This project had the aim to investigate fluxes and pools of photosynthetically fixed carbon (C) within the fine root systems of individual Abies alba trees. Therefore, pulse-labellings with 13 CO2 were applied to aboveground parts of trees in order to follow the 13C-label within the root systems. The various forms of labelled C within the fine roots, non-structural (starch) as well as structural carbohydrates (cellulose), were recorded in dependency on time. Relative 13 C amounts (excess 13 C) in the fine roots increased significantly after the 3rd day of labelling as 13 C bulk and as 13 C cellulose. Highest values were recorded 1 and 6 months after labelling. Highest 13 C values were also recorded in the fine root diameter class 1.5-2.0 mm, whereas lowest values were in the fine roots diameter class less than 0.5 mm. A first 13 C-signal in the soil was recorded in the dissolved organic carbon (DOC) after 1 month. In the soil organic matter (SOM), however, a first 13 C-signal was observed only 12 months after the labelling. The label in the SOM occurred either in the floating SOM-particles after the water extraction or in the occluded and less dense fraction in the polytungstate. Applying the pulse-labelling either in spring, summer, or autumn, a clear seasonal effect was obvious by observing the incorporation of the 13 C-label into the root systems mainly in summer and autumn. However, after the spring pulse-labelling, a stronger signal was observed for clipped plants compared to the control plants. Measuring the 13 C-label in the starch of roots, it was obvious that the plants invested their photosynthetic products into root-starch mainly in the autumn. This highly labelled starch was mainly localised in the very young and newly formed fine roots.