How invisible pollutants are threatening America's most treasured landscapes
Picture this: you're standing on a scenic overlook in the Great Smoky Mountains National Park, but instead of the famous "smoke" that gives the park its name—a natural fog caused by vegetation—you're peering through a different kind of haze. This haze isn't natural; it's pollution that has traveled hundreds of miles, invading one of America's most treasured landscapes. This isn't a hypothetical scenario. A startling 2024 report revealed that 97% of national parks have unsatisfactory air quality, with hazy skies obscuring the very views visitors come to see 7 .
of national parks have unsatisfactory air quality
of parks still face ozone pollution
Our national parks, those magnificent protected lands we often consider pristine wilderness, are under constant assault from an invisible enemy: atmospheric pollutants. While these areas are protected from development and resource extraction, they remain vulnerable to what travels through the air—sulfur dioxide, nitrogen oxides, and other harmful compounds that originate far beyond park boundaries 2 4 . These pollutants don't just blur vistas; they trigger cascading effects that compromise entire ecosystems, from the soil fungi that support forest growth to the lichens that serve as early warning systems. By studying how these pollutants harm protected ecosystems, scientists are revealing critical insights about the health of our natural world—and what we stand to lose.
Scientists faced a complex challenge: how to measure the health of something as vast and intricate as a forest? The solution was elegant in its simplicity—track four key organisms that serve as biological barometers for the entire ecosystem 2 :
These colorful, plant-like growths on trees and rocks are particularly vulnerable because they absorb all their nutrients directly from the atmosphere. Sensitive lichen species disappear when pollution levels rise, creating "lichen dead zones" that signal deteriorating air quality. Since many animals eat lichens or use them for nesting material, their loss cascades through the food web 2 .
Even the forest's mightiest inhabitants are vulnerable. Studies of over one million trees across 71 species found that nitrogen or sulfur pollution decreased growth and survival for most species. Paradoxically, some trees initially grow faster but invest less in root development, leaving them vulnerable to drought, high winds, and pests 2 .
The wildflowers and grasses of the understory play crucial roles in biodiversity. Excess nitrogen often favors the aggressive growth of invasive plant species over native ones, reducing plant diversity and increasing fire risk by adding more dry fuel 2 .
Mycorrhizal fungi form symbiotic relationships with tree roots, dramatically expanding their reach for water and nutrients. When excess nitrogen is available, trees may reduce their reliance on these fungal partners. When drought or other stresses occur, the weakened fungal network may be unable to help 2 .
Central to this research is the concept of "critical load"—the specific amount of pollution an ecosystem can absorb without harmful consequences 2 . Think of it like a nutritional threshold: a small amount of nitrogen might act as fertilizer, but beyond a certain point, it becomes poison. When pollution deposition exceeds this critical threshold, scientists observe measurable harm to these sensitive organisms. Determining these thresholds for different ecosystems has been crucial for setting informed environmental policy.
To understand the full scope of air pollution's impact, the National Park Service's Air Resources Division embarked on a comprehensive, large-scale assessment of forest health across the national park system 2 .
The study's approach was as ambitious as it was systematic, spanning hundreds of national parks and collecting data annually from 2000 to 2021 2 .
Researchers compiled information on lichens, trees, non-woody plants, and soil fungi across the parks. For lichens and soil fungi, they used national land cover data, pollution deposition models, and field surveys. For trees and herbaceous plants, they relied on the agency's detailed vegetation maps 2 .
For each of the four ecosystem components in every park, scientists calculated the specific critical loads for nitrogen and sulfur pollution. They then determined the "exceedance"—how much current pollution levels were surpassing what the ecosystem could safely handle 2 .
These annual assessments were added to an online Air Quality Conditions and Trends data visualization tool, allowing park managers, scientists, and the public to see park-specific pollution impacts and track changes over time 2 .
The findings, drawn from this two-decade-long study, revealed the extensive reach of air pollution damage, even in America's most protected lands.
| Ecosystem Component | Parks with Data | Parks Experiencing Harm |
|---|---|---|
| Lichen | 250-300 | 200-250 |
| Soil Fungi | 250-300 | 150-200 |
| Herbaceous Plants | 200-250 | 100-150 |
| Trees | 200-250 | 50-100 |
| Source: National Park Service Air Resources Division 2 | ||
The data shows that in 2021, air pollution continued to threaten fundamental components of forest ecosystems in hundreds of parks, with lichens and soil fungi being the most widely affected 2 . While the study found that sulfur and nitrogen pollution has mostly declined across the U.S. over the past 20 years, the persistence of this damage indicates that recovery is a slow process and many parks remain in a precarious state 2 .
| Pollutant | Primary Sources | Impact on Forest Ecosystems |
|---|---|---|
| Sulfur | Power plants, industry | Causes acidification of soil and water, leaching essential nutrients like calcium; harms aquatic insects and fish. |
| Nitrogen | Vehicles, agriculture, power plants | Causes eutrophication (over-fertilization), leading to invasive plant growth, reduced biodiversity, and altered soil fungi relationships. |
| Ozone | Formed from reactions between nitrogen oxides and volatile organic compounds in sunlight | Damages plant tissues, reduces tree growth, and makes breathing difficult for animals and humans. |
| Source: National Park Service 2 7 | ||
Reduction in sulfur emissions at Great Smoky Mountains National Park since 1990
Reduction in nitrogen emissions at Great Smoky Mountains National Park since 1990
Years of data collection in the ongoing study
To conduct this vast assessment, environmental scientists relied on a suite of specialized concepts and tools.
| Tool/Concept | Function in Research |
|---|---|
| Critical Load | The pollution threshold an ecosystem can tolerate without harm; the benchmark for measuring damage. |
| Exceedance | The amount by which pollution deposition exceeds the critical load; indicates the severity of the impact. |
| Deposition Models | Computer simulations that estimate how and where pollutants fall to earth. |
| National Land Cover Data | Maps of vegetation types used to understand which ecosystems are exposed to pollution. |
| Pollution Monitors | Ground-based instruments that measure actual concentrations of pollutants in the air. |
| Vegetation Inventories | Detailed, on-the-ground surveys of plant species and their health. |
The data from these tools combined created a powerful, park-specific resource. As Jim Renfro, an air resource specialist at Great Smoky Mountains National Park, noted, long-term monitoring data tied to projected critical loads was "essential for restoring and protecting sensitive park ecosystems" 2 . This approach allowed managers to demonstrate the need for pollution controls on distant power plants, contributing to significant emissions reductions.
The research presents a clear verdict: even the most majestic and protected landscapes are not immune to human-made pollution. The harmful effects on lichens, trees, non-woody plants, and soil fungi reveal a web of interconnected vulnerabilities. Yet, the story is not entirely bleak. The same research also shows a path toward recovery. Thanks to the Clean Air Act and its amendments, sulfur pollutants have steadily declined, and many species are beginning to recover 2 . At Great Smoky Mountains National Park, once among the most polluted parks, sulfur emissions have dropped 80% and nitrogen emissions 50% since 1990 2 .
This underscores a powerful message: environmental regulations work. The data collected by scientists has been instrumental in informing policies that drive these improvements. However, with 96% of parks still facing ozone pollution and ongoing threats to nature, the job is far from complete 7 . The study of biological vulnerabilities in our national parks provides not just a warning, but also a blueprint. It shows that by continuing to support science-based policy, invest in clean air protections, and transition away from polluting technologies, we can ensure that these treasured ecosystems regain their health and resilience for generations to come.