Clear—cuttings cause significant changes in boreal forest ecosystems and have long-term effects on populations of understory plants. The aim of our studies was to determine the impact of clear-cuttings on understory mosses and vascular plants populations after clear-cuttings. The species diversity of particular populations in mature stands before cuttings and after one year was determined. Results of investigations corroborated changes in coverage, frequency, and prominence value of predominant Ericaceae plants. We determined the different response of Vaccinium vitis-idaea L., V. myrtillus L. and Calluna vulgaris (L.) Hull. Plant species that are particularly sensitive to clear—cuttings have been identified also. Assessing the viability of moss populations in mature forest stands and deforested areas showed that moss species are most sensitive to environmental changes after clear—cuttings. These investigations could justify the conservation of sensitive forest plant populations and non-wood forest resources.