Date of Award:

5-2012

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Committee Chair(s)

Helga Van Miegroet

Committee

Helga Van Miegroet

Committee

Eugene W. Schupp

Committee

John Stark

Abstract

Certain forms of nitrogen (N) in the atmosphere are pollutants with effects that mimic fertilizer application. If there is too much N, it can become a stressor, and the ecosystem may undergo drastic changes (e.g. certain plant species may decline or disappear). The N load at which a system starts exhibiting negative effects is dependent on the type and location of the ecosystem. Alpine ecosystems (i.e. above 9000 feet in Wyoming) may be particularly sensitive to low levels of atmospheric N input because of short growing seasons, sparse plant cover, and shallow soils that limit their ability to absorb the extra N. It is therefore very useful to have early warning signs of changes in ecosystem N dynamics.

The National Atmospheric Deposition Program (NADP) monitors air quality and pollutant inputs with precipitation using instruments set up at various sites across the U.S., but there are only a limited number of NADP locations in the western U.S., with few locations at high elevation. Therefore, for many locations in the Rocky Mountains, N deposition is often modeled from the few available NADP monitoring sites. Even less is known about N deposition impacts on ecosystems in the Intermountain West, especially in the sensitive alpine ecosystems.

This study focused on N deposition effects on an alpine ecosystem located in the Grand Teton National Park. Modeling of N deposition for the Rocky Mountains has predicted a north to south gradient in the Grand Teton National Park. The objective of this project was to evaluate whether small changes in atmospheric N deposition had detectable effects on alpine plant communities and soils of Grand Teton National Park. First, we wanted to see if there was an actual N deposition gradient from north to south in the park by locating our measurements at three locations predicted to receive of high (Moose Basin), medium (Paint Brush Divide), and low (Rendezvous Mountain) N deposition. Secondly we wanted to investigate whether any of these alpine systems were already showing signs of excess available N. This was achieved by gathering information on how much N was coming from the atmosphere to each location, and by looking at various plant and soil parameters indicative of the N content.

Atmospheric N bonds to water molecules in the air and returns to the Earth with precipitation events such as rain and snow. Alpine ecosystems in the Grand Teton National Park receive most of their annual precipitation in the form of snow. Thus, snow distribution and relative melt rate dictates where the water accumulates in this landscape, creating areas that are either wet or dry in the summer months. This is important to plant and soil communities. Since atmospheric N inputs follow water, we thought that the wet areas would have different amounts of available N compared to dry areas, which in turn, would cause differences in plant and soil properties. To see if there were differences in wet versus dry areas, we compared ways that plants and soils use and/or store N across this presumed N deposition gradient. In summary, we followed N from the atmosphere, into the plants, then the roots, and then to the soil.

Our results show that the Tetons receive modest amounts of atmospheric N (< 2 kg ha-1 yr-1 ) mostly in winter (85%) with very small amounts coming in during the summer (15%, -1 yr-1 ) months. We confirmed that snow pack accumulation and snow melt created wet and dry sites, but these sites were not different in terms of N status.

At the three study locations in Grand Teton National Park, there were small, but significant differences in N availability that were expressed in plant and soil properties. Moose Basin (i.e., high N deposition) showed characteristics of an N-rich site having more N in the soil, more plant biomass, and more N in the plants, while Rendezvous Mountain (i.e., low N deposition) showed characteristics of an N-poor site with less N in the soil, and less ability to process N in the soil. Paint Brush (i.e. medium N deposition) shared similarities with both N-poor and N-rich sites.

This study shows that even small changes in atmospheric N input can cause fundamental changes in ecosystem characteristics. In order to detect these changes, it is important to look at both plant and soils together. We found that some characteristics such as total and extractable N in soils, soil nitrification potential, above and belowground biomass, and N content can be used as early warnings signs of ecosystem N overload.

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This work made publicly available electronically on June 4, 2012.

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