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The studies initiated by Włoczewski in 1936 revealed changes in the species composition of forest stands in the Białowieża National Park. This led to the question about what these changes would be in the future. The aim of our study was to predict changes in the volume of tree species in the so called Strict Reserve in the Białowieża National Park over the period of 40 years. A size−class growth model was developed for this purpose. The average merchantable timber volume for each of the eleven taxa was calculated for the successive 10−year intervals, and the increase in standing volume as well as the volume of tree loss (mortality) and ingrowth were calculated for these intervals. The empirical material was taken from the permanent and temporary sample plots. The permanent plots (160) were measured in 2000, 2002, 2004 and 2011. The empirical data were used to determine, separately for each species, the relationship between trees dbh and mortality (fig. 1), diameter growth (fig. 2) or the rate of saplings (height of 0.3−1.3 m) growth into the first dbh class (0.1−1.9 cm). The temporary sample plots (460) were measured in 1995 and 2005, and the data were used to validate the size−class growth model and predict timber volume in the period 1995−2005 (tab.). The results of the validation may indicate significant changes in standing volume for the majority of tree species in the period 2005−2045 (fig. 3). In 2045, hornbeam will dominate. The volume increment in the period 2005−2045 will remain at almost the same level, the volume of tree loss will decrease and the volume of ingrowth will slightly increase, beginning from 2015 (fig. 4). Only five taxa will have a significant share in the ingrowth throughout the period 2005−2045, of which approx. 85% will be hornbeam in the period 2035−2045 (fig. 5). The observed changes in the species composition of forest stands in the Białowieża National Park may be the result of global, regional as well as local factors such as the pressure of herbivorous ungulates on young trees. Anticipating changes in the structure of forest stands may be useful in planning the protection of forest areas by reducing the impact of local factors on forest stands (regeneration) in an indirect way.
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