Background and aims Analysis of the variability of organogenesis and extension (two basic processes of primary growth) is needed to improve our understanding of architectural plasticity in trees. An example of such plasticity is the difference in size between fully grown shoots found at different positions on the tree. The contribution of preformation processes to these differences was explored by determining the intraspecific variability of shoot preformation of main stems of understorey saplings of Juglans regia. Methodology In 2007, two samples of annual shoots that differentiated in 2006 were taken from saplings growing in a Mediterranean mixed forest in the south of France. At the first sampling, the terminal winter bud of each parent shoot was dissected and the nodes of these buds were counted. For the second sampling, annual shoot growth was measured after the 2008 growing season. The percentage of light transmitted through the understorey was determined by hemispherical photographs. Principal results The effect of plant development (ontogeny) on the number of preformed leaves and the annual shoot extension was strong. Light availability also contributed to explaining differences in the length of annual shoots. The impact of light on the number of preformed leaves was minimal. Conclusions Ontogeny (onset of branching) and reduced irradiance both affected leaf preformation in buds of annual shoots, with ontogeny being the more influential. The results suggest that the shoot preformation did not contribute to the plasticity of morphology of annual shoots. Light availability was more influential and promoted annual shoot extension.