Air Quality at Carlsbad Caverns National Park
Most visitors expect clean air and clear views in parks. Carlsbad Caverns National Park (NP), New Mexico, is a wonder of canyons, shrublands, and more than 100 caves beneath the surface. Although the park is rural and surrounded by the Chihuahuan Desert, there are some nearby and regional sources of air pollution, including oil and gas operations, mineral extraction and processing, agricultural activities, refineries, and power plants. 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 Carlsbad Caverns 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 change community composition. Ecosystem sensitivity to nutrient N enrichment at Carlsbad Caverns National Park (CAVE) 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 CAVE relative to other national parks is very high (Sullivan et al. 2016); to search for acid-sensitive plant species, see: NPSpecies.
From 2017-2019 total N deposition in CAVE ranged from 3.6 to 4.0 kg-N ha-1 yr-1 and total S deposition ranged from 1.2 to 1.4 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 at CAVE.
Arid ecosystems and grasslands have shown variable responses to excess N. About a third of CAVE is covered in grasslands. Increases in N have been found to promote invasions of fast-growing exotic annual grasses and forbs (e.g., Russian thistle) at the expense of native species (Brooks 2003; Allen et al. 2009; Schwinning et al. 2005). N may also increase water use in plants like big sagebrush (Inouye 2006).
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 21.8 km2 (11.5%) of the land area of Carlsbad Caverns 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, Carlsbad Caverns National Park contains:
- 4 N-sensitive tree species and 14 N-sensitive herbaceous species.
- 5 S-sensitive tree species and 10 S-sensitive herbaceous species.
Airborne mercury, and other toxic air contaminants, when deposited are known to harm birds, salamanders, fish and other wildlife, and cause human health concerns. These substances enter the food chain and accumulate in the tissue of organisms causing reduced reproductive success, impaired growth and development, and decreased survival.
Bats – the most famous mammal at Carlsbad Caverns NP – may be vulnerable to toxic accumulation given their voracious appetite for insects. Findings from Clark (2001) indicate that DDT played a major role in the severe population decline of Brazilian (Mexican) Free-tailed Bats at Carlsbad Caverns since 1936. Furthermore, a mercury risk assessment based on water chemistry and physical parameters rated the potential for methylmercury production in the park very high compared to other NPS units (Krabbenhoft, In Review).
Many visitors come to Carlsbad Caverns NP to experience the large cave chambers deep underground, and to enjoy panoramic vistas of the Guadalupe Mountains as well as one of the few protected portions of the northern Chihuahuan Desert ecosystem. 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. Significant improvement in visibility on the clearest days has been documented since the late 1980’s. Still, regional visibility has not improved significantly on the haziest days and is a long way from the Clean Air Act goal of no human caused impairment.
- Reduced visibility, at times, due to human-caused haze and fine particles of air pollution, including dust
- Reduction of the average natural visual range from about 175 miles (without pollution) to about 90 miles because of pollution
- Reduction of the visual range to below 55 miles on high pollution days
Visit the NPS air quality conditions and trends website for park-specific visibility information. The NPS has been monitoring visibility at Guadalupe Mountains NP, Texas since 2000 these data are considered representative of regional visibility conditions for Carlsbad Caverns 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 plant tissues making it harder for plants to produce and store food. It also weakens plants making them less resistant to disease and insect infestations. Some plants are more sensitive to ozone than others. There are a few ozone-sensitive plants in Carlsbad Caverns NP including Rhus trilobata (skunkbush), Artemisia ludoviciana (white sagebrush), and Pinus ponderosa (ponderosa pine). A risk assessment that considered ozone exposure, soil moisture, and sensitive plant species concluded that plants in Carlsbad Caverns NP were at low risk of damage to plant leaves (see network report: Kohut 2004). Generally, dry conditions in the park during peak ozone concentrations are likely to limit ozone uptake by plants. However along springs and seeps, where conditions are wetter, plants may have higher ozone uptake and injury (Kohut et al. 2012). Search ozone-sensitive plant species found at Carlsbad Caverns NP.
Visit the NPS air quality conditions and trends website for park-specific ozone information. Carlsbad Caverns NP has been monitoring ozone since 2006. Check out the live ozone and meteorology data from Carlsbad Caverns NP and 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
Clark, D.R., Jr. 2001. DDT and the decline of free-tailed bats (Tadarida brasiliensis) at Carlsbad Cavern, New Mexico. Arch Environ Contam Toxicol 40(4):537–543.
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 Sonoran Desert Network. Available at https://irma.nps.gov/DataStore/Reference/Profile/2181558
Kohut, B., C. Flanagan, E. Porter, J. Cheatham. 2012. Foliar Ozone Injury on Cutleaf Coneflower at Rocky Mountain National Park, Colorado. Western North American Naturalist 72(1): 32–42.
Krabbenhoft, D. P. Modeling Surface-Water Methylmercury in National Parks. In Review.
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
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. Available at 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.