## 1 Introduction  
Forests offer a wide variety of ecosystem services, such as provisioning services, support services, cultural services, and regulation services [1; 2; 3]. Above all, forest plants absorb and store carbon dioxide through photosynthesis, thereby mitigating the greenhouse gas effects on the earth's climate and affecting temperature and humidity on local and global scales [1; 4]. However, since the Industrial Revolution, forests have been affected by various human activities. Acid rain, for example, has been found to have a significant negative effect on forests in the past several decades [5; 6]. Recently, ozone, nitrogen, and sulfur have been reported to become increasing threats to forest ecosystems [7; 8; 9; 10]. Sulfur oxide and ozone are typical air pollutants that are affecting plant growth [8; 9; 10]. However, in the case of sulfur oxide, its concentration is gradually decreasing due to various environmental regulations and it is assumed that the concentration will continue to decrease into the future [7]. In particular, tropospheric ozone is a secondary pollutant, which is produced by photochemical reactions involving precursors, such as nitrogen oxides (NO\(x\)), volatile organic compounds (VOCs), and carbon monoxide (CO), and modulated by meteorological conditions [11; 12; 13; 14; 15]. The concentration of ozone will increase in the future [16]; ozone has a higher toxicity than other substances, which directly affects plants and causes considerable damage to them [17; 18]. When ozone enters the plant body, damage first occurs in the photosynthetic organ by which the plant takes in the ozone, and it is here where the most serious damage occurs [19; 20]. However, studies on the effects of ozone on forests have been limited to indigenous forests or a select group of plant species [17; 21; 22]. Research exists on the damage that is caused by ozone in a variety of conditions on some specific species, but studies on a more macroscopic scale are rare. Also, most literature on the relationship between net primary productivity and ozone looks at either the past or the present, and projections concerning the effects of ozone in the future are rarely considered. It is necessary to pay more attention to the future impacts of ozone concentration on forests as long-term forest sustainability becomes increasingly important. Increasing global greenhouse gas emissions, furthermore, are likely to have a greater impact on the production of air pollutants, such as ozone going forward [23; 24; 25; 26; 27; 28]. Climate change, such as changes in wind patterns and the amount and intensity of precipitation, and an increase in temperature all have a direct impact on the frequency and intensity of air pollution and can increase the production of air pollutants by forcing the use of heaters or air conditioners in affected areas [23; 24]. Urban heat-island effects are likely to produce secondary pollutants, such as ozone and increase natural air pollution sources due to soil erosion or fires [25; 26; 27; 28]. Climate change is also likely to produce air pollutants because oxidation reactions occur more easily at elevated temperatures [25]. In the regions of East Asia, especially, ozone concentrations have increased dramatically due to China's economic growth when compared to other regions in the world [29; 30; 31]. Korea, which is located to the East of China, is the one of the countries which is most affected by China's emissions [32], and the ozone concentration, which has been increasing steadily since the 1980s, has more than doubled over the last 30 years in Korea [33]. Also, if the long-distance migration of ozone and ozone precursors from China increases [34], the ozone concentration in Korea will continue to increase in the future. When considering these conditions, we decided that it is important to understand the relationship between ozone and the forests of Korea. Many studies are conducted in transboundary air pollution and its impacts [35; 36; 37].