as a precursor of O3. In addition, we should make more effort to reduce the emission of CO, NOx, VOC, etc. as O3 precursors [22, 64]. Elevated ground-level O3 can adversely affect plants and inhibit their growth and productivity, threatening ecological health via pollination [38, 40, 41] food security, etc. Therefore, it is important to develop ways to protect plants against O3-induced negative effects [13, 107]. Of course, we should choose tree species of non-emitter of BVOC as a precursor of O3 as shown by screening experiments of tree species [18]. It would be effective to create a space where the wind can easily pass through the city plan to discharge O3 [13, 46]. In addition, we expect that new studies should be green chemistry for the continuous support of green infrastructure for city residents.
