Conservation of Caladenia (Orchidaceae) under a Changing Climate
Creators: name: Janissen, Brendan
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 410401
subject: Conservation and biodiversity

description: The Orchidaceae is one of the most diverse and threatened plant families globally, with almost 3.5% listed as threatened and many more being data deficient likely underestimating the number of the threatened orchid species. This is partly due to their reliance on Orchid Mycorrhizal Fungi (OMF) for germination, growth and further development and their complex environmental niches and microsite requirements that make them sensitive to environmental change. Climate change is a major, and overlooked, threat to orchids, with only 17% of International Union for Conservation of Nature (IUCN) listed species considered under threat by climate change.
Re-introduction and translocation are common conservation practices utilised in threatened orchid management and recovery plans. However, their effectiveness is widely debated, with many re-introductions and translocations failing to produce self-sustaining populations. Furthermore, there are limitations to success at each stage of the re-introduction/translocation process; for example: poor in-vitro germination success, poor survival rates when transferred to pots or poor survival rates after re-introduction. The low success varies case-to-case but is likely a result of a lack of consideration for the specific microsite conditions for the orchid and its OMF. This thesis presents novel approaches to improve conservation outcomes in Caladenia (and possibly other orchid) species by improving our understanding of microsite condition in situ, including environmental responses, to inform the re-introduction/translocation process with future climate predictions in mind.
Long-term (12 years) vital rates of wild, re-introduced and translocated populations of the endangered terrestrial orchid, Caladenia amoena (Charming Spider Orchid), were compared, which demonstrated that re-introduction and translocation were insufficient at establishing self-sustaining populations during the 12-year study period. When relating vital growth rates to climate, emergence and flowering declined as mean maximum temperatures rose, whereas flowering increased with rainfall. Both emergence and flowering were positively correlated with the length of the growing period, which decreased by &gt;33% during the study, highlighting it as a key concept and possibly the biggest challenge in preventing further population decline.
Similar negative correlations between emergence and flowering and temperature were observed in endangered Caladenia robinsonii (Frankston Spider Orchid), suggesting declines will continue given the current predictions of +1 to +3°C of warming by 2051 at its only known location. Using Principal Components Analysis (PCA) and General Linear Modelling (GLM) to compare soil properties and vegetation characteristics among re-introduced and wild populations identified greater nitrate concentration, graminoid cover and canopy tree distance and an open small shrub layer as common characteristics in the better performing populations.
Understanding in-situ conditions of C. robinsonii helped to improve in-vitro germination and identified three critical factors for germination, namely conducive conditions, ‘ready-to-germinate’ seed and effective mycorrhizal fungi. The positive effects of warm stratification on germination and seedling development were demonstrated for the first time, more than tripling germination and stage 4 seedling development compared to non-stratified seed. When comparing germination and OMF growth, there is a narrow overlap between the most favourable conditions for the orchid seed and OMF growth and only within this overlap can most seeds germinate and seedlings develop.
Elucidating OMF responses to climate was investigated using real-time polymerase chain reaction (qPCR) and a Serendipita-specific primer (C. robinsonii’s OMF). Soil abundance of Serendipita changed with season but not in response to mean monthly maximum (MMmax) and mean monthly minimum (MMmin) air and soil temperature or mean monthly (MM) rainfall. Serendipita OMF abundance in soil was less in declining populations than in stable/increasing populations. This is the first study to investigate the response of a free-living OMF in soil to seasonal variation in OMF abundance and soil moisture, temperature and rainfall, and suggests that the predicted changes in temperature and rainfall will probably not directly affect this free-living OMF. If free-living OMF abundance is not expected to be impacted by climate, focus should be given to other factors such as microsite conditions, particularly those favourable to in-situ recruitment, which is essential for establishing self-sustaining populations and to halt declines.
Microsite factor differences between recruiting and declining clusters of C. robinsonii were investigated using a variety of statistical methods including linear regression, GLM and PCA. Soil moisture, ground cover and OMF abundance were important factors for seedling recruitment. A soil OMF abundance of ≥1.5 ng μL-1, bryophyte cover of 60%, Banksia marginata leaf litter cover of 5-10% and a soil water content of 6-8 VWC were favourable to establishment. These results provide key factors that should be considered when choosing future re-introduction sites and during the management of existing C. robinsonii populations.
The works presented in this thesis provide novel insights that advance the field of orchid conservation in Australia and the world. The current re-introduction methods for C. robinsonii and C. amoena have not been successful at establishing self-sustaining populations. Our new understanding of re-introduced C. robinsonii populations in situ has led to improved in-vitro germination (increases up to 37% in plant numbers for re-introduction) and new on-ground management tools which could greatly improve the chances of halting population declines. Furthermore, this research could be transferrable to other Caladenia species and potentially other orchid genera.