Modelling individual variations in plant productivity and resource-dependent mortality is a key issue in population dynamic models. The present study examined two classical approaches to account for plant productivity in functional-structural plant models (i.e. the up-scaling of a leaf photosynthesis model, and the down-scaling of a canopy production model) and compares them in their ability to account for the size-structure of a population of alfalfa plants competing for light. The two models differed mainly in their formulation of the plant carbon balance. Only the leaf approach included a respiration sub-model and was able to predict self-thinning and changes in radiation use efficiency among plants. Variations in plant mass were however mainly explained by differences in light interception. The two models behaved quite well to simulate the mass distribution of surviving plants, the leaf model being clearly more difficult to calibrate.