High mountain environments are often characterised by low temperatures and short growing seasons, yet support high plant endemism and biodiversity. While such ecosystems are considered among the most vulnerable to climatic warming, the impacts of climate change on diversity and composition can be complex. To develop a deeper understanding of these dynamics, changes in vegetation over time along five alpine summits that are part of the Global Observation Research Initiative in Alpine Environment (GLORIA), were assessed including species richness, α-diversity, β-diversity, vegetation and growth form cover as well as composition. The five summits of Mount Clarke in the Australian Alps were surveyed in 2004, 2011 and 2019. Despite increases in species richness over time, there was an overall decline in diversity through biotic homogenisation across the summits. Near complete vegetation cover was recorded in 2004 but increased over the 15 years via in-filling and densification, driven by increasing cover of graminoids and shrubs. Consequently, there was also a shift in species composition which was greatest at higher elevations. The results indicate that there has been a shift towards more competitive and thermophilic composition, which may have implications for flammability in a warming and drying climate. Further assessments will be required to more fully explore the effect of climate variation from climate change, and implications for conservation of this and other alpine floras globally.