It’s in the Air: The Ecological Effects of Nitrogen Deposition in Rocky Mountain National Park

U. S. National Parks are reserves of natural, healthy-functioning ecosystems supporting diverse animal and plant life, protected by law from the impacts of human-generated activities and pollution. However, encroaching development, overuse, and air- and waterborne contaminants from outside park boundaries are causing noticeable changes to water quality and ecosystem health and functioning.

Long-term human-caused effects on alpine and subalpine ecosystems have attracted much media attention in the southern Rocky Mountains, especially in one of its crown jewels, Rocky Mountain National Park. Located an hour’s drive from Denver at the top of the Colorado Front Range, the park is a popular year-round destination for more than three million people each year. The Front Range metropolitan area from Fort Collins south to Colorado Springs includes 75% of Colorado’s population and its most productive agricultural lands. It is rich in natural gas resources, supports a thriving agricultural industry, and has a rapidly growing urban population. Pollutant emissions from the Front Range are the highest in Colorado. Nitrogen oxides and ammonia from transportation, construction, energy production, and agriculture combine with air pollutants from outside the region to affect visibility, regional haze, ozone, and especially atmospheric nitrogen deposition in the mountains to the west. Deteriorating air quality in and around the park is a growing concern.

USGS Fort Collins Science Center (FORT) researcher Jill Baron has been studying ecosystem processes in Rocky Mountain National Park for over 24 years. In that time, she and her team at the Colorado State University Natural Resource Ecology Laboratory have found that changes due to atmospheric deposition, particularly of nitrogen, are affecting the Park’s physical and living resources.

Close-up of filter equipment with chlorophyll on filter paper, used for measuring algal primary productivity.	Lead investigator Dr. Jill Baron prepares to extract water from a soil lysimeter, a sampling device that captures soil water. The chemical composition of soil water gives important information on whether the forest area sampled is saturated with nitrogen.

Nitrogen acts initially as a fertilizer and ultimately as an acidifier, both unnatural alterations in mountain ecosystems. Climatic and geologic conditions in Rocky Mountain National Park make it especially responsive to atmospheric deposition. The cold, harsh, windy climate makes for short growing seasons. Exposed and slow-weathering granitic bedrock, shallow soils, and old-growth forests cannot take up and store most of the nitrogen that is deposited. The alpine vegetation, adapted to short growing seasons and low-nitrogen conditions, cannot absorb it all. Consequently, the excess leaches into park lakes and streams. These lakes and streams can become acidic once nitrogen uses up the buffering chemicals in the water and soils. In other sensitive regions of the world, acidic waters have been shown to interfere with trout reproduction and survival.

Nitrogen deposition has increased in the park at an average rate of about 2 percent per year for the past 24 years. Research has documented chemical changes in surface waters, soils, trees, and other vegetation on the east side of the park. Shifts in species composition have occurred in lakes (diatoms), and in the tundra, studies show that excess nitrogen from atmospheric deposition has increased the type and number of grasses and sedges. This is a concern because grasses can outcompete and replace flowering plants as nitrogen accumulates in soils. Research into the past, using lakebed sediments as records of change, has shown the progression of atmospheric deposition of nitrogen, metals, and organic chemicals through time.

Researchers prepare to take soil samples in permanent vegetation plots in the alpine tundra.	This soil core sample from alpine tundra will be analyzed for different types of soil acidity and soil acid neutralizing capacity.

Dr. Baron and her research team use models to predict how long, at current or different deposition rates, before acidification occurs in the park. They also “hindcast” back in time to postulate changes in atmospheric deposition and effects since the year 1900. Park managers, Colorado Department of Public Health and Environment officials, and Environmental Protection Agency, Region 8 representatives are using this information to make science-based policy decisions to reverse current deposition trends and ultimately conserve the integrity of these fragile mountain ecosystems.