Relationships of CO 2 assimilation rates with exposure- and flux-based O 3 metrics in three urban tree species.

The relationships of CO ₂ assimilation under saturated-light conditions (A _sat ) with exposure- (AOTX, Accumulated Ozone exposure over a hourly Threshold of X ppb) and flux-based (POD _Y , Phytotoxic Ozone Dose over a hourly threshold Y nmol·m ^-2 ·s ^-1 ) O ₃ 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 (g _max ) and experiencing lower cumulative O ₃ uptake along the experiment, but in contrast it was the most sensitive to O ₃ . POD _Y was slightly better than AOTX metric at estimating relative A _sat (R-A _sat )for PO and RB but not for FC. The best fittings obtained for the regressions between R-A _sat and AOTX for FC, PO and RP were 0.904, 0.868, and 0.876, when the thresholds of X were 60ppb, 55ppb and 30ppb, respectively. However, AOT40 performed also well for all of them, with R ² always >0.83. For POD _Y , the highest R ² values for FC, PO and RB were 0.863, 0.897 and 0.911 at thresholds Y=7, 5 and 1nmolO ₃ m ^-2 s ^-1 , respectively. Given the potentially higher O ₃ 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 O ₃ levels are high. Parameterization and modeling of stomatal conductance for the main urban tree species may provide reliable estimations of the stomatal uptake of O ₃ 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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