Population Persistence and Soil Microbial Communities of a Serpentine Endemic Plant Outside Its Historic Elevation Range.

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 describe: (1) how transplanted populations persisted outside of their current range in micro- (cooler aspects) or macro- (higher elevations) climatic refugia; and (2) soil microbial communities that may have helped or hindered population persistence in climatic refugia. Location: Siskiyou Mountains of southwestern Oregon (USA). Taxon: (Rosaceae), Angiosperms; Mycota (Fungi); Monera (Bacteria). 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. 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 (alpha and beta diversity) and a higher mutualist:pathogen ratio than the unsuccessful sites. Additionally, the mutualist:pathogen ratio did not differ between rhizosphere and bare ground at the successful site, suggesting that the persistence of this population was potentially enhanced by soil mutualists that were already present at that site. Taken together, these results highlight that the success of species range shifts into climatic refugia may be influenced by the presence of suitable soil microbial communities, with a potentially outsized role of mycorrhizal mutualists, emphasizing the need to consider soil microbial communities in future range predictions of highly specialized plants such as serpentine endemics.