Factors influencing survival and success of wild and re-introduced populations of the endangered orchid Caladenia robinsonii (Frankston spider orchid) (Orchidaceae)

Context Re -introduction to augment surviving populations of endangered orchid species is commonly used in efforts to prevent extinction. This strategy was adopted for the federally endangered orchid Caladenia robinsonii at its only extant site, in south-eastern Australia. Aims The aims of this study were to predict the likely effects of climate change on the future survival of C. robinsonii and to understand why different re-introductions varied in success at the site of the surviving wild plants. Methods Ex situ-grown plants were used for five re-introductions from 2008 to 2017, into the same reserve as the wild plants. Individual survival (re-emergence), flowering and recruitment (seedlings) were measured annually from 2008 to 2020. Soil was analysed for fertility and vegetation was analysed for cover and floristics. Results Re -emergence in all original (wild and re-introduced) plants declined by 57% (excluding recruits) and 27% (including recruits) as temperature increased and rainfall declined during critical growing seasons. Results varied with planting position on site: plants in the recruiting group (Group R) had greater re-emergence and recruitment than those in the declining group (Group D). Group R was associated with more fertile soil and greater cover from graminoids and small to medium shrubs than Group D. Conclusions Success in re-introduction of C. robinsonii was favoured by the planting site having more fertile soil, shade from graminoids and small to medium shrubs, and planting in wetter years with cooler temperatures. Implications Caladenia robinsonii will continue to decline at the current rate of climate change and so continual augmentation will be required to prevent its extinction.