Detecting the sensitivity of desert herbaceous plant responses to co-occurring atmospheric compounds

Cities occupy a small land area globally, yet atmospheric compounds generated from human-dominated ecosystems have significant impacts on protected lands within and beyond urban boundaries.  Decades of research have shown that ecosystem processes are sensitive to elevated concentrations of atmospheric carbon dioxide (CO₂), reactive nitrogen (N) or ozone (O₃) alone, as they function as both resources and stressors for primary producers.  However, few studies have explored the combined impacts of CO₂, O₃ and N on ecosystem processes at realistic concentrations in and around cities, as well as in more remote regions.  Using the Central Arizona Phoenix (CAP) LTER site as a model system, we are examining the net effect of elevated, but realistic concentrations of co-occurring CO₂, O₃ and N deposition on the sensitivity of ecological responses and thresholds.  In a preliminary dose response experiment, we investigated growth and physiological responses of common native and non-native Sonoran Desert winter herbaceous species to elevated O₃ and nitric acid.  In growth chambers, plants were exposed for six weeks to a range of O₃ or nitric acid concentrations realistic to Phoenix while maintaining ambient CO₂.  We expected decreased plant production, growth rate and physiological functioning in elevated O₃ and nitric acid treatments.  Under unlimited water conditions, the aboveground biomass, growth rate and physiological parameters of the non-native species (Schismus arabicus) did not significantly differ among control, ozone or nitric acid treatments.  On the other hand, native species, Pectocarya recurvata and Plantago ovata, showed more variability among treatments suggesting the native species are more sensitive to ozone stress.  However, the pattern of decreased biomass and growth rate in higher ozone concentrations was not significant, possibly reflecting the experimental design with small incremental changes between O₃ treatments (~10, 30, 65, 100ppb O₃).  Finally, we unexpectedly found that Plantago ovata, a native of the Sonoran Desert, had greater aboveground biomass, growth rate, and maximum quantum yield (Fv/Fm) in the nitric acid treatment than plants grown under control conditions.  Our next steps are to conduct a multi-factor dose-response experiment, exposing the desert herbaceous species to combinations of elevated CO₂, O₃ and N in order to examine their sensitivity to multiple factors.   This research will provide empirical evidence for developing a multi-factor critical load that can be used in future management to preserve native ecosystems within the “airshed” affected by co-occurring pollutants.