Undergraduate Honors Theses

Thesis Defended

Spring 2011

Document Type

Thesis

Department

Ecology and Evolutionary Biology

First Advisor

Dr. William M. Lewis, Jr.

Abstract

Watershed disturbances (a land area sharing a single drainage point for all surface and groundwater flow paths) are often accompanied by a significant increase in the export of nitrogen from the terrestrial to the aquatic ecosystem, mainly in the form of nitrate (NO3-). Unlike other watershed disturbances, infestation by the mountain pine beetle (Dendroctonus ponderosae Hopkins) is not associated with elevated nitrate concentrations in the stream channel (Leigh Cooper, Personal Communication). The lodgepole pine ecosystem of Colorado is nitrogen-limited (Fahey et al. 1985. Biogeochemistry 1: 257-275). Across a landscape, outbreaks of the mountain pine beetle leave a patchy distribution of surviving trees. Surviving trees are nonviable hosts to the beetle. Trees of a smaller diameter at breast height are not host to the beetle, as they do not provide a sufficient food source to developing larvae in the inner bark (Cole and Amman 1969 USDA Forest Service Research Paper: INT-95). Younger, smaller lodgepole pine and spruce that survive an outbreak are, therefore, resilient to mountain pine beetle infestation. Resilient trees are potential nitrogen sinks for any release of inorganic soil nitrogen as a result of neighboring tree death. Foliar nitrogen concentrations were used as a proxy for plant nitrogen uptake of surviving trees. Of variables estimated, neighboring tree mortality was a strong predictor of foliar nitrogen concentrations of surviving trees (p<0.001, R2 = 0.17). The data suggest increased nitrogen uptake by resilient trees in response to neighboring tree death. Across a watershed, resilient trees that are no longer competing for nitrogen with neighboring trees represent a nitrogen sink for available inorganic soil nitrogen, mitigating significant nitrate loss from the terrestrial to the aquatic ecosystem. Belowground measurements of soil nitrogen availability and the partitioning of nitrogen between the microbial and plan community could further explain the relative importance of resilient trees in nitrogen retention of watersheds affected by the mountain pine beetle.

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