Air Pollution

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Niwot LTER researchers Isabel Ashton and John Murgel measure species composition in plots near Chapin Pass, Rocky Mountain National Park, as part of a project comparing vegetation responses to N deposition in the park with NWT LTER. Results of this project are informing land managers involved with determining limits on emissions of reactive N in Colorado.
Bill Bowman, NWT LTER

Alpine environments are sensitive indicators of air pollution. By combining monitoring of high-elevation ecosystems and field experiments, NWT scientists have determined that current levels of nitrogen pollution associated with industry and agriculture are altering alpine plant diversity and are polluting lakes and streams, and may soon acidify soils.

Long-term ecological measurements and field experiments by NWT LTER scientists have shown that high-elevation ecosystems are particularly sensitive to air pollution. NWT LTER is an alpine ecosystem located in the Colorado Rocky Mountains and extends to more than 4,000 m at the Continental Divide. While remote from emission sources of air pollution, the harsh environmental conditions characteristic of these alpine environments suggest that organisms in high-elevation catchments are notably sensitive to air pollution when compared to lower-elevation areas near the pollution sources.

Emissions of reactive nitrogen (N) by agricultural development and fossil fuel combustion have increased the deposition of N on a global basis more than threefold over the past 160 years. The environmental impact of these N inputs on terrestrial ecosystems varies according to vegetation type, chemistry of the soil parent material, and the rate and accumulated input of N deposition. Enhanced net primary production may occur where N remains the limiting resource for plant growth. Greater N availability often results in losses of plant diversity and greater establishment of invasive species. Once plants, microorganisms, and soils cease to take up N entering system from deposition, leaching of nutrient base cations, increases in soluble aluminum, and soil acidification can occur, eventually leading to decreases in plant growth and further losses in diversity.

Symptoms of ecological changes due to N deposition are showing up in more remote regions, including national parks and wilderness areas in the Rocky Mountains and Sierra Nevada in the U.S. Mountain ecosystems are especially sensitive to inputs of N deposition due to thin soils and low rates of net primary production, limiting the uptake and stabilization of reactive N. Once thresholds of ecological change are reached reversibility of the changes in diversity and soil neutralizing capacity are uncertain and probably limited. Thus it is important to develop metrics to predict and detect ecological change to prevent the onset of undesirable conditions such as soil and surface water acidification.

We have used a combination of experimental N additions and long-term monitoring plots to empirically predict and detect ecological responses to N deposition in the alpine on Niwot Ridge, Rocky Mountain National Park, and the Western Tatra Mountains of Slovakia. These studies have determined that

  • Biodiversity of alpine ecosystems in the Front Range of the Rocky Mountains are changing as a result of current rates of deposition
  • Doubling of the current rates of deposition and/ or long-term sustained deposition at current rates will initiate acidification of alpine soils with granitic parent material
  • Greater mobilization of toxic aluminum and iron will occur with sustained inputs of N. The results of this research have been used by the National Park Service and EPA to consider air quality standards in Colorado.
Cover (a, symbols are means+s.e.m., n=5) and rate of change in cover (b, each plot represented by a triangle) forCarex rupestrisin an alpine dry meadow in Rocky Mountain National Park subjected to ambient deposition (control), and additions of 5, 10, and 30 kg N ha-1 yr-1. The extrapolation of the line to the x-axis in b provides an estimate of the threshold of N input at which significant increases in cover ofC. rupestrisoccurs due to N deposition. (from Bowman, WD, J. Murgel, T. Blett, and E. Porter.
In rev. Journal of Environmental Management) Nitrogen Critical Loads for Alpine Vegetation and Soils in Rocky Mountain National Park)
For further reading: 
Lieb, A.M., A. Darrouzet-Nardi, and W.D. Bowman. 2011. Nitrogen deposition decreases acid buffering capacity of alpine soils in the southern Rocky Mountains. Geoderma 164: 220-224.
Bowman, W.D., C.C. Cleveland, L. Halada, J. Hreško, and J.S. Baron. 2008. Negative impact of nitrogen deposition on soil buffering capacity. Nature Geoscience 1: 767-770
Bowman, W.D., J.L. Gartner, K. Holland, and M. Wiedermann. 2006. Nitrogen critical loads for alpine vegetation and terrestrial ecosystem response - Are we there yet? Ecological Applications 16: 1183-1193.
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