Abstract
In the western United States Populus tremuloides Michx., hereafter referred to as aspen, is considered an important forest cover type because of its contribution to local and regional biodiversity, as well as its use for habitat and food for a variety of large and small mammals, songbirds, and game birds. These critical ecosystem services provided by aspen have made many ecologists and resource managers concerned with recent trends of aspen decline across the western U.S. Many ecologists have for decades predicted a trajectory of disappearance while more recently others have suggested that aspen persistence is contingent on local disturbance regimes, management, and or geographical location. The greatest threats to aspen persistence in the West have been identified by researchers as competition with succeeding conifers, browsing by ungulates, and acute drought. In this dissertation, I investigate the relationship between aspen persistence and regeneration ecology with these identified threats focusing primarily on drought and soil moisture but accounting for successional conifer density and browsing pressure. My efforts focus on aspen stands in the Caribou-Targhee National Forest (CNF). In chapter 2, I explore the importance of precipitation as a proxy for potential soil moisture limitation on aspen growth across the CNF using dendrochronological methods. Specifically, I applied a new dendrochronological technique, Blue Intensity (BI), for the extraction of a climate signal in aspen latewood to explore the importance of mid-summer precipitation on aspen growth. Blue intensity is an effective and inexpensive proxy for wood density that has been found to correlate more accurately with climate factors (precipitation, temperature) than ring width, especially in latewood. The results of this analysis showed a positive correlation between mid-summer precipitation and latewood density that supports the first part of my hypothesis; aspen growing on sites with expected soil moisture limitations will produce a climate signal that correlates with precipitation; and the second part of my hypothesis that this signal reveals latewood development is driven by and dependent on soil moisture availability based on precipitation. From these results I conclude that late season growth for aspen on exposed high elevation sites is dependent on precipitation and growth is thus limited by soil moisture availability. In Chapter 3, I investigate the relationship between soil moisture availability and aspen stand persistence by comparing the structure, composition, and regeneration densities of nine aspen stands with nine paired upland aspen stands. Considering the numerous studies published in the last two decades that show evidence of drought being the main inciting factor of aspen decline on xeric sites, I hypothesized that the proximity to perennial streams will lead to higher soil moisture availability and thus increase the probability of aspen persistence on the Caribou-Targhee National Forest landscape. My results for this study partially supported my hypothesis that riparian areas support aspen stands that show evidence of a higher probability of persistence relative to upland aspen stands. The most compelling evidence comes from the significantly higher aspen regeneration densities at the seedling layer (< 1m height) on average in riparian areas relative to upland aspen stands. In the sapling layer (>1m height) aspen regeneration densities were still considerably higher in the riparian areas than in the upland areas but this difference was not significant. In this study I used the term “seedling layer” and “sapling layer” to refer to aspen regeneration in different height classes and did not differentiate between regeneration originating from seed versus regeneration originating from suckering. Building off of the results in chapter 3 of significant differences in the density of aspen in the regeneration layer between riparian and upland aspen stands, in chapter 5 I developed a model path analysis using structural equation modeling to explore the relative impacts and interactions of physiographic and ecological factors that affect aspen regeneration densities. Before any analysis of the direct factors influencing soil moisture availability on aspen regeneration, it was important to characterize the clonal diversity of each site. To do this, in chapter 4 I used a traditional approach, with modern techniques, based upon leaf morphology to determine the clonal diversity on each site to be used in a structural equation model (SEM). Overall, I found that there were differences in mean clonal diversity between riparian and upland aspen stands, with riparian stands tending to have higher clonal diversity. The results of the SEM analysis (chapter 5) support my hypothesis that factors affecting soil moisture availability have the strongest effect on regeneration. Specifically, the exogenous factor with the strongest direct effect on aspen regeneration was incident radiation (heatload), and the endogenous factor with the strongest direct effect on regeneration was the percent cover of competitive plant species. Overall, the results of each chapter support my global hypothesis that factors reducing soil moisture availability and increasing site susceptibility to drought have a negative effect on aspen growth and regeneration. My results, however, also emphasize that context is important in determining the collective effect of these factors. Thus, proper assessment of aspen vulnerability in the West requires analyses at multiple scales that can incorporate the relative weights and interactions of elements influencing aspen persistence.