COS 155-5 - Surface ozone trends and plant physiological inhibition in National Parks in the western US

Abstract: Surface ozone (O₃) resulting from human activities has increased worldwide in recent decades. The economic and ecosystem consequences of human-caused O₃ are widespread, yet little is known about how current ambient O₃ levels may be impacting plant species on public lands in the western US. Here, we examine nearly 30 years of ground-level O₃ data at four National Parks across the Colorado Plateau region of the western US. We also examine nitric oxide (NO_x), a common O₃ precursor, near one of our focal parks with high oil and gas activity. We test the relationship of O₃ with temperature, the number of automobiles in parks, and NO_x data to better understand the potential influence of regional energy production and tourism on O₃ trends in recent years. We combine air quality data during the summer growing season and physiological field measurements of a common O₃ bioindicator species, Rhus aromatica, to model the relative influence of ambient O₃ on plant performance. We then use a seedling fumigation experiment to quantify leaf-level response to short-term O₃ exposure and potential recovery.

All parks had at least one day when mean ambient O₃ was ≥ 60 ppb (i.e., the lower end of the US Environmental Protection Agency’s recommended air quality standard) and patterns have mostly trended downward. Still, parks had between 110–1198 annual hours when O₃ levels reached 60 ppb, up to 395 hours ≥ 80 ppb, and 129 hours ≥ 100 ppb. Reductions in oil and gas industry operations during the COVID-19 pandemic led to reduced NO_x emissions. Monthly mean O₃ levels were partially explained by the number of vehicles in parks and mean maximum temperature from 2009–2022. When we measured Rhus aromatica individuals in the field in 2019, O₃ levels were best explained by the number of vehicles across our focal parks. Modeled net photosynthetic rates (A_net) and leaf carbon to nitrogen ratios decreased with increasing O₃ levels, whereas integrated water use efficiency and nitrogen isotope discrimination increased at higher O₃ levels; suggesting physiological stress and that plants are likely acquiring nitrogen from different sources in parks during the growing season. All R. aromatica individuals responded similarly to short-term O₃ exposure but A_net rates declined overall, and the limited recovery of photosystems 11-minutes post exposure suggests permanent damage. Our work directly informs ecosystem and air quality management related to surface O₃ levels, while describing the physiological impacts of O₃ on plant performance.