The effects of elevated ozone and CO 2 on growth and defense of native, exotic and invader trees

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Elevated ozone and CO₂ can differentially affect the performance of plant species. Variation among native, exotic and invader species in their growth and defense responses to CO₂ and ozone may shape CO₂ and ozone effects on invasions, perhaps in part also due to variation between native and invasive populations of invaders. We manipulated ozone (control or 100 ppb) and CO₂ (ambient or 800 ppm) in a factorial greenhouse experiment in replicated chambers. We investigated growth and defense (tannins) of seedlings of Triadica sebifera from invasive (USA) and native (China) populations and pairs of US and China tree species within three genera (Celtis, Liquidambar and Platanus). Overall, ozone reduced growth in ambient CO₂ but elevated CO₂ limited this effect. T. sebifera plants from invasive populations had higher growth than those from native populations in control conditions or the combination of elevated CO₂ and ozone in which invasive populations had greater increases in growth. Their performances were similar in elevated CO₂ because native populations were more responsive and their performances were similar with elevated ozone because invasive populations were more susceptible. Compared to other species, T. sebifera had high growth rates but low levels of tannin production that were insensitive to variation in CO₂ or ozone. Both China and US Platanus plants reduced tannins with increased CO₂ and/or ozone and US Liquidambar plants increased tannins with the combination of elevated CO₂ and ozone. The growth results suggest that intraspecific variation in T. sebifera will reduce the effects of CO₂ or ozone alone on invasions but increase their combined effects. The tannin results suggest that defense responses to CO₂ and ozone will be variable across native and exotic species. The effects of CO₂ and ozone on growth and defense of native and exotic species indicate that the benefit or harm to species from these global change drivers is an idiosyncratic combination of species origin and genus.