The relevance of the study is determined by a high conservation value of activities on the reintroduction of forest reindeer (Rangifer tarandus tarandus) listed in the Red Data Book of Russia, in the territory of the Kerzhensky State Nature Reserve (Nizhny Novgorod Region). The paper aims to estimate the first results of studying the environmental-forming role of the forest reindeer in the forest bog complexes of the reserve (using the example of an adaptation enclosure). Our purpose is to assess the location character of reindeer trails in forest and bog habitats, the extent of grazing pressure on the vegetation in summer; to assess changes in the number of plants consumed by reindeer mainly in summer; to assess the foraging activity of reindeer in winter under condition of snow cover; to analyze possible consequences of reindeer impact on ecosystems. To solve these problems, vegetation mapping was carried out in the territory of an adaptation enclosure before the animals were placed there, reindeer trails were mapped to identify the features of their distribution throughout the enclosure, and the reindeer impact on ecosystems was estimated. It is shown that due to the environment-forming activity of the reindeer, the biological diversity of bog ecosystems increases, possible stagnation in forest succession are eliminated, and the fire hazard in lichen pine forests decreases.