Résumé
While phenotypic plasticity is considered the major means that allows plant to cope with environmental heterogeneity, scant information is available on phenotypic plasticity of the whole-plant architecture in relation to ontogenic processes. We performed an architectural analysis to gain an understanding of the structural and ontogenic properties of common buckthorn (Rhamnus cathartica L., Rhamnaceae) growing in the understory and under an open canopy. We found that ontogenic effects on growth need to be calibrated if a full description of phenotypic plasticity is to be obtained. Our analysis pointed to three levels of organization (or nested structural units) in R. cathartica. Their modulation in relation to light conditions leads to the expression of two architectural strategies that involve sets of traits known to confer competitive advantage in their respective environments. In the understory, the plant develops a tree-like form. Its strategy here is based on restricting investment in exploitation structures while promoting major vertical exploration and is probably key to species survival in the understory. Under an open canopy, the second strategy leads the plant to adopt a shrub-like shape. It develops densely branched exploitation structures and flowers abundantly and rapidly. This strategy perfectly matches its aggressive behaviour observed in full sunlight. We propose, as hypotheses, that these two light-related strategies are implicated in the ability of R. cathartica to outcompete the surrounding vegetation in a range of environmental conditions.