RangeX seeks to better understand the processes and impacts of plants that are expanding their ranges following climate warming, and to use this knowledge to inform the development of policy regarding range- expanding plant species. We focus on mountain ecosystems as an ideal model system to address our research questions. Mountains are themselves of crucial conservation value, as hotspots of biodiversity, refugia for biota threatened by climate warming, and as key global sources of water, food, and livelihoods, but are experiencing above-average rates of warming and increasing pressures from invasive species and development, making mountains priority areas for sustainability research.