The occurrence of sycamore as part of the species composition of the natural woodland stands of the lower montane zone in the Western Bieszczady Mountains has been analysed within the left part of the upper catchment of the River San (from the source to the mouth of Wołosate). The studies have been based on the climate and habitat diversity The analyses took account of the description of the biological features and ecological requirements found in the literature and critical evaluation of the existing scientific research. The archival forest inventory of the Bieszczady National Park and Stuposiany Forest District and also the subsequent works describing the diversity of the geological and soil conditions of the research area have been used for this article. Studies of the role of sycamore in the lower mountain zone of the Western Bieszczady have been based on the assumption that in the natural forests the performance of the tree species in the stands is a result of the implementation of their life strategies under the competitive pressure from the other species. Among the natural stands of the study area covering just over 7,300 ha, the presence of sycamore with a representation of at least 10% has been found in only one fifth of their area. It was therefore necessary to check whether the occurrence is random, or whether certain elements of the diversity of environmental conditions have an impact on it. The studies proved that distribution of sycamore in natural forests has been determined by certain conditions of climate and habitat, defined in this article. They result in the greater competitiveness of sycamore in comparison to the other species of the lower mountain zone stands (especially beech, silver fir and Norway spruce). The results of the above analysis should have been used for the construction of models of the natural forests to be implemented in the conservation of the natural stands. They should also be taken into account in planning of the restoration of the nurse stands within the former agricultural habitats covering one third of the wooded area in Bieszczady Mountains.