Native desert herbaceous species are more sensitive than non-native species to urban air quality
Plants and ecosystems are rarely exposed to a single pollutant, yet research on the effects of co-occurring atmospheric compounds is limited. Carbon dioxide (CO2), ozone (O3), and nitrogen (N) deposition are elevated in and around cities impacting air quality at local to global scales. Despite the ecological relevance of CO2, O3, and N, acting as both stressors and resources for ecosystems, their combined impacts at realistic concentrations are unknown. To address this gap, we ask, what is the sensitivity of ecosystems to co-occurring CO2, O3, and N? Using the Central Arizona Phoenix Long Term Ecological Research (CAP LTER) site as a model system, we examined the net effect of elevated, but realistic concentrations of CO2, O3, and N on growth and physiological responses of dominant native (Pectocarya recurvata) and non-native (Schismus arabicus) Sonoran Desert winter herbaceous vegetation. In a six-week dose response experiment, we fumigated plants in growth chambers with treatment combinations of CO2 (400, 550, and 700ppm), O3 (15, 60, 100ppb), and N (0, 4, 8 kgN ha-1 yr-1 applied as ammonium nitrate). Plants were grown in individual containers to exclude competition and were successively harvested every week throughout the experiment. Herbaceous plants are quick to respond to small environmental changes, thus we expected our study species to be sensitive to elevated co-occurring compounds with overall non-additive responses.
In preliminary results, the non-native species was less sensitive to combinations of O3, CO2, and N than the native species. Among ozone treatments, aboveground biomass, growth rate and physiological parameters of non-native S. arabicus did not significantly differ. On the other hand, the native P. recurvata was more sensitive to ozone stress. Controlling for CO2 and N, P. recurvata had a higher mortality rate and lower growth rate (measured by change in leaf area/day) in high O3 (1.8 cm2/day) than in ambient O3 (4.1 cm2/day). As expected, elevated CO2 had a positive effect on growth rate and partially mitigated the negative effects of O3. P. recurvata had the highest growth rate (5.5 cm2/day) under conditions of ambient O3, high CO2 and elevated N. The non-native species’ resistance to elevated ozone and changing urban air quality may have long-term effects on community composition of native ecosystems surrounding cities. Our research will provide empirical evidence for a multi-factor critical load that will help managers preserve native ecosystems within airsheds that are affected by co-occurring pollutants.