Climate refugia, isolated areas of suitable habitat buffered from climate change, are increasingly threatened not only by rising temperatures but also by competition from warm-adapted species that are expanding their ranges. To safeguard climate refugia, we need to identify the climatic conditions that inhibit the ‘invasion’ of warm-adapted species into them. We studied boreal epixylic bryophyte communities and two southern species, Nowellia curvifolia and Hypnum cupressiforme. These two species are expanding poleward into epixylic communities, providing a natural system to test which climatic conditions restrict warm-adapted invaders and help maintain climate refugia. At 45 sites across the trailing edge of the boreal forest, we transplanted H. cupressiforme and N. curvifolia onto multiple logs and recorded their growth annually from 2022 to 2025. At each site, we also measured microclimate conditions and surveyed the species richness of bryophyte communities. We modelled yearly growth of both species as well as epixylic species richness as a function of the microclimate. We found that epixylic bryophytes, N. curvifolia, and H. cupressiforme occupied distinct microclimates: epixylic richness was highest in cold, wet sites; N. curvifolia grew most in warm, wet sites; and H. cupressiforme grew most in cold, dry sites. We recommend that forestry management prioritise practices that preserve cool, moist microclimates in these habitats, otherwise they risk becoming invaded by warm-adapted species should they become warmer or drier.