Impacts of a Mixed Ungulate Community on Aspen Forests: From Landscape to Leaf

• Main Author: Rhodes, Aaron C.
• Format: Others
• Published: BYU ScholarsArchive 2017
• Subjects: Populus tremuloides; mule deer; American elk; cattle; herbivory; defense chemistry; Plant Sciences
• Online Access: https://scholarsarchive.byu.edu/etd/6615; https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=7615&context=etd

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