A tale of two sides of the mountain in Rocky Mountain National Park

By Jacob Hagedorn, Univ. of Maryland Center for Environmental Science

This article, and others in the series "Parks in Science History", was written by a graduate student at the University of Maryland. The articles highlight the roles that national parks have played in the history of science and, therefore, the world's intellectual heritage. More articles and videos will be produced in the future.

Rocky Mountain National Park

NPS Photo / Crystal Brindle

How do ecosystems change over time and what causes those changes? This important basic science question can be difficult to answer because ecosystems are large and complex. And it’s not easy to perform experiments on something the size of, say, a mountain range. Unlike medicine in which the effect of a drug is determined by comparing groups of people who receive different treatments, it’s not always easy to compare two groups of ecosystems, half of which experience a ‘treatment’ and half that do not.

That said, scientists sometimes have success making such comparisons. And one early example featured a well-known national park.

In 1981 a group of scientists led by Dr. Jill Baron, an ecologist at the US Geological Survey (USGS), designed a study in Rocky Mountain National Park, in Colorado. Baron and her colleagues wanted to understand how mountain ecosystems respond to air pollution. The park was an ideal study site because the 13,000-14,000 foot high continental divide, which runs north-south, splits the park into distinct eastern and western halves. Air on one side of the divide rarely mixes with air on the other side, so effects of air pollution can be studied by comparing the two sides -- analogous to comparing two groups of patients.

Relying on gravity to filter water samples for nitrogen analysis

COURTESY BARON LABORATORY

On the west side of the park, the air is relatively pollution-free because there are not many urban centers or agricultural lands nearby or in upwind regions. But on the east side of the park, urban areas, industry, and agriculture inject various pollutants into the air. The central purpose of Baron’s study was to measure if and how the pollutants in the air changed the chemistry of lakes and soils. Baron chose to focus on nitrogen that gets onto a landscape via rain and snow because it is a major component of industrial and agricultural emissions and is easily measured in soil and water. Nitrogen is an essential nutrient for plant growth, and small amounts fertilize ecosystems. In larger amounts nitrogen contributes to acid rain. Neither fertilizers nor acid rain are welcome in national parks where ecosystems are protected in a natural state.

On both sides of the park, Dr. Baron and her team measured the concentration of nitrogen pollutants in air, soil, and lakes over many years. They found higher concentrations of nitrogen on the east side than the west side. It would have been easy to say that the only cause explaining these observed differences is nitrogen deposited by rainfall, but there are other processes that could result in increased nitrogen levels. Baron and her team explored these alternative explanations. For example, non-native trout introduced into lakes on the east side can raise nitrogen concentration in lakes through their feces. But the time lag between fish introduction and increased nitrogen in lakes suggests fish introductions were not the primary cause of the observed changes in nitrogen levels.

Dr. Baron and her colleagues examined numerous alternative explanations for the observed differences between the east and west side of the park, but their data ruled out those alternatives. The process that drives the increased nitrogen deposition on the east side is industrial and agricultural airborne pollution east of the mountains.

Dr. Baron’s study was one of the first to evaluate the effects of airborne nitrogen pollution at the large scale of a whole ecosystem. It also showed how ecosystems respond to small progressive changes in airborne pollution over time. This specific paper, as part of a long-term research program lasting decades, has been cited in other studies over 300 times and more generally Dr. Baron’s work has significantly contributed to the fundamental understanding of human impacts on biogeochemical cycling in mountain ecosystems.

Dr. Baron’s scientific contributions were made possible by the unique geography and atmospheric conditions of Rocky Mountain National Park. This park, like others, has played and continues to play an important role in the development of scientific knowledge generally.