This dissertation explores the differential effects of an ungulate community (cattle, mule deer, and American elk) on aspen (Populus tremuloides) regeneration pre- and post-fire disturbance. It's first chapter examines the differential effects of cattle, mule deer, and American elk on aspen regeneration across variable topography, climate and forest type; aspen dominant to conifer dominant subalpine forests. We found that each ungulate species, if sufficiently numerous, can cause aspen regeneration failure. Also, high elevation, southern aspect and high winter snow pack increase resilience to ungulate herbivory. Chapter two compares the efficacy of four methods of estimating ungulate impact on aspen. We found that direct methods of estimation (meristem removal, and defoliation) are better indicators of ungulate impact than indirect methods (fecal and camera counts). Therefore, we suggest to management that removal of apical meristems be an indicator for relative ungulate use. In chapters 3 and 4 we use differential ungulate exclosure fencing similar and camera traps to model the year to year and monthly use of aspen by ungulates. We model the per unit animal impact on aspen using photo counts. Our results show that ungulates on a per animal basis utilize aspen similarly. However, when adjusted for body size, native ungulates select aspen more often than cattle. Also, we found that while aspen is more palatable in June it is relative use, as measured by removal of apical meristems is significantly higher throughout July and August. We conclude that aspen is relatively more palatable than grass and forb species late season, and becomes and important forage species late season when forbs and grass species nutrient content drops. In chapter five, we quantify the physiological effects of ungulates on aspen regeneration after fire in order to elucidate the physiological mechanisms underlying plant:animal interactions. We show that ungulate herbivory induces high concentrations of defense chemistry (Phenolics), and reduces palatability through lower foliar nonstructural carbohydrates, and that this high investment in defense and lower growth potential related to low palatability causes severe reductions in aspen vertical growth leading to aspen regeneration failure. Together, we examine how biotic and abiotic factors at the landscape level, as well as defense and physiological function at the leaf chemistry level mediate how ungulates influence aspen regeneration and recruitment

Human activities and rapid global climate change are altering fire regimes with potential threat to the stability of aspen ecosystems in North America. Aspen is an early successional species that plays an important role in post-fire forest reestablishment, but chronic browsing on juvenile aspen by large ungulate herbivores after fire can be detrimental and lead to regeneration failure. Although larger and more severe fires are expected to become more prominent, whether and how this may influence aspen and ungulate communities remains unclear. The objective of this research was to examine how the relationship between aspen and ungulate communities might be influenced by variation in fire severity and size. In 2012, we examined browse patterns, growth responses and defense chemistry (phenolic glycoside and condensed tannins) concentrations of regenerating aspen that experienced variable burn severity in the 2010 Twitchell Canyon Fire, Utah, USA. We found that greater light availability in higher severity burn environments enhanced aspen tolerance and resistance against herbivory by increasing growth potential and defense chemistry concentrations of aspen. These results suggest that burn severity influences plant-herbivore interactions through bottom-up and top-down forces, and that higher fire severity increases post-disturbance vegetation recruitment potential by increasing resilience to herbivory. In 2013, we characterized aspen and ungulate patterns of 25 fires that spread across five National Forests (Uinta-Wasatch-Cache NF, Ashley NF, Fishlake NF, Dixie NF, and Manti-La Sal NF) in the state of Utah. We identified interaction effects between fire size and severity that strongly influenced aspen and ungulate densities. Fire size and severity are important ecological filters that can interact to affect forest reestablishment and community response. This information is useful in developing decision-making tools for wildfire and ungulate management that can more effectively increase the long-term resilience of forests systems.

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.