| dc.description.abstract | Spiranthes parksii Correll is a rare terrestrial orchid endemic to only thirteen Texas counties, and is a federally listed endangered species in the United States due to habitat loss and fragmentation. Individual plants of S. parksii show irregular patterns of appearance aboveground and may exhibit vegetative dormancy for one or more seasons, even several years. Their transient behavior poses research difficulties and to determine effective conservation practices, the biology and ecology of the species must be assessed further. This thesis presents data for a population of S. parksii and its sympatric congener, Spiranthes cernua (L.) Rich, in eastern Grimes County that was monitored from 2014 to 2018 to determine if (1) population survival is affected by annual weather and consistency in aboveground presence as rosettes and flower stalks, (2) growth and survival are affected by the type and timing of herbivory, and (3) scheduled summertime watering events will decrease the prevalence of summer rosette dormancy and increase survival and inflorescence growth in the fall. A detailed analysis of demographics on these two species was conducted and results suggest the sample population presence is declining over time. Summer temperatures appeared to be negatively correlated with reproductive presence proportions and S. parksii flower stalk size, which indicated potential threats by climate change. Precipitation in previous and current years largely accounted for variations in rosette and reproductive proportions, and high and low precipitation thresholds possibly dictated stalk height and number of flowers. Contrary to previous research and regardless of the overall sample population decline, over 50% of the sample population flowered in three or more years during the study period, and most plants that flowered returned as rosettes each spring. Variability in seedling presence was perhaps also caused by variability of weather conditions and its effects on germination and soil moisture. Experimentally mesh-protected plants that allowed only minute invertebrate access had the greatest presence proportions at all life stages and frequently exhibited the lowest herbivory rates while plants exposed to both vertebrates and invertebrates consistently sustained the greatest herbivory. Rosette herbivory did not affect flower stalk growth except in 2015 when the study site received unusually low precipitation, which indicated negative interactive effects of weather and herbivory on plant vigor. In general, greater rosette herbivory led to a greater probability of reproductive absence. Minimum fall season herbivory rates by treatment did not coincide with maximum flower stalk growth, therefore, the timing rather than type of herbivory appeared to have a greater impact on growth. Historically small and large plants that received supplemental water in 2017 both showed reduced summer dormancy when compared to controls but reproductive growth was not affected. Results suggested that water-treated individuals were also less likely to forgo reproductive season dormancy and instead return aboveground as a fall rosette. Reproduction is considered a costly process that can diminish subsequent growth, but data here indicated that large flower stalks generally became large rosettes. Counter to expectations, there were no differences in dormancy by species yet soil around S. parksii presented lower volumetric water content, deeper claypans, and less slope than that of S. cernua. It is expected that microhabitat parameters have some influence on dormancy as large plants in this study resided on steeper slopes and exhibited a reduced tendency toward summer and fall dormancy than small plants, but results will benefit from more detailed soil analyses and the inclusion of genetic factors. | en |
