Big Sagebrush (Artemisia tridentata) in a Shifting Climate Context: Assessment of Seedling Responses to Climate

The loss of big sagebrush (Artemisia tridentata) throughout the Great Basin Desert has motivated efforts to restore it because of fire and other disturbance effects on sagebrush-dependent wildlife and ecosystem function. Initial establishment is the first challenge to restoration, and appropriateness of seeds, climate, and weather variability are factors that may explain success or difficulties in big sagebrush restoration efforts. This project provided several ways of assessing climate responses of big sagebrush seedlings during the critical establishment phase post-fire. We evaluated eleven different seed sources of big sagebrush from all three subspecies, dissimilar climates-of-origin, and different ploidy levels to assess how subspecies, cytotype, and climate-of-origin affect initial establishment of sagebrush in a common garden study. We assessed ecophysiological-climate adaptation as it relates to seedling performance using a suite of dependent variables, including: survival, growth, water balance, photosynthesis, and threshold freezing responses. Results indicate the importance of minimum temperatures to seedling establishment, and reveal a gradient of physiological responses to freezing that inform big sagebrush adaptation and functional diversity. We then used in-situ experimental warming to isolate minimum temperatures, and test the effects of warming on seedling physiological performance for the three dominant subspecies of big sagebrush: A.t. tridentata, A.t. vaseyana, and A.t. wyomingensis. Experimental warming further supported our minimum temperature hypothesis, indicating that warming may alter seedling freezing response thereby affect growth and survival. In a third experiment, we evaluated how initial establishment of big sagebrush is influenced by management treatments on the herb layer, as post-fire rehabilitation frequently involves alterations of the plant community and soil. Results suggest that drill seeding combined with land management treatments that cause disturbance of the herb layer and soil surface may negatively affect sagebrush during the establishment phase. Also, seedlings from local seed or faster-growing populations had greater survival than seedlings from climates that differed from the experimental site. In summary, we provide experimental evidence for the importance of minimum temperatures and seed sources to big sagebrush ecology and management of sagebrush systems. As the climate warms, selection for population-specific freezing resistance mechanisms may alter subspecies distributions. Our data indicated that warming could increase relative abundance of A.t. tridentata compared to A.t. wyomingensis at our Birds of Prey National Conservation Areas study site on the lower Snake River plain. The underlying mechanism for this is greater stress overcome by changes in resource allocation from freezing protection to growth, as well as an extraction of deeper soil water resources in A.t. tridentata. Mortality of A.t. vaseyana appeared to relate to drought stress and greater vulnerability to minimum temperature exposure. Understanding differences in big sagebrush populations’ ability to compete with different types and abundances of herbs as well as variation in freezing resistance mechanisms will contribute to appropriate seed selection for particular restoration sites. The implication is that selection of seed is critical for big sagebrush restoration success