Phenology and developmental rhythmicity of the Cerberiopsis species (Apocynaceae): The growth trajectories of C. candelabra, a monocarpic pioneer tree

International audience

English. Introduction: Monocarpy, the strategy of a plant that flowers once then die is rare among long-lived tree species. Only known in tropical forests, it concerns three genera worldwide including the genus Cerberiopsis (Apocynaceae), endemic to New Caledonia. The genus comprises three species, of which only one, Cerberiopsis candelabra, is a monocarpic pioneer tree. The tree can reach about thirty meters high, and could live for more than a century before flowering massively and dying.Objectives: This study aims to describe the growth trajectories and rhythmicity of Cerberiopsis species in order to (i) compare the phenology of the processes underlying the development of individuals and (ii) identify morpho-anatomical markers in C. candelabra for initiation of the unique flowering episode.Methods: 6 C. candelabra, 3 C. neriifolia and 3 C. obtusifolia at adult stage in natural conditions were investigated. The trunk and crown phyllotactic spires of all individuals were traced. For each spire, we measured (i) the internode lengths and (ii) 10 axial and foliar morpho-anatomical markers. Data were coded in MTG format to retrospectively reconstruct the tree’s growth trajectories. A growth monitoring of 135 seedlings was also conducted from June 2020 to February 2022.Results: We identify morpho-anatomical marker complexes in Cerberiopsis species that (i) vary synchronously (number of growth rings, internode lengths, pith area, branching location) and (ii) indicate a rhythmic primary growth with polycyclism during establishment phase. Growth monitoring of seedlings confirms that rhythmicity is annual in C. candelabra. Besides, we noticed in all three species a significant fluctuation of the pith surface along axes, especially during branching and flowering.Implications: This study highlights the growth trajectories of Cerberiopsis species. Our results suggest that in C. candelabra there is a minimum pith surface threshold for branching and flowering. This threshold could reflect optimal conditions associated with flowering induction: the size of apical meristem and the amount of mobilizable carbon resources. Moreover, the rhythmic growth of C. candelabra and the annual nature of wood rings production allows us to retrospectively date the studied axes and the age of trees. These results offer promising perspectives to better understand (i) the life history strategy of this rare species and (ii) the triggers of flowering.