Our research, which has yielded unique findings, is dedicated to the study of the dynamics of the number of flora species in a forest-protected area (Mordovia State Nature Reserve, European Russia) for the period 1936-2023. During this period, there was an increase in the number of species of both native (from 525 to 695 species) and alien (from 38 to 174 species) flora. Based on the above data, we conclude that the composition of the native flora of the MSNR is more than 99% identified. In the first years of the flora inventory, about 70% of the native flora is identified. The spectrum of families of the native flora of the MSNR has changed little over time. After the first inventory, the number of families in the flora does not add up to much (78 in 1936, 87 in 2023). There is mainly an increase in the number of identified genera in already known families and species within identified genera. In the spectrum of families of alien flora there are constant changes, new genera from different families, species within genera are added. The floristic data analysis revealed that the natural flora exhibited characteristics of a forest, despite the presence of a forest-steppe zone adjacent to the southern border of the MSNR. The primary indicators of systematic structure, composition of life forms, and ranges of species of native flora collectively indicate that the Mordovia State Nature Reserve forest area is situated within the natural zone of mixed (coniferous-broadleaved) forests. A comparison of long-term data from the MSNR flora study revealed that the process of formation of the alien flora of the MSNR, its structure, and chorological groups has not been completed yet. It will continue until the cultivation of plants from different continents and natural areas in protected areas is banned.