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Park Air Profiles - Saguaro National Park

Air Quality at Saguaro National Park

Most visitors expect clean air and clear views in parks. Saguaro National Park (NP), in Arizona, is a desert and mountainous landscape and home to North America's largest cacti—the giant saguaro. Air quality in the park is affected at times by, upwind urban and industrial sources, including the Tucson and Phoenix metropolitan areas. Pollution sources in Mexico can also affect the park. Air pollutants blown into the park can harm natural and scenic resources such as soils, surface waters, plants, wildlife, and visibility. The National Park Service works to address air pollution effects at Saguaro NP, and in parks across the U.S., through science, policy and planning, and by doing our part.

Nitrogen and Sulfur

Bobcat sitting on a saguaro cactus
Visitors come to Saguaro NP to enjoy scenic views of desert and mountain landscapes, giant saguaro cacti, and wildlife.

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 Saguaro National Park (SAGU) relative to other national parks is very high (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 SAGU relative to other national parks is high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.

From 2017-2019 total N deposition in SAGU ranged from 3.1 to 4.6 kg-N ha-1 yr-1 and total S deposition ranged from 0.5 to 0.9 kg-S ha-1 yr-1 based on the TDep model (NADP, 2018). See the conditions and trends website for park-specific information on N and S deposition in SAGU.

Arid ecosystems have shown variable responses to excess N. In nearby desert ecosystems, increased N has been found to promote the spread of fast-growing exotic 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). Buffelgrass is another invasive grass of particular concern and studies in similar ecosystems show N favors buffelgrass over native species (Lyons et al. 2013). Interactions between N, non-native annual grasses, and fire have profound implications for changes to biodiversity in non-fire adapted ecosystems like the Sonoran Desert. Increased N may also exacerbate water use in plants like big sagebrush (Inouye 2006).

Small streams with steep-sided canyon walls in higher elevations of SAGU have little ability to retain nutrients and water, or to buffer potentially acidic run-off. Perennial pools in lush desert oases in the park may be sensitive to acid deposition. Fortunately, there is no evidence that acidification has occurred in park streams or pools as many areas of the park are well-buffered from acidification.

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 79 km2 (21.2%) of the land area of Saguaro National Park.

  • N deposition exceeded the 3.1 kg-N ha-1 yr-1 critical load to protect N-sensitive lichen species richness in 100% 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, Saguaro National Park contains:

  • 3 N-sensitive tree species and 14 N-sensitive herbaceous species.
  • 7 S-sensitive tree species and 11 S-sensitive herbaceous species.

Persistent Pollutants

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.

Some dragonfly larvae sampled at Saguaro NP had mercury concentrations at moderate or higher impairment levels. Dragonfly larvae have been sampled and analyzed for mercury from eight sites in the park; 59% of the data fall into the moderate (100-300 ng/g dw) and 35% fall into the high (300-700 ng/g dw) impairment categories for potential mercury risk. An index of moderate impairment or higher suggests some fish may exceed the US EPA benchmark for protection of human health (Eagles-Smith et al. 2018, Eagles-Smith et al. 2020). The data may not reflect the risk at other unsampled locations in the park.

Visibility

Desert landscape with saguaro cacti
Clean, clear air is essential to appreciating the scenic vistas at Saguaro NP.

Visitors come to Saguaro NP to stand amidst giant saguaro cacti and to enjoy a scenic backdrop of the Tucson and Rincon Mountain ranges. Park vistas 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. Modest improvements in park visibility on the haziest days have been documented since the mid 2000’s, but visibility in the park requires significant improvement to reach the Clean Air Act goal of no human caused impairment.

Visibility effects:

  • Reduced air clarity, at times, due to human-caused haze and fine particles of air pollution, including dust;
  • Reduction of the average natural visual range from about 160 miles (without pollution) to about 100 miles because of pollution at the park;
  • Reduction of the visual range to below 70 miles on high pollution days.

Visit the NPS air quality conditions and trends website for park-specific visibility information. Saguaro NP has been monitoring visibility since 1988. Explore air monitoring »

Ground-Level Ozone

Ponderosa Pine Tree
Ponderosa Pine trees are one of the ozone sensitive species found at Saguaro NP.

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. A risk assessment that considered ozone exposure, soil moisture, and sensitive plant species concluded that plants in Saguaro NP are at low risk of ozone injury (Kohut 2007; Kohut 2004). However, ozone concentrations and cumulative doses at the park are high enough to damage plant leaves on sensitive vegetation under certain conditions (NPS 2010). Surveys in the early 1990s found slight ozone injury on ponderosa pines (Miller et al. 1996), but the typical dry conditions in the park limit ozone uptake by plants. Some plants are more sensitive to ozone than others. Ozone sensitive plant species at the park include Apocynum androsaemifolium (Spreading dogbane), Rudbeckia laciniata (cut-leaf coneflower), and Pinus ponderosa (ponderosa pine). Search for more ozone-sensitive plant species found at Saguaro NP.

Visit the NPS air quality conditions and trends website for park-specific ozone information. Saguaro NP has been monitoring ozone since 1982. Explore air monitoring »

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.

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

Eagles-Smith, C.A., J.J. Willacker, S.J. Nelson, C.M. Flanagan Pritz, D.P. Krabbenhoft, C.Y. Chen, J.T. Ackerman, E.H. Campbell Grant, and D.S. Pilliod. 2020. Dragonflies as biosentinels of mercury availability in aquatic food webs of national parks throughout the United States. Environmental Science and Technology 54(14):8779-8790. https://doi.org/10.1021/acs.est.0c01255

Eagles-Smith, C.A., S.J. Nelson., C.M. Flanagan Pritz, J.J. Willacker Jr., and A. Klemmer. 2018. Total Mercury Concentrations in Dragonfly Larvae from U.S. National Parks (ver. 6.0, June 2021): U.S. Geological Survey data release. https://doi.org/10.5066/P9TK6NPT Inouye, R.S. 2006. Effects of shrub removal and nitrogen addition on soil moisture in sagebrush steppe. Journal of Arid Environments. 65: 604–618.

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 R.J. 2007. Ozone Risk Assessment for Vital Signs Monitoring Networks, Appalachian National Scenic Trail, and Natchez Trace National Scenic Trail. NPS/NRPC/ARD/NRTR—2007/001. National Park Service. Fort Collins, Colorado. Available at https://www.nps.gov/articles/ozone-risk-assessment.htm

Kohut, B. 2004. Assessing the Risk of Foliar Injury from Ozone on Vegetation in Parks in the Sonoran Desert Network. Available at https://irma.nps.gov/DataStore/Reference/Profile/2181558.

Lyons, K.G., Maldonado-Leal, B.G., Owen, G. 2013. Community and Ecosystem Effects of Buffelgrass (Pennisetum ciliare) and Nitrogen Deposition in the Sonoran Desert. Invasive Plant Science and Management 6: 6(1): 65–78. In Press.

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. Natural Resource Report NPS/NRSS/ARD/NRR—2021/2299. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/nrr-2287254.

Miller, P.R. 1996. Extent of Ozone Injury to Trees in the Western United States. U.S. Forest Service Pacific Southwest Research Station. General Technical Report PSW–GTR–155–Web. Available at http://www.fs.fed.us/psw/publications/documents/gtr-155/01-miller.html

[NADP] National Atmospheric Deposition Program. 2018. NTN Data. Accessed January 20, 2022. Available at http://nadp.slh.wisc.edu/NADP/

[NPS] National Park Service, Air Resources Division. 2010. Air quality in national parks: 2009 annual performance and progress report. Natural Resource Report NPS/NRPC/ARD/NRR—2010/266. National Park Service, Denver, Colorado. Available at https://irma.nps.gov/DataStore/Reference/Profile/662783.

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 Colorado 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.

Part of a series of articles titled Park Air Profiles.

Saguaro National Park

Last updated: December 7, 2022