Date of Award:

8-2012

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Wildland Resources

Committee Chair(s)

Karen E. Mock

Committee

Karen E. Mock

Committee

James N. Long

Committee

Paul G. Wolf

Abstract

Quaking aspen has the largest natural distribution of any tree native to North America, ranging from Alaska through the breadth of Canada and south to mid-Mexico. Recent studies suggest a general decline of aspen throughout much of the range since at least the mid-20th century, though these findings remain inconclusive. Regardless, factors such as climate change, periods of drought, soil nutrient deficiencies, pathogens, insects, and encroachment by other tree species all pose risks to the health and maintenance of aspen across the continent. This situation is exemplified in the western United States where climate change is forecasted to have an extreme impact on the availability of suitable habitat for aspen. Facing all of these potential challenges, it is important for aspen to be able to adapt to changes in order to persist on the landscape. A critical factor in aspen’s ability to adapt to change is genetic diversity. Surprisingly, range-wide patterns of genetic variation in aspen have never been described. Using a sample set representing the full longitudinal and latitudinal extent of aspen’s distribution, I have assessed levels and patterns of genetic diversity in the species. To do this, I used a set of highly variable molecular markers, known as microsatellites, to identify individuals based on unique genotypes. I identified two genetically distinct clusters. One cluster is made up entirely of individuals from the western United States and Mexico, the southwestern portion of the range. This cluster also displays significantly less within-site genetic diversity than is seen in the rest of the range but an increase in regional diversity for the Southwest as a whole. This pattern of genetic diversity coupled with the detection of a distinct genetic cluster in the Southwest suggests that not only do southwestern populations represent a stable edge that has persisted through multiple climate oscillations, but also the presence of a distinct aspen ecotype. I was also able to identify “hot spots” of genetic diversity which, combined with knowledge of glacial history, sheds light on potential glacial refugia and routes of postglacial recolonization. This information will be important as we move toward understanding how aspen responded to dramatic climate fluctuations in the past, and to predict how the species will respond to climate change in the future.

The results of this study are important in designing long-term conservation strategies for aspen. My results suggest that a high conservation value can be placed on the “hot spots” of diversity in order to preserve diversity for the range as a whole. Also, the finding of a stable edge and distinct ecotype in the Southwest highlights the need for unique conservation strategies for populations in this region.

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This work made publicly available electronically on July 30, 2012.

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