The aim of the study is to analyze changes in the species composition and in the diameter structure of the main forest−building species in the Roztocze National Park (central−eastern Poland). The research was carried out on 315 permanent monitoring plots. The range of collected data included: type of protection (strict and active protection), forest site type, tree species and their dbh (>7 cm over bark). Measurements and observations were made in 2007 and repeated in 2017. There was a significant reduction in the number of all tree species, except of beech Fagus sylvatica, regardless the protection status. Despite this, the basal area for the strict and active protection zone was similar both in 2007 (37.77 and 36.39 m²/ha, respectively) and 2017 (37.17 and 37.85 m²/ha, respectively). In the whole park, the number of large trees (mainly beeches) with high individual basal area increased, while the number of young generations of all species decreased. In the strict protection zone, the admixture species (decrease by 27.3%), as well as hornbeam Carpinus betulus (decrease by 22.8%), are not particularly competitive with beech. A similar tendency, although mitigated by active silvicultural treatments, was also observed in the active protection zone, in which admixture species have lost 24.8% while hornbeam 16.2%. In the strict protection area, there was a significant reduction in the number of fir Abies alba (by 14%). In the active protection zone fir increased its range and basal area, and maintains its population practically unchanged. There was also a decrease in the number of young generations, as evidenced by changes in the frequency in the 7−16 cm dbh class. The highest level of reduction in this class was found for pine Pinus sylvestris (by 64.8%), rare species (40.4%), hornbeam (31.2%), oak Quercus robur (26.6%), and spruce Picea abies (20.5%). Only beech increased the abundance in this class (by 1.4%) as well as in the next 17−26 cm class (even by 14.2%). The presented significant changes in the species composition and structure of basal area of the stands in the Roztocze National Park that occurred in the 10−year period, indicate a significant role of tending cuts in maintaining proper species composition, appropriate diameter structure of stands, their stability of natural richness and high level of diversity of forest ecosystems.
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