Abstract: To better understand the effects of ground-level ozone (O₃) on nutrients and stoichiometry in different plant organs, urban tree species Celtis sinensis, Cyclocarya paliurus, Quercus acutissima, and Quercus nuttallii were subjected to a constant exposure to charcoal-filtered air (CF), nonfiltered air (NF), or NF + 40, 60, or 80 nmol O₃ mol^–1 (NF40, NF60, and NF80) starting early in the summer of the growing season. At the end of summer, net CO₂ assimilation rate (A), stomatal conductance (g_s), leaf mass per area (LMA), and/or leaf greenness (SPAD) either were not significantly affected by elevated O₃ or were even higher in some cases during the summer compared with the CF or NF controls. LMA was significantly lower in autumn only after the highest O₃ exposures. Compared to NF, NF40 caused a large increase in g_s across species in late summer and more K and Mn in stems. At the end of the growing season, nutrient status and stoichiometric ratios in different organs were variously altered under O₃ stress; many changes were large and often species-specific. Across O₃ treatments, LMA was primarily associated with C and Mg levels in leaves and Ca levels in leaves and stems. NF40 enriched K, P, Fe, and Mn in stems, relative to NF, and NF60 enhanced Ca in leaves relative to CF and NF40. Moreover, NF resulted in a higher Ca/Mg ratio in leaves of Q. acutissima only, relative to the other O₃ regimes. Interestingly, across species, O₃ stress led to different nutrient modifications in different organs (stems + branches vs leaves). Thus, ambient and/or elevated O₃ exposures can alter the dynamics and distribution of nutrients and disrupt stoichiometry in different organs in a species-specific manner. Changes in stoichiometry reflect an important defense mechanism in plants under O₃, and O₃ pollution adds more risk to ecological stoichiometries in urban areas.

Key words: Air pollution, Ground-level ozone, Urban green, Micronutrients, Nutrient cycling