The effects of elevated ozone and CO2 on growth and defense of native, exotic and invader trees
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.