Native desert herbaceous species are more sensitive than non-native species to urban air quality

Background/Question/Methods

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, O_3, 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, O_3, 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.

Results/Conclusions

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.