Air Quality at Bryce Canyon National Park
Most visitors expect clean air and clear views in parks. Bryce Canyon National Park (NP), Utah, is home to the largest concentration of hoodoos—totem-shaped rock spires resistant to erosion—in the world. The park enjoys relatively good air quality given its remote location on the Colorado Plateau. However, upwind urban and industrial sources, including large power plants and mines, can harm the park’s natural and scenic resources such as vegetation, surface waters, and visibility. The National Park Service works to address air pollution effects at Bryce Canyon NP, and in parks across the U.S., through science, policy and planning, and by doing our part.
Nitrogen and Sulfur
Nitrogen (N) and sulfur (S) compounds deposited from the air may have harmful effects on ecosystem processes. Healthy ecosystems can naturally buffer a certain amount of pollution, but once a threshold is passed the ecosystem may respond negatively. This threshold is the critical load, or the amount of pollution above which harmful changes in sensitive ecosystems occur (Porter 2005). N and S deposition change ecosystems through eutrophication (N deposition) and acidification (N + S deposition). Eutrophication increases soil and water nutrients which causes some species to grow more quickly and changes community composition. Ecosystem sensitivity to nutrient N enrichment at Bryce Canyon National Park (BRCA) relative to other national parks is low (Sullivan et al. 2016); for a full list of N sensitive ecosystem components, see: NPS ARD 2019. Acidification leaches important cations from soils, lakes, ponds, and streams which decreases habitat quality. Ecosystem sensitivity to acidification at BRCA relative to other national parks is moderate (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.
From 2017-2019 total N deposition in BRCA ranged from 2.9 to 3.6 kg-N ha-1 yr-1 and total S deposition ranged from 0.6 to 0.7 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). BRCA has been monitoring atmospheric N and S deposition since 1985, see the conditions and trends website for park-specific information.
Arid ecosystems have shown variable responses to excess N. Invasive grasses tend to thrive in areas with high N deposition, displacing native vegetation adapted to low N conditions. Increases in N have been found to promote the spread of fast-growing non-native annual grasses (like cheatgrass) and forbs (like Russian thistle) at the expense of native species (Brooks 2003; Allen et al. 2009; Schwinning et al. 2005). In contrast, a recent study showed little vegetation response to fertilization, but did see a decline in the stability of the soil crust community (Phillips et al. 2021). N may also increase water use in plants like big sagebrush (Inouye 2006).
Given the abundance of base cations in park soils and rocks, surface waters in BRCA are generally well-buffered from acidification. However, the park’s seeps and springs may be sensitive to acid inputs. Additionally, small streams with steep-sided canyon walls in the park have little ability to retain nutrients and water, hindering their ability to buffer potentially acidic run-off.
Epiphytic macrolichen community responses
Epiphytic macrolichens grow on tree trunks, branches, and boles. Since these lichens grow above the ground, they obtain all their nutrients directly from precipitation and the air. Many epiphytic lichen species have narrow environmental niches and are extremely sensitive to changes in air pollution. Epiphytic lichen communities are less diverse in arid areas, but are still impacted by air pollution. Geiser et al. (2019) used a U.S. Forest Service national survey to develop critical loads of nitrogen (N) and critical loads of sulfur (S) to prevent more than a 20% decline in four lichen community metrics: total species richness, pollution sensitive species richness, forage lichen abundance, and cyanolichen abundance.
McCoy et al. (2021) used forested area from the National Land Cover Database to estimate the impact of air pollution on epiphytic lichen communities. Forested area makes up 90 km2 (61.7%) of the land area of Bryce Canyon National Park.
- N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 41.8% of the forested area.
- S deposition was below the 2.7 kg-S ha-1 yr-1 critical load to protect S-sensitive lichen species richness in every part of the forested area.
For exceedances of other lichen metrics and the predicted decline of lichen communities see Appendices A and B of McCoy et al. (2021).
Additional modeling was done on 459 lichen species to test the combined effects of air pollution and climate gradients (Geiser et al. 2021). A critical load indicative of initial shifts from pollution-sensitive toward pollution-tolerant species occurred at 1.5 kg-N ha-1 yr-1 and 2.7 kg-S ha-1 yr-1 even under changing climate regimes.
Plant species response
Plants vary in their tolerance of eutrophication and acidification, and some plant species respond to nitrogen (N) or sulfur (S) pollution with declines in growth, survival, or abundance on the landscape. Horn et al. (2018) used the U.S. Forest Service national forest survey to develop critical loads of N and critical loads of S to prevent declines in growth or survival of sensitive tree species. Clark et al. (2019) used a database of plant community surveys to develop critical loads of N and critical loads of S to prevent a decline in abundance of sensitive herbaceous plant species. According to NPSpecies, Bryce Canyon National Park contains:
- 4 N-sensitive tree species and 16 N-sensitive herbaceous species.
- 7 S-sensitive tree species and 14 S-sensitive herbaceous species.
Pollutants like mercury and pesticides are concerning because they are persistent and toxic in the environment. These contaminants can travel in the air thousands of miles away from the source of pollution, even depositing in protected places like national parks. In addition, while some of these harmful pollutants may be banned from use, historically contaminated sites continue to endure negative environmental consequences.
When deposited, airborne mercury and other toxic air contaminants are known to harm wildlife like birds and fish, and cause human health concerns. Many of these substances enter the food chain and accumulate in the tissue of organisms causing reduced reproductive success, impaired growth and development, and decreased survival.
- Microplastics were found in park precipitation samples. These microplastics, thought to be distributed by atmospheric transport, consisted mostly of clothing fibers like cotton, polyester, and nylon. Bryce Canyon NP is estimated to have an annual deposition rate of 0.22-0.26 metric tons of plastic per year (Brahney et al. 2020).
Visitors come to Bryce Canyon NP to experience the spectacular terrain of the Bryce Amphitheater against the forests and meadows of the Paunsaugunt Plateau, and the spectacular night sky. Park vistas, while usually quite clear, are sometimes obscured by haze, reducing how well and how far people can see. Visibility reducing haze is caused by tiny particles in the air, and these particles can also affect human health. Many of the same pollutants that ultimately fall out as nitrogen and sulfur deposition contribute to this haze. Organic compounds, soot, dust, and wood smoke reduce visibility as well.
Smoke from wildfires often affects visibility on the haziest days. At night, pollution can make stargazing more difficult because it scatters artificial light — increasing the impact of light pollution. Significant improvement in park visibility on the clearest days has been documented since the 1990’s, but visibility on the haziest days has not changed significantly. Overall, visibility in the park still needs improvement to reach the Clean Air Act goal of no human caused impairment.
- Reduction of the average natural visual range from about 175 miles (without pollution) to about 140 miles because of pollution at the park
- Reduction of the visual range to below 95 miles on high pollution days
Visit the NPS air quality conditions and trends website for park-specific visibility information. Bryce Canyon NP has been monitoring visibility since 1988. Check out the live air quality webcam and explore air monitoring »
At ground level, ozone is harmful to human health and the environment. Ground-level ozone does not come directly from smokestacks or vehicles, but instead is formed when other pollutants, mainly nitrogen oxides and volatile organic compounds, react in the presence of sunlight.
Over the course of a growing season, ozone can damage the leaves of plants, reducing their growth rate and making them less resistant to disease and insect infestations. An ozone risk assessment concluded that plants in Bryce Canyon NP were at low risk of foliar ozone injury (Kohut 2004). However, estimated ozone concentrations and cumulative doses at the park are high enough to induce foliar injury to sensitive vegetation under certain conditions.
Generally, dry conditions in the park during peak ozone concentrations are likely to limit ozone uptake by plants. However along streams and seeps, where conditions are wetter, plants may have higher ozone uptake and injury (Kohut et al. 2012). Ozone sensitive plants at the park include Apocynum cannabinum (common dogbane) and Populus tremuloides (quaking aspen). Past surveys at the park found probable ozone injury on Sambucus caerulea (blue elderberry) (NPS 2000). Some plants are more sensitive to ozone than others. Search ozone-sensitive plant species found at Bryce Canyon NP.
Visit the NPS air quality conditions and trends website for park-specific ozone information.
Allen, E. B., L. E. Rao, R. J. Steers, A. Bytnerowicz, and M. E. Fenn. 2009. Impacts of atmospheric nitrogen deposition on vegetation and soils in Joshua Tree National Park. Pages 78–100 in R. H. Webb, L. F. Fenstermaker, J. S. Heaton, D. L. Hughson, E. V. McDonald, and D. M. Miller, editors. The Mojave Desert: ecosystem processes and sustainability. University of Nevada Press, Las Vegas, Nevada, USA.
Brahney, J., M. Hallerud, E. Heim, M. Hahnenberger, and S. Sukumaran. 2020. Plastic rain in protected areas of the United States. Science 368(6496): 1257-1260. https://www.science.org/doi/10.1126/science.aaz5819
Brooks, M.L. 2003. Effects of increased soil nitrogen on the dominance of alien annual plants in the Mojave Desert. Journal of Applied Ecology. 40:344–353.
Clark, C.M., Simkin, S.M., Allen, E.B. et al. Potential vulnerability of 348 herbaceous species to atmospheric deposition of nitrogen and sulfur in the United States. Nat. Plants 5, 697–705 (2019). https://doi.org/10.1038/s41477-019-0442-8
Geiser, Linda & Nelson, Peter & Jovan, Sarah & Root, Heather & Clark, Christopher. (2019). Assessing Ecological Risks from Atmospheric Deposition of Nitrogen and Sulfur to US Forests Using Epiphytic Macrolichens. Diversity. 11. 87. 10.3390/d11060087.
Geiser, Linda & Root, Heather & Smith, Robert & Jovan, Sarah & Clair, Larry & Dillman, Karen. (2021). Lichen-based critical loads for deposition of nitrogen and sulfur in US forests. Environmental Pollution. 291. 118187. 10.1016/j.envpol.2021.118187.
Horn KJ, Thomas RQ, Clark CM, Pardo LH, Fenn ME, Lawrence GB, et al. (2018) Growth and survival relationships of 71 tree species with nitrogen and sulfur deposition across the conterminous U.S.. PLoS ONE 13(10): e0205296. https://doi.org/10.1371/journal.pone.0205296
Inouye, R. S. 2006. Effects of shrub removal and nitrogen addition on soil moisture in sagebrush steppe. Journal of Arid Environments. 65: 604–618.
Kohut, B. 2004. Assessing the Risk of Foliar Injury from Ozone on Vegetation in Parks in the Northern Colorado Plateau Network. Available at https://irma.nps.gov/DataStore/Reference/Profile/2181489.
Kohut, B., C. Flanagan, E. Porter, J. Cheatham. 2012. Foliar Ozone Injury on Cutleaf Coneflower at Rocky Mountain National Park, Utah. Western North American Naturalist 72(1): 32–42.
McCoy K., M. D. Bell, and E. Felker-Quinn. 2021. Risk to epiphytic lichen communities in NPS units from atmospheric nitrogen and sulfur pollution: Changes in critical load exceedances from 2001‒2016.
[NADP] National Atmospheric Deposition Program. 2018. NTN Data. Accessed January 20, 2022. Available at http://nadp.slh.wisc.edu/NADP/
Natural Resource Report NPS/NRSS/ARD/NRR—2021/2299. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/nrr-2287254.
[NPS] National Park Service. 2000. Results of 1999 ozone injury surveys at Bryce Canyon NP, Cedar Breaks NM and Zion NP. Memorandum. Available at https://irma.nps.gov/App/Reference/Profile/581126.
Phillips, M. L., D. E. Winkler, R. H. Reibold, B. B. Osborne, and S. C. Reed. 2021. Muted responses to chronic experimental nitrogen deposition on the Colorado Plateau. Oecologia 195:513-524.
Porter, E., Blett, T., Potter, D.U., Huber, C. 2005. Protecting resources on federal lands: Implications of critical loads for atmospheric deposition of nitrogen and sulfur. BioScience 55(7): 603–612. https://doi.org/10.1641/0006-3568(2005)055[0603:PROFLI]2.0.CO;2
Schwinning, S., B. I. Starr, N. J. Wojcik, M. E. Miller, J. E. Ehleringer, R. L. Sanford. 2005. Effects of nitrogen deposition on an arid grassland in the Utah plateau cold desert. Rangeland Ecology and Management. 58: 565–574.
Sullivan, T. J. 2016. Air quality related values (AQRVs) in national parks: Effects from ozone; visibility reducing particles; and atmospheric deposition of acids, nutrients and toxics. Natural Resource Report NPS/NRSS/ARD/NRR—2016/1196. National Park Service, Fort Collins, CO.