Climate warming and its interactions with other drivers of global change is impacting the biodiversity and functioning of plant communities globally. Species are shifting their distributions in response to global warming and through biological invasions, many expanding their ranges across elevation and latitudinal gradients. But they are doing so at widely different rates, leading to both a taxonomic and functional reassembly of ecological communities. Community reassembly could have even more profound, yet until now poorly understood, impacts on the future of biodiversity, ecosystem functioning, and nature's benefits to people. In particular, changes to species interactions in novel communities could mediate effects of climate warming on biodiversity, and on key ecosystem functions like carbon cycling and pollination, which could in turn feedback to climate warming. To anticipate and respond to these challenges and opportunities, we require both a process-based understanding of range expansions, and a shared understanding of this issue between researchers, natural resource managers and policy-makers. To reach these overarching objectives, RangeX asks four key questions: 1. What processes explain variation in species’ range expansion with climate warming? 2. How do range-expanding species impact biodiversity and key ecosystem functions, and how will these impacts feed back to changing climate? 3. Can we predict which species are most/least able to expand their ranges with climate warming, and their impact on ecosystem processes? 4. Can better ecological understanding of range expansions be synthesised with stakeholder knowledge to improve policy and management of range expansions and biodiversity? To tackle these questions we focus on elevation gradients in mountains, as both ideal natural laboratories to test hypotheses about the proximate drivers and impacts of range expansions, and potentially highly suitable “early warning systems” to detect the changes in biodiversity and ecosystem functioning. We adopt a hierarchical approach, nesting detailed experimental and comparative studies within a global network of researchers and managers using standardized methodology to study plant range expansions in mountains (the Mountain Invasion Research Network; MIREN). In this way we will tease-apart mechanisms underlying range expansions and their impacts, and interrogate their generality by performing analyses that integrate our experimental results with detailed survey data available from up to 22 mountain regions around the world. We will work with national and international stakeholders through participatory workshops and surveys to synthesise knowledge and co-develop a joint problem understanding of range-expanding species, and co-produce outputs and targeted dissemination strategies to improve awareness and integration of range-expanding species in regional and global policy.