Developmental trajectories and Ecology of Monocarpic Trees: Insights from the genus Cerberiopsis (Apocynaceae)

Semelparity is the ability of an organism to reproduce once and then die. In plants, this life history strategy is also named monocarpy. After a unique flowering event, the whole plant dies. This strategy inherently characterizes annual and biennial herbaceous, and frequently occurs in bamboo and palms, but remains extremely rare in woody branched plants such as shrubs and trees. Branched plants are long-lived organisms and present multiple meristems (growing points) that are as many opportunities to flower at different times. Therefore, the persistence of the monocarpic strategy in such plants is questionable since a premature death of the individual in the decades before the flowering will prevent any descendants. About twenty-nine monocarpic canopy tree species are recorded worldwide all included in two tropical and subtropical genera: Tachigali (Fabaceae) and Cerberiopsis (Apocynaceae). This latter is endemic to New Caledonia and consists of three species with different habits. While C. neriifolia and C. obtusifolia are respectively a polycarpic (multi-flowering) treelet and shrub, C. candelabra is a large monocarpic rainforest tree.
In this thesis, we investigate the morpho-anatomical bases of monocarpy, its underlying ecological implications, and functional adaptations. (1) Through a comparative analysis, we highlight that the variation of only a few architectural traits has led to the emergence of this strategy within the genus Cerberiopsis, and flowering in C. candelabra is independent of tree age, size, or senescence. (2) Based on growth monitoring, we show that C. candelabra exhibits high survival and growth rates at the juvenile stage, and forms monodominant regenerations. These two studies pave the way to (3) an integrative retrospective analysis: we reconstruct the developmental trajectories of the Cerberiopsis species and demonstrate that their growth phenology is sensitive to climate seasonality. C. candelabra exhibits strong structural and temporal regularities, as well as a high degree of synchronization between all developmental processes. These particularities might have been preconditions for the evolution of its monocarpic strategy. Finally, my work introduces (4) a new type of annual growth ring that results from the production of circular tension wood, a potential adaptive feature related to the cyclonic season of New Caledonia. The processes underlying monocarpic flowering and its triggers are discussed throughout the different chapters of this thesis. We suggest that flowering depends on endogenous preconditions and is triggered in C. candelabra by large-scale disturbances such as fires and tropical cyclones.

Composition du jury :
Dr Hans Beeckman, Directeur de recherche, Muséum Royal d'Afrique Centrale - Rapporteur
Dr Sylvain Delagrange, Professeur des Universités, Université du Québec - Rapporteur
Dr Géraldine Derroire, Ingénieur de recherche, CIRAD - Examinatrice
Dr Isabelle Chuine, Directrice de recherche, CNRS - Examinatrice
Dr Sandrine Isnard, Chargée de recherche, IRD - Co-encadrante
Dr Patrick Heuret, Chargé de recherche, INRAE - Directeur