Rusconi, Olivia

2022, Rusconi, Olivia, Rasmann, Sergio, Mitchell, Edward

The current epoch, the Anthropocene, is witnessing a generalized and rapid extinction of species worldwide. As the causes of such extinctions, often related to global change drivers (e.g., habitat destruction, pollution, climate change), might not be hampered immediately, conservation actions should focus on preserving, restoring and regenerating the disappearing species through concrete action plans. Rare and endangered plant species, for instance, can be preserved by reintroducing seedlings in the same habitat, or introducing them into yet unoccupied habitats to generate novel populations. While this approach holds several good promises, it often generates mixed results, likely due to the fact that the current or novel habitat do not actually offer the optimal conditions for the target species to grow and thrive. With this thesis, we have tackled the problem of how to better characterize the ecological requirements of an endangered plant species in order to increase the success rate of future (re)introduction actions and to develop more efficient and targeted conservation measures. By focusing on the iconic and endangered orchid species Cypripedium calceolus (Lady’s Slipper) as a model species, we have taken a broad-range perspective, including both geomatics and field work to study the links between multiple ecological factors (soil properties, climate, association with local vegetation and health of 34 C. calceolus populations across Switzerland). In the first chapter, we propose a two-step approach to identify (re)introductions sites for endangered plant species using C. calceolus as a model species. The first step involves modelling its niche and its distribution with bioclimatic and topographical predictors. The second step consists in refining these bioclimatic predictions by analysing stationary ecological factors, such as edaphic conditions, and relating them to populations-level fitness values. The first results show that climatic predictions alone were not precise enough, but nevertheless could highlight a likely decline of the species range during the next 50 years due to predicted climate warming. Moreover, when incorporating topographical layers for the modelling approach at the regional scale, we show that the species is more likely to occur near forest edges. Finally, by analysing in situ soil factors, we show that soil organic matter, cation-exchange capacity and pH correlated most strongly with C. calceolus population fitness variables as described by multivariate function trait space. We thus advocate for the combination of modelling tools with fine scale on-site ecological surveys to identify suitable reintroduction sites for this, and potentially other, endangered plant species. In the second chapter, we aimed to determine the relationships between the performance of C. calceolus populations and soil and vegetation factors, to improve this species conservation and but also to advance the theoretical underpinning of which facets of an ecosystem most influence this species fitness. By studying C. calceolus functional traits of 34 sites across Switzerland, we found that large (>20 individuals) populations of C. calceolus displayed a specific assemblage of measurable characteristics that discriminate them from small (<10 individuals) populations, indicating that it is possible to assess the health of a population of a rare plant species by measuring a specific set of traits. While we could not direct predict population health status from vegetation (phytosociological alliances) and soil types, we show that a unique combination of companion plants and several edaphic variables, such as soil organic matter (SOM), CaCO3, pH, and P could be used to potentially assess the optimal sites to implement (re)-introduction actions for this emblematic and patrimonial orchid species. In the third chapter, we addressed the pollination ecology of C. calceolus by studying the links between plant vegetative and floral traits and local pollinator biodiversity. Moreover, we studied potential variation at the population level of how plants attract their pollinators and subsequently how this variation relates to the overall reproductive effort of C. calceolus individuals. We found that small C. calceolus populations occur in less diverse sites, both in terms of surrounding plants and insects. However, while plants from larger populations were physically larger and produced more flowers per capita, they produce the same number of seeds per capita (i.e., seed set) as small populations. This occurred despite the lower pollinator diversity present at the small population sites, meaning that plants from small populations must compensate by providing a stronger attraction to pollinators. Indeed, we discovered that floral volatile organic compounds are produced in higher quantity from the flowers of plants from small populations, suggesting that small populations compensate for low pollinator diversity by producing a stronger floral scent. Two additional perspective chapters deal with the genetic structure of the Swiss C. calceolus populations and their association with mycorrhizal fungi. While these data opens new interesting venues, these topics merits further investigations in the future. In summary, our results do not only increase the fundamental knowledge about C. calceolus biology and ecology but also allow to establish concrete conservation measures and to select more appropriate translocation sites. Moreover, this thesis emphasizes the complexity of orchid ecology and supports the use of integrated research and practical protection measures for this family of plants, and likely others. In a context of global climatic changes and due to the fragility of orchid ecological needs, this kind of approach is more relevant than ever. From a broader perspective, the method we developed is transposable to other plant taxa. Therefore, we advocate for the use of comprehensive methodologies based on multiple aspects of plant biology and ecology, as presented here, to study and protect a broad range rare and endangered plants.