The use of a recently constructed soil Biotron (a rhizotron set in an ecological context) to assess root growth is described. The assumptions needed to allow observational data from such a source to be used to estimate the flow of carbon into the root system are specified. Data are presented for two of the species which occur commonly beside the observation panels of the Pellston Biotron, Prunus pumila and Pteridium aquilinum. These have root systems which vary in morphology and root length density and hence contribute differently to the soil carbon budget. The value of facilities such as that at Pellston to research programmes on global change is discussed.