The angiosperm family Orchidaceae has the highest number of rare taxa with limited distributions due to their specialized ecological requirements including mycorrhizal symbiosis. Understanding their ecological niche requirements that could drive plant distributions and population persistence, especially under the changing climatic conditions, is fundamental for both plant ecology and conservation. We used an orchid occurring in the coastal scrub habitat within the California Floristic Province to ask: 1) Do the specialized interactions with mycobionts drive the rarity of an orchid? and 2) Does the mycobiont diversity change through the phenological development of the species? Orchid Mycorrhizal Fungi (OMF) were sampled from the roots of Platanthera (Piperia) cooperi to represent both the peninsular (PLF and PLE) and island (SCE and SCW) populations. Roots were sampled from seedlings, vegetative, and flowering individuals across two years at each of the four sites to amplify fungal nuclear ribosomal internal transcribed spacer (nrITS) locus for quantification and comparison of OMF communities.
We generated 190 sequences from roots of 83 individuals; 155 sequences were identified as Tulasnellaceae, and 35 were identified as Ceratobasidiaceae. Altogether, twenty-one operational taxonomic units (OTUs) were recovered, of which eight belonged to Tulasnellaceae and 13 belonged to Ceratobasidiaceae. Both Tulasnellaceae and Ceratobasidiaceae were documented at all sites except SCW which exclusively associated with Ceratobasidiaceae regardless of sampling year or phenological stage of the sampled individual. Kruskal Wallis test revealed similar OTU alpha diversity across sites, phenological stages, and years (p>0.05). PERMANOVA based on pairwise unweighted Bray-Curtis distances revealed differences in OTU richness across sites, phenological stages, and their interaction (p<0.05), though OTU richness across years was similar. Clustering based on distance-based Redundancy Analysis (db-RDA) clustered PLF and SCE sites together, and these two sites segregated from both PLE and SCW. The clustering results appear to explain population demography because among the four sampled populations only PLF and SCE hosted flowering plants, and OTUs found in flowering plants at these sites were not recovered from seedling or vegetative plants sampled at PLE and SCW. Our data suggest that P. cooperi is specific towards few narrow clades of mycorrhizal fungi and its populations with different demographies display distinct OMF communities.