Seasonal variations in the amount of black carbon particles deposited on the leaf surfaces of nine Japanese urban greening tree species and their related factors.

As black carbon (BC) particles can be deposited on the leaf surfaces, urban greening is considered to be effective in purifying urban air. However, little information on the seasonal variations in the amount of BC particles deposited on the leaf surfaces (BC amount on the leaves) is available in Japanese urban greening tree species. Therefore, we investigated seasonal variations in the BC amount on the leaves of evergreen (Quercus glauca, Quercus myrsinaefolia, Osmanthus fragrans and Ilex rotunda) and deciduous (Zelkova serrata, Styrax japonica, Magnolia kobus, Cornus kousa and Cornus florida) broad-leaved tree species. The BC amount on the leaves tended to increase from April for different periods, and then reached a saturated state in the tree species, excluding M. kobus. In the 4 evergreen broad-leaved trees, the seasonal variation was positively correlated with the atmospheric concentration of BC particle. In the 5 deciduous broad-leaved trees, the seasonal variation was negatively and positively correlated with the water-repellence (water droplet contact angle) and the amount of epicuticular wax on the leaf surface, respectively. Therefore, the BC amounts on the leaves of evergreen and deciduous broad-leaved urban tree species are considered to be mainly regulated by environmental factors and leaf surface characteristics, respectively.

This is the first paper that reports the seasonal variations in the amount of BC particles deposited on the leaves of Japanese urban greening tree species and their related factors such as environmental conditions and leaf surface characteristics. This study will provide the basic and novel information for the phytoremediation of urban air pollution induced by BC particles in Asia.

Relationship between the amount of black carbon particles deposited on the leaf surface and leaf surface traits in nine urban greening tree species.

To select urban greening tree species suitable for the purification of the atmosphere polluted by black carbon (BC) particles, it is necessary to clarify the determinants of the amount of BC particles deposited on the tree leaves. In the present study, we investigated the relationship between the amount of BC particles that were deposited from the atmosphere and firmly adhered to the leaf epicuticular wax, and leaf surface traits in seedlings of nine tree species grown for two years under natural conditions (Fuchu, Tokyo, Japan). There was a significant interspecific difference in the maximum amount of BC particles deposited on the leaf surface, and the order was as follows: Ilex rotunda > Cornus florida > Osmanthus fragrans > Cornus kousa > Quercus glauca ≒ Quercus myrsinifolia > Magnolia kobus ≒ Zelkova serrata ≒ Styrax japonicus. In the nine tree species, significant highly positive correlations were observed between the amount of BC particles deposited on the leaf surface, and the hydrophobicity of leaf epicuticular wax determined by its chemical composition. Therefore, we concluded that the hydrophobicity of leaf epicuticular wax is an important determinant of the amount of BC particles deposited on the leaf surface of urban greening tree species.

This is the first paper that shows that the hydrophobicity of leaf epicuticular wax is an important determinant of the amount of BC particles deposited on the leaf surface of urban greening tree species. This study will provide the basic and novel information for the selection of urban greening tree species suitable for the purification of the air polluted by BC particles.

Strategic roadmap to assess forest vulnerability under air pollution and climate change.

Although it is an integral part of global change, most of the research addressing the effects of climate change on forests have overlooked the role of environmental pollution. Similarly, most studies investigating the effects of air pollutants on forests have generally neglected the impacts of climate change. We review the current knowledge on combined air pollution and climate change effects on global forest ecosystems and identify several key research priorities as a roadmap for the future. Specifically, we recommend (1) the establishment of much denser array of monitoring sites, particularly in the South Hemisphere; (2) further integration of ground and satellite monitoring; (3) generation of flux-based standards and critical levels taking into account the sensitivity of dominant forest tree species; (4) long-term monitoring of N, S, P cycles and base cations deposition together at global scale; (5) intensification of experimental studies, addressing the combined effects of different abiotic factors on forests by assuring a better representation of taxonomic and functional diversity across the ~73,000 tree species on Earth; (6) more experimental focus on phenomics and genomics; (7) improved knowledge on key processes regulating the dynamics of radionuclides in forest systems; and (8) development of models integrating air pollution and climate change data from long-term monitoring programs.

Assessing critical loads and exceedances for acidification and eutrophication in the forests of East and Southeast Asia: A comparison with EANET monitoring data.

Spatial variations in sulfur (S) and nitrogen (N) deposition have changed in East and Southeast Asia in recent decades. Nevertheless, in this region, including the tropics, regional-scale assessments of the long-term risk of acidification and eutrophication (N saturation) for terrestrial ecosystems using a critical load approach have not been updated since 2001. To evaluate future risks, maps of critical loads and exceedances were updated using recently acquired spatial datasets of soil properties, soil minerals, climate, tree plantations, and the annual S and N depositions estimated using the Community Multiscale Air Quality (CMAQ) model. The resulting maps were verified using data on long-term trends in soil pH and nitrate concentration in surface water acquired by the Acid Deposition Monitoring Network in East Asia (EANET). It was found that N deposition exceeded the critical load for eutrophication not only in East Asia but also in some parts of the tropical monsoon and humid regions in Southeast Asia, whereas S deposition partly exceeded the critical load for soil acidification in China and small parts of the tropical monsoon region. The high-risk areas for eutrophication coincided well with the EANET sites, where the increase in nitrate concentration in the surface water was significant over the last 20 years. Hence, the estimated map of the critical load exceedance for eutrophication is more plausible for assessing the risk in East and Southeast Asia than that for acidification, although the critical load exceedance for acidification would be sufficiently significant as an updated risk map based on the latest input values. This update also suggests that increased N deposition around megacities, water discharge, and tree plantations may play an important role in the spatial variability of eutrophication risks in the tropics of Southeast Asia.

Development of science and policy related to acid deposition in East Asia over 30 years.

Scientific and public interest in acid deposition and its ecological impacts have increased throughout 1990s in East Asia (Northeast and Southeast Asia). After being established in 2001, the Acid Deposition Monitoring Network in East Asia (EANET) celebrates the 20th anniversary in 2021, and is now being expanded in scope reflecting the shifting social concern from acid deposition to broader air quality and climate change in recent years. This paper reviews the past 30 years of development of scientific research and policy related to acid deposition in East Asia. Since the onset of the twenty-first century, East Asia has had the highest SO2 and NOx emissions in the world by continents, with substantial economic developmental inequality among countries. An overview of studies on sulfur and nitrogen deposition, the acidification of inland water and forest soil, and forest decline reveal that although limited acidification of inland water and forest soils have been documented, no decline in the populations of fish and other aquatic biota has been reported in East Asia. After a review of policy-oriented modeling studies on source receptor relationships and the critical loads of sulfur and nitrogen in East Asia, the history of EANET and its success and challenges are discussed. Finally, the importance of epistemic communities as the interface between science and policy in the region is discussed. Regional governance and cooperation are essential for reducing the emission of greenhouse gases, especially short-lived climate pollutants and atmospheric pollutants to realize the co-benefits of global climate change mitigation and improved air quality.

Air pollution monitoring and tree and forest decline in East Asia: A review.

Air pollution and atmospheric deposition have adverse effects on tree and forest health. We reviewed studies on tree and forest decline in Northeast and Southeast Asia, Siberia, and the Russian Far East (hereafter referred to as East Asia). This included studies published in domestic journals and languages. We identified information about the locations, causes, periods, and tree species exhibiting decline. Past air pollution was also reviewed. Most East Asian countries show declining trends in SO2 concentration in recent years, although Mongolia and Russia show increasing trends. Ozone (O3) concentrations are stable or gradually increasing in the East Asia region, with high maxima. Wet nitrogen (N) deposition was high in China and tropical countries, but low in Russia. The decline of trees and forests primarily occurred in the mid-latitudes of Japan, Korea, China, and Russia. Long-term large N deposition resulted in the N saturation phenomenon in Japan and China, but no clear forest health response was observed. Thereafter, forest decline symptoms, suspected to be caused by O3, were observed in Japan and China. In East Russia, tree decline occurred around industrial centers in Siberia. Haze events have been increasing in tropical and boreal forests, and particulate matter inhibits photosynthesis. In recent years, chronically high O3 concentrations, in conjunction with climate change, are likely have adverse effects on tree physiology. The effects of air pollution and related factors on tree decline are summarized. Recently, the effects of air pollution on tree decline have not been apparent under the changing climate, however, monitoring air pollution is indispensable for identifying the cause of tree decline. Further economic growth is projected in Southeast Asia and therefore, the monitoring network should be expanded to tropical and boreal forest zones. Countermeasures such as restoring urban trees and rural forests are important for ensuring future ecosystem services.

Seasonal variation in the atmospheric deposition of inorganic constituents and canopy interactions in a Japanese cedar forest.

The seasonal changes in throughfall (TF) and stemflow (SF) chemistry and the canopy interactions of K+ and N compounds were studied in a Japanese cedar forest near the Sea of Japan. The fluxes of most ions, including non-sea-salt SO4(2-), from TF, SF, and rainfall showed distinct seasonal trends, increasing from autumn to winter, owing to the seasonal west wind, while the fluxes of NH4+ and K+ ions from TF+SF might have a large effect of canopy interactions. The contact angle (CA) of water droplets on leaves decreased with leaf aging, suggesting that surface wettability increases with leaf age. The K+ concentration in TF was negatively correlated with the CA of 1-year-old leaves, while the NH4+ concentration was positively correlated with the CA. The net fluxes of NH4+ and NO3(-) from TF were positively correlated with the CA. The increase in wettability may accelerate leaching of K+ or uptake of NH4+.