Air Pollution Impacts on Plants in East Asia

Gaseous pollutants in East Asia are explained in this chapter. Sulfur dioxide (SO₂) emissions in this region of the world are now decreasing, although these emissions in China are still the highest in the world. SO₂ emission in South Korea decreased very quickly; in Japan, it took about 20 years to reduce the atmospheric SO₂ concentration from 50 ppb to less than 5 ppb, whereas it took only 10 years in Korea to reduce the SO₂ concentration by the same amount, owing to a switch in fuel sources from coal to natural gas.

The emission of nitrogen oxides (NOx = NO + NO₂) is still increasing in China, and this emission causes an increase in tropospheric ozone. The transboundary transport of ozone and its precursors is affecting not only the countries surrounding China, but also North American countries.

Trends in ozone concentrations over the East China Sea were analyzed, based on aerial observations for up to 20 years. A clear increase in ozone was found, particularly in the boundary layer, lower than 1,500 m above sea level.

Acid deposition is a regional issue in Asia. Emission levels of acidic substances, such as SO₂ and nitrogen oxides (NO_X), are still very high, although SO₂ emissions in China have started declining. The Acid Deposition Monitoring Network in East Asia (EANET), the regional network covering Northeast and Southeast Asia, has been monitoring acid deposition and its effects on forests and inland waters. Wet deposition levels of sulfur and nitrogen in EANET countries are significantly higher than those in Europe and the United States, although total deposition, as well as dry deposition, has not been sufficiently evaluated in the region, particularly in forested areas. Soil acidification is a potential risk for plant growth in Asia. Soil acidification has been observed in areas receiving high acid deposition in China and Japan. Observational data on acid deposition and soil chemical properties should be accumulated for forest areas to ascertain the risk for plants in the region.

Field surveys and experimental studies of the effects of ozone on Japanese agricultural crops have been conducted since the early 1970s. We review these research studies comprehensively, in chronological order. In the 1970s, most studies of the effects of ozone on agricultural crops, and the evaluation of ozone-induced foliar injuries, were field surveys. Since the 1980s, there has also been considerable research using various facilities, such as open-top chambers and controlled-environment growth cabinets; these studies are concerned with the response to and mechanism of ozone influence on various crops. In addition, since the last half of the 1990s, informative studies have been carried out to assess and map critical levels of ozone in relation to Japanese agricultural crops.

The effects of ozone (O₃) on tree species in Japan have been studied since the 1970s. Based on the results from O₃ fumigation studies, current ambient levels of O₃ have negative impacts on the growth and physiological functions of Japanese forest tree species, although there is a big variation of O₃ sensitivity between species. Stomatal O₃ uptake is one of the key factors that can explain the differences in O₃ sensitivity between species, and modeling of this factor has been intensively studied during the past decade. Although O₃ generally induces stomatal closure, less efficient stomatal control, so-called stomatal sluggishness, is also induced by chronic exposure to O₃. These opposite phenomena result in complex responses of stomata to O₃. Detailed gas exchange analysis has revealed that O₃-induced reductions in the photosynthetic rate of Japanese forest tree species were mainly due to a biochemical limitation in chloroplasts, but not due to stomatal closure. Risk assessments of the O₃ impact on Japanese forest tree species, based on the results of experimental studies, national monitoring data of air pollutant concentrations, and vegetation surveys, indicate that the areas with high O₃-induced reduction in growth do not necessarily correspond to the areas with relatively high O₃ exposure. Free-air O₃ fumigation systems in Japan were developed in 2011. Studies with this novel technology have clarified differences in leaf O₃ sensitivities between canopy positions, and have estimated the effects of O₃ on whole-canopy carbon budgets. As future perspectives, not only we need clarification of the physiological mechanisms of O₃ impact, but we also need clarification of the effects of interactions between trees and other biotic factors such as diseases, herbivores, and symbiotic microbes.

Plant responses to ozone (O₃) are highly dependent on other environmental factors. Elevated atmospheric CO₂ concentrations may reduce stomatal O₃ uptake in some tree species in certain environmental conditions. In addition, changes in the carbon availability and allocation pattern within a plant body under elevated CO₂ may confer compensative capacity against O₃ stress. The influence of the soil nitrogen load on growth and photosynthetic response to O₃ is highly species-specific. The soil nitrogen load enhanced the negative impact of O₃ on Fagus crenata, whereas lower O₃-induced growth reduction of Larix kaempferi was observed under a higher nitrogen load. It is predicted that soil-water stress would induce lower stomatal O₃ uptake due to stomatal closure, and therefore the negative impacts of O₃ on trees would decrease under water-stress conditions. In fact, some antagonistic effects of O₃ and soil water stress were observed in F. crenata, although these effects may depend on the severity of the water stress. The number of studies on the combined effects of O₃ and other environmental factors on Japanese forest tree species is limited and further research is crucial to accumulate the required datasets.

Air pollution is a regional issue in Asia. To assess air pollution levels and their actual impacts on plants, environmental monitoring using indicator plants has been conducted since the 1970s. In particular, in Japan, during the 1970s/1980s when air pollution levels were relatively high, various kinds of indicator plants were used for monitoring air pollution. Some vascular plants, such as morning glory and petunia, were used as indicator plants for monitoring the pollution of photochemical oxidants (Ox), including O₃ and peroxyacetyl nitrate (PAN). In the middle of the 1970s, nationwide surveys using morning glory revealed Ox pollution in 37 of the 47 prefectures in Japan. Epiphytes, such as bryophytes and lichens, were used as bio-indicators for SO₂ pollution. The “epiphyte desert” observed in urban and industrial areas showed severe SO₂ pollution in large cities in Japan during the 1970s. An improvement in the distribution of epiphytes resulted in reduced SO₂ pollution in Japan during the 1980s. Similar environmental monitoring using bryophytes and lichens as bio-indicators was conducted in other Asian countries. Common tree species, such as cedar, can also be used as indicator plants for environmental monitoring. Further application of such monitoring in Asian countries should be promoted.

Ground-level ozone (O₃) levels are expected to increase over the twenty-first century, particularly in the region of East Asia. We performed an O₃ flux-based risk assessment of C sequestering capacity in an old cool temperate deciduous forest, consisting of O₃-sensitive Japanese beech (Fagus crenata), and in a warm temperate deciduous and evergreen forest dominated by O₃-tolerant Konara oak (Quercus serrata), based on long-term CO₂ flux observations. Light-saturated gross primary production, as a measure of C sequestering capacity, declined earlier in the late-growth season with increasing cumulative O₃ uptake, suggesting an earlier autumn senescence in the O₃-sensitive beech forest, but not in the O₃-tolerant oak forest.

Decline of the mountain birch (Betula ermanii) has been observed at the somma of Lake Mashu in northern Japan. To clarify its causes, ozone (O₃) and fog chemistry were evaluated. O₃ concentration was lower in summer, but higher in spring, with a monthly mean of 50–60 ppbv. The results of on-site open-top-chamber (OTC ) experiments using birch seedlings suggested the possibility that O₃ decreased the leaf photosynthetic capacity and shifted biomass allocation to the roots. The results of fog measurements showed that the fog acidity was not serious; however, nitrogen input via fog deposition was estimated to be 30 meq m^-2 in the plant-growing season, and this could be the main nitrogen source for trees in this area. These results indicated that anthropogenic air pollutants have influenced plant growth in this area. However, there is still great uncertainty about O₃ effects on the mountain birch, such as its effects causing stomatal sluggishness, water stresses, and these effects combined with nitrogen input. Further evaluation of these factors is necessary to assess the tree decline.

The Tanzawa Mountains are located in the southwestern part of the Kanto District of Japan, and many hikers and climbers visit the area from several gateways, leading to the overuse of trails and the retreat of vegetation. Siebold’s beech (Fagus crenata) forests are distributed in the high-elevation areas. Beech growth on the southern slopes, along the ridgeline, and around peaks has declined significantly. Recent ozone monitoring data suggest that high ozone concentrations may be a possible chronic cause of the loss of beech vitality. 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 destroy ground vegetation and the related community balance. The factors affecting beech decline in the Tanzawa Mountains are complicated, and further scientific research activities in various fields are required to understand the phenomena and to recover the beech forest vegetation.

Current ambient O₃ concentrations in China are high, as shown by observations of typical O₃ symptoms in some plant species and crop yield losses, as detected by the use of chemical protectants against O₃. Experiments with artificially elevated O₃ concentrations have shown the effects of O₃ on growth processes, grain yield, grain quality, CH₄ emissions, and soil microbiology. The experiments have facilitated estimations of the yield losses in wheat and rice caused by current and future O₃ concentrations at the national scale. Further studies are warranted on the interactions between O₃ and other environmental changes, such as increasing CO₂ concentrations, increased nitrogen deposition, aerosol loading, and climatic changes. Future needs for research include improvement of O₃ impact models and the development of an O₃ monitoring network to cover the vast areas of crop production in China. The establishment of an air quality standard for protecting crops from O₃ damage is of critical importance for food security in China.

This chapter reviews the effects of elevated ozone on tree species in China, based on the results of studies in the past two decades. The high ozone concentration in summer in most parts of China has induced typical ozone symptoms in urban and mountain forest tree species. In experiments using open-top chambers, elevated ozone affected the growth, gas-exchange rate, foliar microscopy, antioxidant systems, and biogenic volatile organic compound (BVOC) emissions in trees. The effects of ozone on biomass accumulation depended on the ozone concentration, tree species sensitivity, and exposure duration. The ozone uptake of individual tree species was also investigated by the sap flow technique. Further studies were conducted on the interactions between O₃ and other environmental change factors, such as increasing CO₂ concentrations, increased nitrogen deposition, and drought. Future needs for research include the development of an O₃ flux model for the most widely used tree species and the assessment of ozone removal by urban forests on a regional and a national scale.

Most results with simulated acid-rain exposure experiments have suggested that rain acidity below pH 3.0 could induce direct or indirect deleterious effects on crops and other herbaceous plants, with visible injury development and reductions in growth and yield; however, plants show different sensitivities to acidity below pH 3.0 in terms of visible acute injury or reductions in growth and yield. Current rain acidity, which is regarded as over pH 4.0, may not induce direct adverse effects on field-grown plants. However, acidic precipitation may have a potential threat to induce increasing availability and toxicity of rare earth and heavy metals in plants.

We summarized the results obtained from experimental studies on the growth and physiological responses of Japanese and Chinese tree species to simulated acid rain, mist, or fog. Based on the studies conducted in Japan and China, exposure to simulated acid rain, mist, or fog at pH 4.0 or above for one to three growing seasons did not induce any adverse effects on the growth and physiological functions of Japanese and Chinese tree species. Current acidic rain monitored in Japan and China, where the lowest acidity of ambient precipitation was around pH 4.0, was not found to induce any direct significant effects in the tree species examined.

We summarized the results obtained from experimental studies on the combined effects of concurrent or sequential exposure to simulated acid rain, mist, or fog, and ozone, on the growth and physiology of Japanese and Chinese tree species. Based on the limited number of studies conducted in Japan and China, we found that the combined effects of acid precipitation and ozone on Japanese and Chinese tree species were additive or greater than additive. Moreover, there is a possibility that acid precipitation with a relatively low acidity increased the extent of ozone-induced inhibition of assimilate translocation from the shoot to roots, and this ozone effect on trees exacerbated by acid precipitation may be associated with nitrate input from precipitation.

Acid deposition is a serious environmental problem in Asia and may adversely affect Asian tree species due to soil acidification, the leaching of nutrients essential for normal plant growth from rhizospheric soil, and enhancement of the solubility of toxic metals such as aluminum (Al) and manganese (Mn) in the soil solution. The results of limited experimental studies indicate that soil acidification increases the concentrations of Al and Mn in the soil solution and reduces the growth, physiological functions such as net photosynthetic rate, and concentrations of nutrients such as Ca and Mg in several Asian forest tree species such as Cryptomeria japonica, Pinus densiflora, Betula platyphylla, Fagus crenata, and Pinus massoniana. A relatively low pH of culture solution below 3.5 reduces fresh and dry-weight growth and increases the concentrations of Ca and Mg in the shoots of Asian tree species such as C. japonica. Excessive Al or Mn in soil solution or water culture medium reduces fresh and dry-weight growth, shoot water content, and concentrations of Ca and Mg in the shoots and roots of Asian tree species such as C. japonica and P. densiflora. The extent of the negative effects of Al on growth, physiological functions, and water and nutrient status is greater than that of Mn when the concentrations of both elements in water culture medium are the same. Because the atmospheric deposition of acid substances is increasing in Asian countries, further studies are needed to elucidate the effects of soil acidification on many Asian forest tree species.

The effects of increasing nitrogen (N) load on Asian forest trees have been studied mainly in Japan and China during the past decade. In this chapter, we summarize the expected mechanisms of the harmful effects of excessive N load on the eco-physiological function of trees, and introduce experimental studies on the growth responses and foliar nutrient status of young Asian tree seedlings to increasing N loads. By comparing the growth responses to various N loads in 12 tree species in Japan and China, we confirmed that: (1) the threshold of the N load that induced growth reduction was quite different among the species; (2) the threshold value ranged between 50 and 100 kg N ha⁻¹ year⁻¹ in relatively sensitive (low-tolerance) species. Furthermore, a significant relationship between foliar nutrient balance and growth indicated that (3) the threshold N/P ratio for growth reduction was slightly higher than that in European tree species, and (4) an Mn/Mg ratio of over 0.8 was observed, together with growth reduction, in the sensitive tree species.

Because aerosol particles in the atmosphere scatter and absorb incoming solar radiation, they can indirectly affect plant productivity by reducing incoming solar radiation. Conversely, aerosol particles affect plants after deposition from the atmosphere. In this chapter, we introduce both the possible indirect effects of aerosol particles on plant productivity, via reducing incoming solar radiation, and the direct effects of particulate matter deposited on the leaf surface. A wide range of experimental studies has shown the physical effects of particulate matter deposited on the leaf surface – leaf shading, increased leaf temperature, stomatal plugging, and interference with stomatal closure – that can occur in the field in Asia. Leaf shading by particulate matter reduces the net photosynthetic rate under low light conditions. The increase in leaf temperature caused by particulate matter increases or decreases the net photosynthetic rate under air temperatures below or above the optimum for photosynthesis, respectively. The light extinction coefficient of particulate matter is an important factor in the increase in leaf temperature. Stomatal plugging decreases gas diffusivity during the light period, and this can reduce the net photosynthetic rate. Interference with stomatal closure induces unexpected water loss at night, which can reduce drought tolerance. Although the chemical effects of dust depend on its chemical properties (e.g., pH), the effect of dust-induced injury cannot be explained by the effect of a single major dust component only, suggesting that dust components have an interactive effect. Currently, however, little information is available on the combined effects of the components of particulate matter.

Asian countries face the problem of transboundary air pollution, including that of particulate matter with a diameter smaller than 2.5 μm (PM_2.5). The deposition velocities of PM_2.5 particles, such as black carbon (BC) and ammonium sulfate (AMS), onto forests are substantially high. In this chapter, we summarize the dynamics of BC and AMS deposited on leaf surfaces and we introduce experimental studies of their long-term effects on four representative Japanese forest tree species. Experimental studies of the acute effects of BC on plants have revealed its physical effects – stomatal plugging, leaf shading, and increased leaf temperature. These effects were not observed in an experimental study on the long-term effects of BC on four Japanese forest tree species, possibly because of the low amount of BC deposited on the leaf surfaces. The amounts of BC deposited on leaf surfaces differed considerably between experimental and field observation studies. Long-term AMS exposure did not significantly affect the growth of the four representative Japanese forest tree species, but significantly increased and decreased the net photosynthetic rate in current-year and previous-year needles, respectively, of Cryptomeria japonica seedlings. Hygroscopic particles such as AMS deposited on the leaf surface can establish a continuous liquid water connection between the leaf interior and surface, which enables the bidirectional transport of water and solutes between them. AMS exposure significantly increased the concentrations of NH₄ ⁺, free amino acids, and total soluble protein in the current-year needles of C. japonica, suggesting that AMS deposited on the needle surfaces deliquesced, was absorbed into the needles, and was metabolized.

Recent knowledge of the dry deposition of aerosols onto forests, focusing especially on field measurements in East Asia, is summarized with respect to aerosol impact on the vegetation in the region. Direct measurements of aerosol deposition in Japan by means of the gradient method and the relaxed eddy accumulation method were introduced together with their principles. They indicate that the levels of deposition velocity of PM_2.5 sulfate were in agreement with experimental results in North America and Europe and were higher than those calculated by theoretical models. The deposition velocity was probably influenced by humidity due to the growth in size of hygroscopic aerosols. The calculations taking into account the effect of growth of hygroscopic aerosols were in reasonable agreement with the measurements. A complex terrain contributed to increasing the range of measured deposition velocities. However, the measurements agreed with calculated deposition velocities on average for about 2 weeks. Measurements of vertical profiles of PM_2.5 components in a forest indicate that the dry deposition of NH₄NO₃ particles was more efficient than that of (NH₄)₂SO₄ particles, probably due to the volatilization processes of NH₄NO₃. Finally, a case study on the dry deposition of elemental carbon estimated by the inference method in a tropical forest in Thailand was introduced.