The results of a model experiment on the evaluation of environmental effects of the forest plantation based on transgenic forms of aspen are analyzed. The system of simulation models EFIMOD, which combines both populational and mass-balance approaches, was used to investigate changes of the biological cycles of carbon and nitrogen as a result of the replacement of natural forests by aspen (Populus tremula L.) plantations (including plantation of aspen trees with modified growth parameters and increased nitrogen content in tissues). The model experiment was carried out using the data on climate and soils that are typical for spruce forests of North-West of Leningrad region. Short-rotation plantations (30 yrs) on the place of spruce forests after clear cutting were simulated. The results obtained showed a greater decrease of C stock in soils under plantations of modified aspen trees. The total pool of N in soils under plantations slightly decreased in spite of the nitrogen fertilizer use. Losses of C and N in forest soils are explained by the consumption of nutrient elements by fast-growing stands of transgenic and non-trangenic aspen that not compensated by the return of the elements with the annual litter fall.