Altitudinal range shifts due to global warming create novel plant species communities and interactions, which have been shown to be stronger pressures on the autochthonous alpine plant communities than warming itself. However, it seems unlikely that this effect simply stems from the lowland identity of the colonizing species but rather from the morphological and functional characteristics of certain colonizing plant species that render these superior competitors in a warmed alpine environment. To investigate this hypothesis, we performed a full-factorial warming and transplant experiment in four subalpine grassland sites in Southern Norway, using OTCs to warm vegetation plots by ca. 0.8 °C and individual transplants of three lowland species with novel and more competitive trait values compared to the autochthonous alpine vegetation and three lowland species with extant alpine trait values. In the vegetation plots, we monitored the plant community composition as well as the population dynamics of two alpine target species of low competitiveness, Veronica alpina and Sibbaldia procumbens. We found that the overall plant community at the plot-scale increased in richness in the presence of warming and lowland colonizers, irrespective of their traits, as some species became more common while some few, rare species disappeared. For our target alpine species populations, warming alone was positive but the combination of warming with lowland colonizers was detrimental, and especially so with novel traits colonizers. These results point in the direction of a homogenization of alpine vegetation under warming and colonization, leading to a plot-scale increase of richness, and thus masking the disappearance of species which are outcompeted especially by colonizers with novel and more competitive traits.