Climate warming has increasingly severe impacts on sensitive mountain ecosystems. Many low- and mid-elevation species are shifting their climatic niche to higher elevations. This shift is directly affected by warming and indirectly by biotic interactions as the extant vegetation may hinder the invasion of novel species. In the RangeX project, we combine an upslope transplantation of lowland plants with experimental warming using Open Top Chambers (OTCs) to experimentally accelerate and disentangle these processes. The aim is to understand the mechanisms behind range expansions and their impact on the alpine ecosystem. We studied the reproductive phenologies of the lowland range expanders to directly measure their success under current or future climates and biotic conditions. Specifically, we recorded the amount and timing of reproductive structures (buds, flowers, infructescences) throughout one growing season. Flowering start, duration, peak and end can reveal important aspects regarding the competition with the native vegetation and local pollinators. Here, results of the Norwegian and Swiss sites are presented. Upslope transplantation delayed onset of flowering, but unexpectedly, experimental warming did not advance phenologies of the transplants in the cold site. Competition decreases the number of reproductive units produced, independent of transplantation and warming. A general pattern shows that competition outweighs temperature in shaping the successful performance of lowland species in novel climatic conditions. These insights can be used to predict who the winner and loser species of the future will be.