Saving threatened plant species: Reintroduction of Hill's thistle (Cirsium hillii. (Canby) Fernald) to its natural habitat.

Hill's thistle (Cirsium hillii (Canby) Fernald) is a perennial plant endemic to the Great Lakes region of North America. Hill's thistle is listed as threatened in Ontario and Canada where it is found in globally rare alvar habitats. The main objective of this study was ex-situ conservation of Hill's thistle using in vitro culture techniques and reintroduction of micropropagated plants back to their natural habitat in Bruce Peninsula National Park, Ontario, Canada. Two out of twenty-nine available seeds were successfully germinated under in vitro condition. An efficient micropropagation protocol was optimized with 100% survival during acclimatization of plantlets in the greenhouse. Three hundred micropropagated plants were reintroduced to twelve different sites within Bruce Peninsula National Park in June and July 2017. Plants were monitored for survival, rosette growth, and flowering on all sites from 2017-2019. After four months of planting, 67 to 99% of the plants were alive in different sites and 90 to 99% of them survived over winter. In the following years, shoot regeneration and flowering were observed on most sites. This study further confirms the benefit of plant tissue culture techniques to ensure revival of Hill's thistle ecological biodiversity through the reintroduction of micropropagated plants. This approach consisting of the components of conservation, propagation, and reintroduction (CPR) may potentially serve as a model for saving and enriching other species at risk.