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Leaf ozone uptake through the stomata is an important index for the ozone risk assessments on trees. Stomatal conductance (g s ) and ozone concentration ([O 3 ]), determinants of the leaf ozone uptake, are known to show vertical gradients within a tree canopy. However, less is known about the within-canopy vertical gradient of leaf ozone uptake. This study was aimed to elucidate how the vertical gradient of [O 3 ] and g s affect needle ozone uptake within a canopy of mature Cryptomeria japonica trees in a suburban forest at Tokyo, Japan. For this purpose, a multilayer gas exchange model was applied to estimate the vertical gradient of needle g s and the accumulated ozone uptake during the study period (POD 1 , Phytotoxic Ozone Dose above a threshold of 1 nmol m -2 s -1 ). In addition, we also tested several scenarios of vertical gradient of [O 3 ] within the canopy for sensitivity analysis. The POD 1 was declined from the top to the bottom of the canopy. This tendency strongly depended on the vertical gradient of g s and was hardly affected by the changes in simulated vertical reductions of the [O 3 ]. We further assessed the photosynthesis of sunlit needles (needles absorbing both direct and diffuse light) and shaded needles (needles only absorbing diffuse light). The photosynthesis of shaded needles in the upper half of the canopy made a great contribution to the entire canopy photosynthesis. In addition, given that their POD 1 was lower than that of sunlit needles, ozone may affect sunlit and shaded needles differently. We concluded that these considerations should be incorporated into modeling of the calculation of ozone uptake for mature trees to make accurate predictions of the ozone effects on trees at the canopy scale., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Ozone uptake through the stomata in tree leaves is an important process for improving air quality by urban trees. Stomatal conductance (g s ) is a key determinant of stomatal ozone uptake. The parameterization of g s models for estimating stomatal ozone uptake of trees has mainly been carried out using g s data measured in seedling leaves although the leaf traits may differ between mature trees and seedlings. In the present study, we compared stomatal ozone uptake estimated by g s models parameterised with data from mature trees and seedlings of Zelkova serrata. We measured g s in leaves of mature trees and seedlings of Z. serrata using a leaf porometer for 3-4 growing seasons. The Jarvis-type g s model was parameterised with data from mature trees and seedlings, separately. The maximum g s , and the functions of the seedling g s estimation model regarding the response to air temperature, vapour pressure deficit and atmospheric ozone concentration were the factors inducing lower stomatal ozone uptake. In contrast, the function of the seedling g s estimation model regarding the response to irradiance resulted in a higher estimated stomatal ozone uptake. The estimated stomatal ozone uptake for one growing season (April-September) by the seedling g s estimation model was 27% lower than that by the mature tree g s estimation model. These results indicate that leaf gas exchange traits of Z. serrata were different between mature trees and seedlings, and that estimating ozone uptake in mature tree leaves using a model based on seedling g s measurements results in an underestimation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Ozone (O 3 ) is an air pollutant that is toxic to trees. O 3 reduces steady-state net photosynthetic rate (A), and the adverse effects of O 3 are mitigated under elevated CO 2 condition. However, the combined effects of O 3 and elevated CO 2 on dynamic photosynthesis under variable light conditions have not yet been clarified. In this study, we investigated the effects of O 3 and elevated CO 2 on dynamic photosynthesis in the leaves of Fagus crenata seedlings under variable light conditions. The seedlings were grown under four gas treatments comprising two levels of O 3 concentration (lower and two times higher than the ambient O 3 concentration) and two levels of CO 2 concentration (ambient and 700 ppm). Although O 3 significantly decreased steady-state A under ambient CO 2 concentrations, no significant decrease was observed under elevated CO 2 concentrations, indicating the mitigating effect of elevated CO 2 on O 3 -induced adverse effects on steady-state A. During photosynthetic induction, the response of A to the change in photosynthetic photon flux density (PPFD) from 50 (low light) to 1000 μmol m -2  s -1 (high light) showed that the increase in A was slowed by O 3 and accelerated by elevated CO 2 . Under fluctuating light condition of repeating low light for 4 min and high light for 1 min, A at end of each high light period gradually decreased in all treatments, and O 3 and elevated CO 2 accelerated the reduction of A. In contrast to steady-state A, no mitigating effect of elevated CO 2 was observed for any parameters related to dynamic photosynthesis. We conclude that the combined effects of O 3 and elevated CO 2 on A of F. crenata are different under steady-state and variable light conditions, and the O 3 -induced decrease in leaf A may not be mitigated by elevated CO 2 in the field under variable light conditions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

The amount of ozone absorbed by the tree leaves is a critical factor determining the ozone effects on forest trees. Stomatal ozone uptake of a forest canopy can be estimated from the ozone concentration and canopy conductance (g c ) determined by the sap-flow-based method. This method measures sap flow as a metric of crown transpiration and then derives g c . The thermal dissipation method (TDM) has been used to measure sap flow in most studies adopting this approach. However, recent studies have indicated that TDM may underestimate sap flow, especially in ring-porous tree species. In the present study, the accumulated stomatal ozone uptake (AF ST ) of a stand of Quercus serrata, a typical ring-porous tree species in Japan, was estimated by measuring sap flow using species-specific calibrated TDM sensors. Laboratory calibration of the TDM sensors revealed that the parameters (α and β) in an equation converting outputs from the sensors (K) to sap flux density (F d ) were substantially larger for Q. serrata than those originally proposed by Granier (1987). The F d measured in the Q. serrata stand using calibrated TDM sensors were significantly larger than those obtained using non-calibrated sensors. The diurnal average of g c and daytime AF ST (10.4 mm s -1 and 10.96 mmol O 3 m -2  month -1 ) of the Q. serrata stand estimated by using calibrated TDM sensors in August 2020 were similar to those of forests dominated by Quercus species estimated by micrometeorological measurements in previous studies. In contrast, the g c and daytime AF ST of the Q. serrata stand estimated by non-calibrated TDM sensors were remarkably lower than those estimated by micrometeorological measurements in previous studies, indicating severe underestimation. Therefore, it is strongly recommended that sap flow sensors are species-specifically calibrated when estimating the canopy conductance and ozone uptake of forests dominated by ring-porous trees based on sap flow measurements using TDM., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Ozone (O 3 ) in the troposphere, an air pollutant with phytotoxicity, is considered as a driver of global warming, because it reduces plant carbon fixation. Recently, a process-based plant growth model has been used in evaluating the O 3 impacts on plants (Schauberger et al., 2019). To make the evaluation more rigorous, we developed a plant growth model and clarified the key factors driving O 3 -induced change in the whole-plant carbon fixation amount (C fix ). Fagus crenata seedlings were exposed to three O 3 levels (charcoal-filtered air or 1.0- or 1.5-folds ambient [O 3 ]) with three soil fertilization levels (non-, low-, or high-fertilized), i.e., a total of nine treatments. The C fix was reduced in non- and low-fertilized treatments but was unaffected in high-fertilized treatment by O 3 fumigation. Our plant growth model could simulate C fix accurately (<10% error) by considering the impacts of O 3 on plant leaf area and photosynthetic capacities, including maximum velocities of carboxylation and electron transport (V cmax and J max , respectively), and the initial slope and convexity of the curve of the electron transport velocity response to photosynthetic photon flux density (φ and θ, respectively). Furthermore, the model revealed that changes in V cmax and J max , φ and θ, or leaf area, caused by 1.5-folds the ambient [O 3 ] fumigation resulted in the following C fix changes: -1.6, -5.8, or -16.4% in non-fertilized seedlings, -4.1, -4.4, or -9.3% in low-fertilized seedlings, and -4.6, -7.6, or +5.8% in high-fertilized seedlings. Therefore, photosynthetic capacities (particularly φ and θ) and leaf area are important factors influencing the impact of O 3 on C fix of F. crenata seedlings grown under various fertilization levels. Further, the impacts of O 3 and soil nutrient on these photosynthetic capacities and plant leaf area should be considered to predict O 3 -induced changes in carbon fixation by forest tree species using the process-based plant growth model., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)