The aim of the study was to characterise changes in the natural forest of the Białowieża National Park (BNP) Strict Reserve during a 15-year period. To allow for a more precise determination of the time course, overall duration and intensity of the observed changes, the 15-year period was further divided into shorter 2-7-year intervals. Taken together, the Strict Reserve forest stands cover 4584 hectares and they constitute the oldest part of the Białowieża Forest placed under protection in 1921.
The measurements were carried out in 2000, 2002, 2004, 2011 and 2015 on 160 permanent sample plots, which are systematically distributed throughout the BNP Strict Reserve. During those 15 years, the volume of merchantable timber and the tree density fluctuated only slightly, because the reduction in standing volume due to a spruce and ash decline was compensated for by an increase in the standing volume of lime, hornbeam and alder. The volume increment and tree loss fluctuated slightly, but were nevertheless similar throughout the whole period between 2000 and 2015. The number of trees in the regeneration layer increased. The number of hornbeam trees in this layer increased continuously throughout the whole measurement period, while the number of maple trees started to increase in the second half. All together only five tree species growing in the BNP Strict Reserve progressed from the regeneration layer to the canopy layer in significant numbers. As a result, a gradual decrease in species diversity of forest stands may be expected.
The rather stable, average volume of merchantable timber in the BNP Strict Reserve may be due to the fact that, in a forest with diverse habitats and high species richness, only a few stands are subjected to strong disturbances in a given period of time. Repeated measurements during a relatively short period of time allowed the detecting some fairly quick changes occurring in natural lowland forests.
This paper describes a method of determining the stocking density and volume of forest stands based on airborne laser-scanning data. The aim of this study was to determine the relationship between ground-based measurements of standing volume and tree-density, and those acquired based on the Crown Height Model (CHM) interpolated from airborne laser scanning data. Data were collected from 34 sample plots of two sizes for the CHM analysis: 500 m2 (radius 12.61 m) and 1963.5 m2 (radius of 25.0 m): Trees for sampling were selected using two methods, those whose “centroid” was fully within the sample plot (the tree was considered to be within the sample plots if the centroid of the crown was inside the circle) and those at the “border” (the tree was included in the sample plot if, at least, one part of the contour of the crown was inside the circle). There was a strong relationship (R2 = 0.86) between standing volume measured in sample plots on the ground and the indices produced by the crown elevation model at the locations where the ground-based measurements were performed.