Distribution of mycorrhizal fungus associated with the endangered pink-lipped spider orchid (Arachnorchis (syn. Caladenia) behrii) at Warren Conservation Park in South Australia

Ex situ seed baiting of a population of Arachnorchis (syn. Caladenia) behrii at Warren Conservation Park was conducted to determine the distribution of this orchid's mycorrhizal fungus (or fungi) and whether it was limiting the distribution of the orchid. Forty-five samples of topsoil and organic matter were collected systematically from within and outside the orchid population. All samples were baited with A. behrii seeds and incubated for 8 weeks to assess germination. Seeds from 24 samples out of 45 (53%), from both within and outside the orchid population, reached stage-two germination (seed coat cracked, protocorms swelling), but a fungus later killed five samples. Seeds from three samples (7%) reached stage-three germination (seed coat shed, rhizomes developing). Clearing and staining of seeds revealed that mycorrhizal fungus was present in all samples that had reached stage-two or stage-three germination. Mycorrhizal fungus was distributed independently of the orchids, and, thus, the distribution of A. behrii, which depends on mycorrhizal fungus, did not appear to be limited by the distribution of mycorrhizal fungus. Further research must focus on the inoculum potential and efficacy of th fungus/fungi in situ over a longer time period, as well as other ecological aspects of the orchid's natural history, to identify the reason(s) for its rarity.