Individual and Interactive Effects of Air Warming and Elevated O3 on Carbon Fixation and Allocation in Two Urban Tree Species

•Increased temperature improved plant growth and photosynthetic carbon fixation.•O3 inhibited growth, and Quercus mongolica has a self-repair ability to O3 stress.•The adverse effects of O3 on growth and C fixation were mitigated by air warming.•Urban trees show a species-specific difference in plant functional traits.