We’ve had national parks on the brain a lot lately as we slogged through 16 days of federal shutdown. It's been an economic burden to gateway communities and a frustration to tourists. But a depressingly dysfunctional government isn’t the only thing plaguing our parks. A new study shows that airborne nitrogen pollution is fundamentally changing the ecosystems of many of our public lands.
Nitrogen is a life-giving nutrient in healthy soils and the most plentiful element in the air we breathe. We add it to plants through fertilizers that can transform a lawn to lush green and help tomato plants grow heavy with fruit. But just like you can love a garden to death with too much fertilizer, excess nitrogen in the environment can kill sensitive plants while promoting growth of nitrogen-loving organisms like algae and certain grasses.
Different ecosystems can handle varying amounts of nitrogen, depending on their elevation, soil composition and the living things that have evolved there. Once the amount of a nutrient reaches a tipping point, or “critical load,” it can kill trees, lichens and wildflowers that are sensitive to it. “Some plants love nitrogen and can take in more than others,” says Raluca Ellis, lead researcher and author on the new study published in the science journal, Atmospheric Chemistry and Physics. “But if we keep adding more (nitrogen), we run the risk of changing the ecosystem and setting it up to support more nitrogen-loving species.”
In some places, like Rocky Mountain National Park, officials have been aware of increased nitrogen in the air and studying its effects for years. In 2005, scientists had already recorded nitrogen levels at 20 times what they estimated for the pre-industrial era. And just beyond the park’s borders on Niwot Ridge, nitrogen-loving grasses were replacing native wildflowers. Researchers worry that the shift in plant communities will mean decreased habitat for animals on the ridge as well. Scientists at Harvard, in collaboration with the National Park Service, U.S. Forest Service and the U.S. Environmental Protection Agency, recently determined that a total of 38 national parks already have nitrogen levels that are at or above their critical load levels.
At Yosemite National Park, it's the lichens that have been changing lately. Some 500 species live in Yosemite, but what alarms researchers is the new growth of a beautiful yellow lichen known as Candelaria. “If you were to go to a place with very little air pollution, then you would not be seeing this many of these Candelaria species," botanist Mike Hutton told California's KQED Radio.
Along with lichens, diatoms are among the first parts of an ecosystem to respond to a nitrogen overload. The single-celled organisms found in lakes and streams conduct about a quarter of the world's photosynthesis, making them critical to oxygen release and carbon capture, and a significant base of the food chain. Once such foundational species of an ecosystem start to change, and more nitrogen-loving diatoms become more prevalent, it can trigger a cascade of larger shifts.
In most of the national parks, “we’re nowhere near killing trees and plants, but we’re beginning to see some of the initial changes,” says Bret Schichtel, a scientist with the National Park Service Air Resources Division. Yosemite officials worry that, if these nitrogen levels remain elevated, more native species will die and invasive plants will move in to fill the void. According to the study, however, the source of Yosemite's nitrogen could be clearing over the next few decades.
The two biggest sources of nitrogen pollution are both human-caused: On the coasts, where population density is higher, nearby parks are affected mostly by the nitrogen oxides released in fuel combustion from cars and power plants. In the Interior West, ammonia released from commercial fertilizers and manure is the biggest source of nitrogen pollution. A handful of parks in the West are at or above critical nitrogen levels, and models predict that this kind of pollution will increase, as a growing human population will require a larger food supply, and thus, more fertilizers. Parks near the coasts, however, may see a decrease in nitrogen if vehicle and power plant emissions standards become more strict.
"It’s not a surprise to find elevated levels in the national parks closer to population centers," says Schichtel, "but in the Interior West, in places that we think of as pristine, it is a bit more shocking" to see increasing amounts of human-caused reactive nitrogen. While the new study is focused on parks, communities nearby are likely experiencing similar levels of excess nitrogen, which is a concern for human health as well.
Ellis and Schichtel hope that their research, and similar studies, can find its way into the hands of policy makers and help propel stronger nitrogen regulations. Currently, there are few regulations concerning airborne nitrogen pollution from farms.
Schichtel says that the new science on nitrogen levels in national parks is an important step to answering the question “Do we need to be concerned with these things and do we need to be concerned with them in the future?” And clearly, the model says “yes.”