The evolution of the natural environment of the middle mountains of the Southern Sikhote-Alin is reconstructed for the last 4.9 ka by data of the comprehensive study (botanic, diatom, and pollen analysis) of two peat swamps on the Sergeev Plateau. Radiocarbon dates and the position of the B-Tm marker ash layer of the Baitoushan Volcano, which erupted in the tenth century, are used to determine the chronology of events. An age model is constructed and peat accumulation rates and ash content in the peat are analyzed. It is revealed that the belt of dark coniferous forests at altitudes of about 900 m is relatively stable, while multidirectional climatic variations caused sharp changes in the predominating peat-forming plants. Unstable climatic conditions determined most of the features of the hydrological regime of the swamps. Reduced groundwater level and a greater role of atmospheric precipitation were usually related to cold events. Fires were one of the factors determining the landscape development; their fr