Article ID: 2513
The mycoheterotrophic orchid, Erythrorchis altissima, is a semi-epiphytic plant that depends on wood-decaying fungi for its carbon supply. Although listed as an endangered (IA) species in Japan, little is known about the ecological conditions necessary for its conservation. In this study, we monitored a natural orchid habitat of approximately 300 m×500 m on Okinawa Island for 10 years and confirmed a total of 239 individuals. We studied the characteristics of aerial stems and their host trees, as well as changes in plant distribution. We also conducted in situ seed germination experiments to elucidate the species’ germination ecology. The average length of aerial stems was 3.2 m but that of flowering individuals was 4.6 m, implying that stem elongation is required for the transition to reproduction. Of the above-ground stems monitored, 67% died within 2 years and 82% died within 3 years of being marked, implying a typical stem lifespan of 3 years or less. Host trees included 17 species, with Castanopsis sieboldii being the most common, followed by Cinnamomum yabunikkei and Cinnamomum doederleinii. Of the E. altissima individuals observed, 51% were climbing dead wood and a further 34% were climbing living or partially dead wood, with the remaining 15% growing on the ground without a host tree. Erythrorchis altissima developed mycorrhizal roots in nine observed tree species, with C. sieboldii and Cinnamomum spp. accounting for half of these; thus, in addition to providing physical support, these tree species also serve as primary nutrient sources for E. altissima. Orchids were mainly concentrated in three locations within the study area. For the first time, we successfully germinated seeds in situ and found that stems appeared approximately 1 year after sowing. Our study provides important context for E. altissima conservation and furthers understanding of mycoheterotrophic plants that depend on wood-decaying fungi. Given the potential for local or regional variation, further studies of other populations in tropical Asia will be needed to elucidate fully E. altissima ecology.
