Relationships of CO2 assimilation rates with exposure- and flux-based O3 metrics in three urban tree species

- Three greening species were exposed to 5 O3 concentrations in 15 open-top chambers.
- A stomatal multiplicative model was individually parameterized for the three species.
- Concentration and flux-based O3 response relationships for Asat were established.
- The performance of both types of metrics for risk assessment was similar for any species.
- Current ambient O3 concentration in China damages plant photosynthesis.

The relationships of CO2 assimilation under saturated-light conditions (Asat) with exposure- (AOTX, Accumulated Ozone exposure over a hourly Threshold of X ppb) and flux-based (PODY, Phytotoxic Ozone Dose over a hourly threshold Y nmol·m− 2·s− 1) O3 metrics was studied on three common urban trees, Fraxinus chinensis (FC), Platanus orientalis (PO) and Robinia pseudoacacia (RP). Parameterizations for a stomatal multiplicative model were proposed for the three species. RP was the species showing lower species-specific maximum stomatal conductance (gmax) and experiencing lower cumulative O3 uptake along the experiment, but in contrast it was the most sensitive to O3. PODY was slightly better than AOTX metric at estimating relative Asat (R-Asat)for PO and RB but not for FC. The best fittings obtained for the regressions between R-Asat and AOTX for FC, PO and RP were 0.904, 0.868, and 0.876, when the thresholds of X were 60 ppb, 55 ppb and 30 ppb, respectively. However, AOT40 performed also well for all of them, with R2 always > 0.83. For PODY, the highest R2 values for FC, PO and RB were 0.863, 0.897 and 0.911 at thresholds Y = 7, 5 and 1 nmol O3 m− 2 s− 1, respectively. Given the potentially higher O3 removal capacity of FC and PO by stomatal uptake and their lower sensitivity to this pollutant than RP, the former two species would be appropriate for urban gardens and areas where O3 levels are high. Parameterization and modeling of stomatal conductance for the main urban tree species may provide reliable estimations of the stomatal uptake of O3 and other gaseous pollutants by vegetation, which may support decision making on the most suitable species for green urban planning in polluted areas.

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