Global change is creating novel communities of organisms around the globe as species shift their geographic ranges to maintain favorable climate conditions. Species range shifts create novel plant species interactions and influence relationships between plants and soil organisms in the new range. One well documented range shift in both arctic and alpine biomes is of woody plants, primarily shrubs, moving upwards in elevation or latitude and establishing in traditionally herbaceous dominated ecosystems.

Woody plant range expansion can have significant impacts on belowground ecosystem processes through changes in litter quality and quantity, rooting depth, and interactions with soil microbial communities. In addition, both biotic and abiotic soil conditions can limit the establishment and range expansion of woody plant species. The overarching goal of this dissertation was to understand how plant-soil interactions influence and respond to the range expansion of a sagebrush shrub in the White Mountains of California. I examined multiple belowground processes including soil microbial community structure and function, biogeochemical cycling, and soil feedbacks to other native plant species. I used field and greenhouse studies, plant demographic monitoring, laboratory assays, functional trait analyses, next generation sequencing and diverse statistical modeling techniques to link ecological processes from the microbial to ecosystem scale. Overall, sagebrush expansion increased bacterial diversity, soil moisture, soil organic carbon and nitrogen pools and microbial biomass, which were all associated with increased microbial activity (substrate induced respiration). Sagebrush did not strongly influence overall fungal diversity but increased the abundance of fungal groups including the saprotrophic Agaricomycete fungi, which are important wood and litter decomposers. Soil microbial communities influenced both the performance and leaf functional traits of sagebrush seedlings through changes in microbial diversity and extracellular enzyme activity, which suggested strong plant-microbial competition for soil nutrients. Finally, sagebrush influenced the demography of two co-occurring herbaceous plant species through negative plant-soil feedbacks which overwhelmed the effects of abiotic facilitation by sagebrush. Taken together the results of this dissertation provide a more complete picture of how multiple aspects of the soil environment both respond to changes in plant community composition, and feedback to influence plant performance and plant species interactions.