Ozone (O₃) is a phytotoxic air pollutant, the adverse effects of which on growth and photosynthesis are modified by other environmental factors. In this study, we examined the combined effects of O₃, elevated CO₂, and soil nitrogen supply on Siebold's beech seedlings. Seedlings were grown under combinations of two levels of O₃ (low and two times ambient O₃ concentration), two levels of CO₂ (ambient and 700 ppm), and three levels of soil nitrogen supply (0, 50, and 100 kg N ha⁻¹ year⁻¹) during two growing seasons (2019 and 2020), with leaf photosynthetic traits being determined during the second season. We found that elevated CO₂ ameliorated O₃-induced reductions in photosynthetic activity, whereas the negative effects of O₃ on photosynthetic traits were enhanced by soil nitrogen supply. We observed three-factor interactions in photosynthetic traits, with the ameliorative effects of elevated CO₂ on O₃-induced reductions in the maximum rate of carboxylation being more pronounced under high than under low soil nitrogen conditions in July. In contrast, elevated CO₂-induced amelioration of the effects of O₃ on stomatal function-related traits was more pronounced under low soil nitrogen conditions. Although we observed several two- or three-factor interactions of gas and soil treatments with respect to leaf photosynthetic traits, the shoot to root dry mass (S/R) ratio was the only parameter for which a significant interaction was detected among seedling growth parameters. O₃ caused a significant increase in S/R under ambient CO₂ conditions, whereas no similar effects were observed under elevated CO₂ conditions. Collectively, our findings reveal the complex interactive effects of elevated CO₂ and soil nitrogen supply on the detrimental effects of O₃ on leaf photosynthetic traits, and highlight the importance of taking into consideration differences between the responses of CO₂ uptake and growth to these three environmental factors.

To determine the effects of ozone (O3) on the canopy carbon budget, we investigated photosynthesis and respiration of leaves of Siebold's beech saplings under free air O3 exposure (60 nmol mol−1, during daytime) in relation to the within-canopy light gradient; we then calculated the canopy-level photosynthetic carbon gain (PCG) and respiratory carbon loss (RCL) using a canopy photosynthesis model. Susceptibilities of photosynthesis and respiration to O3 were greater in leaves of upper canopy than in the lower canopy. The canopy net carbon gain (NCG) was reduced by O3 by 12.4% during one growing season. The increased RCL was the main factor for the O3-induced reduction in NCG in late summer, while contributions of the reduced PCG and the increased RCL to the NCG were almost the same in autumn. These results indicate contributions of changes in PCG and RCL under O3 to NCG were different between seasons.

We set up a free-air ozone (O3) exposure system for determining the photosynthetic responses of Siebold's beech (Fagus crenata) and oak (Quercus mongolica var. crispula) to O3 under field conditions. Ten-year-old saplings of beech and oak were exposed to an elevated O3 concentration (60 nmol mol−1) during daytime from 6 August to 11 November 2011. Ozone significantly reduced the net photosynthetic rate in leaves of both species in October, by 46% for beech and 15% for oak. In beech there were significant decreases in maximum rate of carboxylation, maximum rate of electron transport in photosynthesis, nitrogen content and photosynthetic nitrogen use efficiency, but not in oak. Stomatal limitation of photosynthesis was unaffected by O3. We therefore concluded photosynthesis in beech is more sensitive to O3 than that in oak, and the O3-induced reduction of photosynthetic activity in beech was due not to stomatal closure, but to biochemical limitation.

The central mountainous area of Japan is affected by air pollutant emissions from nearby countries such as China and Korea. Sharp increases in the consumption of fossil fuels in the early 21st century, associated with rapid industrialization in China, resulted in long-range transport of pollutants from East Asia and increases in the harmful effects of pollution. However, the air pollutants emissions have decreased since 2006, when air pollution countermeasures were implemented in China. Furthermore, climatic patterns during 2008–2013 reduced tropospheric ozone concentrations around Japan. Such major changes in the social and climatic environment may have had a significant impact on forests. To investigate this, long-term forest monitoring data obtained at Buna-daira (1190 m a.s.l.), Buna-zaka (1090 m a.s.l.) and Arimine (1350 m a.s.l.) were analyzed. Buna-daira and Buna-zaka forests face the continental side of Mt. Tateyama in Toyama Prefecture. In both stands, the girth growth rate of Fagus crenata was found to have increased after 2008; however, such a tendency was not detected at Arimine, which is surrounded by mountains. The growth rates of Cryptomeria japonica, a conifer resistant to air pollution, were found to remain unchanged or decrease. Here, regional long-range transport of air pollution (including ozone and sulfur oxide) has been demonstrated to influence mountain forests in Japan. In particular, recent decreases in regional air pollution may be an important factor controlling increases in F. crenata, likely through changes in interspecific relationships between species sensitive to and tolerant of air pollution.

We aimed to clarify the effects of ozone (O3) on photosynthetic ability of upper and lower canopy leaves of Fagus crenata Blume seedlings grown under different soil nutrient conditions. To accomplish this objective, we analyzed the response of photosynthetic parameters such as maximum carboxylation rate (Vcmax) to cumulative stomatal O3 uptake (ΣFst) and reduction rate of Vcmax per unit ΣFst as an index of detoxification capacity for O3. The seedlings of Fagus crenata were grown for two growing seasons (2014–2015) in nine treatments comprised of a combination of three levels of gas treatments (charcoal-filtered air or 1.0- or 1.5-times ambient O3 concentration) and three levels of soil nutrient treatments (non-fertilized or a supply of relatively low or high concentrations of compound fertilizer). The nutrient supply significantly increased the degree of O3-induced reduction in Vcmax in September. However, nutrient supply did not significantly increase ΣFst and reduce the detoxification capacity for O3. On the other hand, the degree of O3-induced reduction in Vcmax of upper canopy leaves was higher as compared with that of lower canopy leaves in August due to the higher ΣFst. However, the reduction rate of Vcmax per unit ΣFst in lower canopy leaves was higher than that in upper canopy leaves, indicating lower detoxification capacity for O3 in lower canopy leaves. Reduction rate of Vcmax per unit ΣFst over the threshold, which is assumed to be proportional to gross photosynthetic rate, was similar between upper and lower canopy leaves. Therefore, capacity of photosynthetic CO2 assimilation is likely to be associated with detoxification capacity for O3 in upper and lower canopy leaves of F. crenata seedlings grown under different soil nutrient conditions.

To construct stomatal conductance models and estimate stomatal O3 uptake for Fagus crenata, Quercus serrata, Quercus mongolica var. crispula and Betula platyphylla, stomatal conductance (gs) was measured in seedlings of the four tree species. Better estimates of gs were made by incorporating the acute effects of O3 on gs into the models and the models could explain 34–52% of the variability in gs. Although the O3 concentration was relatively high in spring from April to May, COU of F. crenata, Q. serrata and Q. mongolica var. crispula were relatively low and the ratios of COU in spring to total COU in one year were 16.8% in all tree species because of low gs limited mainly by leaf pre-maturation and/or low temperature. The COU of B. platyphylla were relatively high mainly because of rapid leaf maturation and lower optimal temperature for stomatal opening.

A multiplicative stomatal conductance model was constructed to estimate stomatal O3 uptake of Fagus crenata exposed to O3 under different N loads to the soil. Our stomatal conductance model included environmental functions such as the stomatal responses of F. crenata to diurnal changes, chronic O3 stress (AOT0), acute O3 stress (O3 concentration), and nitrogen load to soil. The model could explain 62% of the variability in stomatal conductance. We suggest therefore that stomatal closure induced by O3 and N load-induced soil acidification must be taken into account in developing a stomatal conductance model for estimating stomatal O3 uptake for future risk assessment of O3 impact on Japanese forest tree species such as F. crenata.

We investigated the effects of ozone and leaf senescence on steady-state stomatal conductance and stomatal response to light variation. Measurements were carried out in a free-air ozone exposure experiment on a representative deciduous broadleaved tree species in Japan (Fagus crenata). Both steady-state and dynamic stomatal response to light variation varied intrinsically with season due to leaf senescence. Ozone induced the decrease in steady-state leaf gas exchange and the sluggish stomatal closure progressively. These findings suggest that ozone reduces the ability of plants to adapt to a fluctuating light environment under natural conditions, and therefore impairs plant growth and ability to control water loss.

We examined a performance of the multiplicative stomatal conductance model to estimate the stomatal ozone uptake for Fagus crenata. Parameterization of the model was carried out by in-situ measurements in a free-air ozone exposure experiment. The model performed fairly well under ambient conditions, with low ozone concentration. However, the model overestimated stomatal conductance under enhanced ozone condition due to ozone-induced stomatal closure. A revised model that included a parameter representing ozone-induced stomatal closure showed better estimation of ozone uptake. Neglecting ozone-induced stomatal closure induced a 20 % overestimation of the stomatal uptake of ozone. The ozone-induced stomatal closure was closely related to stomatal ozone uptake rather than accumulated concentrations of ozone exceeding 40 nmol mol⁻¹. Our results suggest that ozone-induced stomatal closure should be implemented to stomatal conductance model for estimating ozone uptake for F. crenata. The implementation will contribute to adequate risk assessments of ozone impacts on F. crenata forests in Japan.

We examined the root production of a set of Fagus crenata (Siebold’s beech) saplings grown in an infertile immature volcanic ash soil (VA) and another set in a fertile brown forest soil (BF) with both sets exposed to elevated CO₂. After the saplings had been exposed to ambient (370–390 μmol mol⁻¹) or elevated (500 μmol mol⁻¹) CO₂, during the daytime, for 11 growing seasons, the root systems were excavated. Elevated CO₂ boosted the total root production of saplings grown in VA and abolished the negative effect of VA under ambient CO₂, but there was no significant effect of elevated CO₂ on saplings grown in BF. These results indicate the projected elevated CO₂ concentrations may have a different impact in regions with different soil fertility while in regions with VA, a higher net primary production is expected. In addition, we observed large elevated CO₂-induced fine-root production and extensive foraging strategy of saplings in both soils, a phenomenon that may partly (a) adjust the biogeochemical cycles of ecosystems, (b) form their response to global change, and (c) increase the size and/or biodiversity of soil fauna. We recommend that future researches consider testing a soil with a higher degree of infertility than the one we tested.