Aim: Here we report on a long-term transplant study of a serpentine endemic plant where individuals were transplanted into cooler macro- and microclimatic refugia (i.e. higher elevations and north-facing aspects) in locations outside of its current range. We asked: 1) Did transplanted populations persist outside of their current range in micro- (cooler aspects) or macro- (higher elevations) climatic refugia? and 2) Was population persistence in climatic refugia helped or hindered by soil microbial communities? Location: Siskiyou Mountains of southwestern Oregon (USA) Taxon: Horkelia sericata (Rosaceae), Angiosperms Mycota (Fungi) Monera (Bacteria) Methods: At each transplant site, we counted surviving individuals (noting reproductive status) and then collected soil from both the rhizosphere of transplanted individuals and from an equal number of areas of nearby bare soil with no plants. Soil bacterial and fungal communities were assessed using next generation sequencing of 16S and ITS-1 marker genes Results: Of the 15 initial transplant sites, one (high elevation) site displayed population persistence (i.e., ‘successful’ site), defined as having surviving individuals with reproductive success. Four sites had surviving individuals but no reproductive success (i.e., ‘unsuccessful’ sites); the remaining 10 sites had no surviving individuals and were excluded from microbial analyses. The successful site had distinct soil fungal and bacterial community composition and a higher mututalist:pathogen ratio than the unsuccessful sites. Additionally, the mututalist:pathogen ratio did not differ between Horkelia’s rhizosphere and bare ground at the successful site, suggesting that the persistence of this population was enhanced by soil mutualists that were already present at that site. Main conclusions: Taken together, these results highlight that the success of species range shifts into climatic refugia may be influenced by the presence of suitable soil mutualists, emphasizing the need to consider soil microbial communities in future ra