Biotic interactions could play a large role in mediating species range shifts in response to climate change, yet their importance is often overlooked. For sessile organisms like plants, interactions with soil microbes and new competitors are likely important determinants of their successful establishment beyond existing range limits. However, for interactions between plants and soil microbes, we know very little of the spatial extent and temporal speed of the dynamics. This makes it difficult to incorporate it into projections of how plant ranges may shift under future climates. In the southwestern US, desert grasslands and shrublands are the two dominant ecosystems. The ecotone region where these two ecosystems meet presents an ideal opportunity to study the role of plant-soil feedbacks and competition in range edge dynamics for the two dominant species, creosotebush and black grama, as well as how these dynamics may shift under future climates. Among the range of climate factors in this system, increasing atmospheric CO2 concentrations is one that could have important cascading effects that we do not understand fully. The different photosynthetic strategies of the dominant plant species (C3 shrub vs. C4 grass) suggests that elevated CO2 could have asymmetric effects on the performance of each plant, as well as their carbon allocation belowground to microbial associates. In a collaboration with Dr. Jenn Rudgers, Dr. Scott Collins, and Prof. Catherine Harris at UNM, we are planning a series of field, greenhouse, growth chamber, and modeling experiments to understand how all of these factors interact to influence range edge dynamics at this grassland-shrubland ecotone. Currently, members of the lab are working on preliminary studies under the theme of this research area. PhD student Melanie Merritt is conducting experiments to understand the effects of CO2 and soil microbiota on creosote and black grama growth and root exudate composition. Postdoc Cristy Portales-Reyes is working on building and testing a individual-based model of ecotone dynamics.