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

Abstract

The relationships of CO₂ 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) 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 (g_max) and experiencing lower cumulative O₃ uptake along the experiment, but in contrast it was the most sensitive to O₃. 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 R² always >0.83. For POD_Y, the highest R² values for FC, PO and RB were 0.863, 0.897 and 0.911 at thresholds Y=7, 5 and 1nmolO₃m^-2s^-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.

Keywords: China; Flux-response relationships; Ozone; Photosynthesis; Urban trees.