Beech (Fagus sylvatica L.) is a shade tolerant species of broadleaved temperate forests from the northern hemisphere. However, how it manages to survive low irradiance conditions is debated. This must involve both carbon balance components (mainly photosynthesis vs. respiration) and efficient utilization / partitioning of the available C resource among the different sinks (mainly growth vs. reserve storage). This survival strategy was investigated through a combined experimental and modelling approach. Four-year-old beech seedlings growing in the understorey of natural Scots pine stands were 3D-digitized in summer for architecture, including woody parts (growth units) and leaves. Then seedlings were harvested in autumn and winter to quantify biomass and carbohydrate reserve contents of each organ, both above and below ground. Roots were separated into 3 classes: taproot, main lateral roots and fine roots, and shoot segments according to their age. These data were used to construct 3D-digital mockups of the young trees and to parameterize and initialize the carbon-based functional-structural model PIAF-1. Additional beech-specific parameters relevant to phenology and gas exchange rates were derived from similar experiments or literature. Meteorological data (daily radiation and temperature) were generated for each tree based on recorded weather data. Transmitted solar irradiance above each seedling was calculated from hemispherical photographs and the global radiation above the stand. The first results suggest that under the most restrictive light conditions, the reserve store was just sufficient for the spring outgrowth. The major factor allowing the seedling to pass the short but critical near-zero-reserve phase appeared to be the leaf growth strategy, with surface area expanding prior to structural hardening. The reserve mobilization strategy showed only as a second factor, with the mobilization rate likely not limiting in contrast to the total reserve store.