Mycorrhizal Associations, Life History, and Habitat Characteristics of the Endangered Terrestrial Orchid Spiranthes parksii Correll and Sympatric Congener Spiranthes cernua: Implications for Conservation



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Spiranthes parksii Correll is a federally listed endangered terrestrial orchid endemic of Texas. The species exhibit a very limited geographic distribution and most remaining populations are threatened by habitat loss and fragmentation. Development of in situ and ex situ conservation strategies is fundamental for the protection and re-introduction of the species in their natural habitat. In this dissertation the life history, symbiotic mycorrhizal fungal associations, and habitat characteristics of S. parksii and sympatric congener Spiranthes cernua were investigated by the use of in situ, and ex situ methods. The central research objective was to investigate the orchid-fungal relationship and its distribution in natural habitats and to provide critical information for conservation of the species.

Population dynamics and reproductive phenology of both species were studied from 2007 to 2013. Demographic parameters, cost of reproduction, and response to environmental conditions were analyzed. Results indicate that probability of flowering is strongly influenced by vegetative size and environmental conditions in the year preceding a flowering event. Reproductive effort resulted in a decrease in plant size, reducing the probability of flowering subsequently. Environmental conditions explained ?70% of the annual variation in the proportion of flowering plants and inflorescence characteristics. Mycorrhizal fungi associations and belowground phenology of both species were studied for three growing seasons. Data suggest that both species form generalist mycorrhizal associations with fungi tentatively classified, as Epulorrhiza sp., and Ceratobasidium sp. Both orchids sustain mycorrhizal colonization throughout their life cycle. However, seasonal changes in mycorrhization reveled a distinctive annual cycle of infection and digestion of fungal hyphae. Seed viability, in situ germination, seedling development, and mycorrhization were examined in situ by using orchid seed baits. Results suggest that S. parksii seeds are short-lived and germination is constrained by rapid reduction in seed viability, prevailing environmental conditions, and distribution of compatible mycorrhizal fungi. Data indicates a small optimal window for recruitment restricted to the first spring after dispersal. Microhabitat characteristics of S. parksii were compared to microsites of sympatric S. cernua, historical S. parksii microsites and vacant microsites currently not supporting either orchid. Vegetation, edaphic, and environmental parameters were evaluated. The study found that microsites currently occupied by S. parksii differ significantly from other types in canopy cover, leaf litter, herbaceous cover, and soil pH. Furthermore, woody encroachment may significantly affect the availability of suitable microsites not only as a result of altered light environments, but also due to changes in community composition, microclimatic, and edaphic conditions.