Soil weathering under mountain pine beetle killed trees, Grand County, Colorado

Denison, Christopher W., author
Sutton, Sally, advisor
Ridley, John, committee member
Stednick, John, committee member
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The objective of this study is to assess differences in soil weathering processes under mountain pine beetle killed trees, as compared to soil weathering under living trees. As pine beetle (Dendroctonus pondersoae) infected trees die, pine needles are shed and accumulate on the forest floor, which may lead to changes in soil pH and soil temperature as the organic horizon thickens and insulates the soils below. Additionally, decomposition of the dropped needles may cause chemical changes in the substrate. These changes in soil pH, temperature, and chemistry are likely to affect weathering of soil minerals. Two hypotheses related to soil weathering processes under beetle infected trees are evaluated: (1) the death of pine trees and accompanying increased pine needle decomposition has increased chemical weathering of the substrate, and (2) an increase in soil weathering under dead pine stands has increased downslope physical migration of weathered material. This study was conducted in the Kauffman Creek watershed in the mountain pine beetle infected Arapahoe National Forest of Grand County, Colorado. Soil samples were collected from a south facing hillslope and from elevations of approximately 9100 to 9400 feet, thus minimizing differences in weathering processes related to hillslope aspect or elevation. Kauffman Creek incises mountainous terrain and the study area is hosted by Paleocene - Eocene sedimentary rocks of the Coalmont Formation. Soils of Kauffman Creek are predominately inceptisols and entisols. The field site was chosen to show a range in pine beetle infestation and health conditions of pine trees on the hillside. On the hillslope there are stands of healthy (green) or recently attacked (brown) pine trees and there are other areas where the pines are in the final stages (gray) of beetle infestation (4+ years post attack) and have dropped most of their needles. A study of soil characteristics (i.e. grain size, inorganic geochemistry, mineralogy, pH, and saturated paste electric conductivity) was conducted to evaluate soil weathering processes. Analytical results indicate that the mean grain size is approximately 1.3 times coarser beneath the gray stands than beneath the healthy-appearing, green pine stands. Major element geochemistry shows average concentrations of Na and K are greater, and average concentrations of Mn and Mg are smaller, in soils beneath the gray pine stands than from those under green stands. The differences in soil chemistry within the soils beneath the gray stands, compared to the soils beneath the green pine stands, suggest increased chemical weathering of soil under the gray stands. Chemical results for soils under the brown stands suggest they also have experienced increased weathering, compared to the green stands. Petrographic results show that the modal percentage of quartz is approximately 1.2 to 1.4 times greater in the soils beneath the gray stands than in the soils under the green stands, while the modal percentages of soil aggregates and micas decrease from the soils under the green stands to the soils under gray trees. The average topsoil pH is lower in the soils beneath the brown and gray trees than in the soils beneath the healthy pines. Soil conductivity data suggests an increase in soil moisture under the brown and gray pine stands. Overall, increased pine needle litter and its decomposition appear to have increased soil weathering. Downslope migration of weathered material was not evident in the results of this study.
2013 Spring.
Includes bibliographical references.
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