This thesis includes three studies. The first study examined how reductions in light availability and changes in soil chemistry that occur as conifers establish in aspen stands, differentially affects the regeneration success of aspen and conifers. We found that aspen were more sensitive to changes in light and soil then subalpine fir. For aspen, reduced light and conifer influenced soils significantly reduced height, biomass, photosynthesis and the production of secondary defense compounds. Subalpine fir seedlings were significantly reduced in photosynthesis, biomass and R:S under lower light conditions but showed no differences in physiology or growth when grown on the contrasting soil types. Subalpine fir seedlings were significantly reduced in photosynthesis, biomass and root:shoot ratio under lower light conditions but showed no differences in physiology or growth when grown on the contrasting soil types. Results from this study suggest that reduction in light and changes in soil chemistry associated with conifer succession place constraints on aspen growth and defense capacity, which may contribute to losses in aspen cover under longer disturbance return intervals. The second study looked at regeneration dynamics of aspen and conifers as forest stands transition from canopy gaps to aspen dominated canopies to conifer dominated canopies. We found that as overstory conifer density increases, aspen decrease in density, basal area, and seedling establishment. Conifers were shown to establish closer to aspen as the canopy increased in conifer density. As this proximity relationship extended into the canopy there is increased mortality in both aspen and subalpine fir, suggesting both facilitation and competition. Our third study looked at the physiological effects of smoke exposure on growth and primary and secondary metabolic responses of deciduous and conifer tree species. Twenty minutes of smoke exposure resulted in a greater than 50% reduction in photosynthetic capacity in five of the six species we examined. Impairment of photosynthesis in response to smoke was a function of reductions in stomatal conductance and biochemical limitations. In general, deciduous species showed greater sensitivity than conifer species. Smoke had no significant affect on growth or secondary defense compound production in any of the tree species examined.

Widespread mortality of quaking aspen (Populus tremuloides Michx.) has occurred over large expanses of the Western US during the 20th century. While much of this decline was due to conifer encroachment into seral aspen, significant aspen losses also occurred in areas of persistent aspen and may have been exasperated by drought conditions. Aspen decline has been especially notable at Cedar Mountain, Utah, an area of mostly private land and extensive persistent aspen coverage. The objectives of this study were to create a time series of live and dead aspen cover on the Cedar Mountain landscape, using remotely sensed imagery, and to test whether water stress correlated to the decline therein. To accomplish these objectives, a decision tree classifier was used to classify the Cedar Mountain area into live and dead aspen cover classes for the years 1985, 1990, 1995, 2001, 2005, and 2008. Thereafter, post-classification change analysis was performed to determine areas and time periods of elevated decline. Regression analyses were performed to ascertain correlations between climatic data and percent change in aspen cover. A topographic analysis using zonal statistics was also performed to determine landscape positions where aspen decline is more prevalent. The time series models indicated that aspen decline followed a step-wise pattern with an overall decrease of 23.57 % in aspen cover during a 23-year period. Considerable aspen decline occurred early in the study time frame, with decreases of 1.38 and 1.36 -1 in 1990 and 1995, respectively. The middle period between 1995 and 2001 had no net change in aspen cover. However, the end of the time series showed the greatest decline with decreases of 1.56 and 1.99 % yr-1 in 2005 and 2008, respectively. There was a correlation between percent change in aspen cover and precipitation, suggesting that drought weakens aspen, making it susceptible to future decline. The topographic zonal statistics revealed that drier landscape positions had greater frequencies of dead aspen. The most significant predictor of aspen decline was elevation, which was significantly greater in the live aspen for three of the five years.

Aspen is a key resource in the Rocky Mountain Region for wildlife forage and habitat, lumber products, scenery, and plays important roles in fire ecology and hydrological processes. There is evidence of aspen decline over much of the Intermountain West for approximately 100 years. In Dixie and Fishlake National Forests, UT, aspen distribution has decreased by nearly half. Causes of this decline are not well understood, although wildlife browsing by ungulates has been implicated as playing a major role. The objective of this research was to examine what soil or plant factors might be involved in wildlife browse choice in aspen. Twenty-two pairs of moderately and intensively browsed sites were studied to identify factors related to browse preferences over two field seasons. In the summer of 2008, sites were sampled in June, July, and August, and in the summer of 2009 sites were sampled in August only. Soils were analyzed for pH, EC, total nitrogen and carbon, and mineral nutrients. Leaf tissue samples were analyzed for defense chemical (tannin and phenolic glycoside) concentrations, mineral nutrients (via acid digestion), acid-detergent fiber, water content, carbon:nitrogen ratio, and non-structural carbohydrate (sugar) concentration. No significant difference in phenolic glycoside concentrations between moderately and intensively browsed sites was found. Tannins were highest in sites with intensive levels of browsing. Iron was significantly higher and zinc lower in intensively than moderately browsed sites. Leaf moisture was also significantly lower in intensively browsed sites. In the absence of differences in phenolic glycosides, ungulates may be selecting browse sites based on iron requirements. Seasonal changes in the studied factors could be identified in 2008. Over the course of the summer, we found significant decreases in nitrogen, phosphorus, potassium, sulfur, zinc, iron, copper, phenolic glycosides, and moisture concentration. Seasonal increases in calcium, sodium, tannins, sugars, acid-detergent fiber, and carbon:nitrogen ratios were observed. The need for large ungulates to obtain specific nutrients may indicate that aspen is in higher demand as a forage at different times of the year, particularly in areas with forages low in these nutrients. Our data suggest that aspen high in iron may be at risk since other factors explaining browsing choice were not significantly different in our study. This information can help identify clones that are at risk and direct resources where and when they are needed most.

This thesis includes two studies: The first examined developmental changes that take place in the physiology of aspen (Populus tremuloides Michx.) and to characterize developmental influences on patterns of phenotypic trait variation among different aged ramets within the aspen clones. We surveyed eight clones, each with 8 distinct age classes ranging from 1 to 170 yrs in age. Using regression analysis we examined the relationships between ramet age and expression of functional phenotypes. Eight of the phenotypic traits demonstrated a non-linear relationship in which large changes in phenotype occurred in the early stages of ramet development and stabilized thereafter. Water and nutrient concentration, leaf gas exchange and phenolic glycosides tended to decrease from early to late development, while sucrose and condensed tannin concentrations and water use efficiency increased with ramet age. We hypothesize that ontogenetically derived phenotypic variation leads to fitness differentials among different aged ramets, which may have important implications for clone fitness. Age-related increases in phenotypic diversity may partially underlie aspen's ability as a species to tolerate the large environmental gradients that span its broad geographical range. Fire is an essential component of many forest ecosystems and fire exclusion policies and other anthropogenic factors have significantly altered disturbance regimes, which has lead to increased aspen succession to conifers. The second study examined how post-fire aspen regeneration success is influenced by increasing conifer abundance under longer fire return intervals. 66 sites were selected from the Sanford prescribed fire complex located in the Dixie National Forest. Slope, aspect, sucker regeneration heights, soil samples, and post and prefire stand densities were measured. Results from this study demonstrated that pre-disturbance conifer abundance and aspen densities are good predictors of aspen sucker regeneration success. Results also found that although conifer densities don't change across aspects, aspen densities are different on north facing slopes. We hypothesize the high levels of aspen regeneration came from a large disturbance size which overwhelmed the high levels of herbivores.