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In Polish forest management planning, the species composition of stands in a forest district is established by adding up the volumes of tree species from individual forest stands. These volumes are calculated according to the estimated share of species cover and the estimated volume of the entire forest stand. The species structure of stands in a forest district, established on the basis of such a method, may be different than that obtained from sample plots. The aim of the study was to compare three ways of determining the tree species structure of stands in a forest district. In Method 1, the species composition was estimated on the basis of the volume of trees measured on sample plots as part of a periodic forest inventory based on stratified sampling. The same data were used in Method 2, but the share of species was calculated on the basis of the basal area. In Method 3, the species structure was estimated according to the volume of trees estimated in each forest stand based on the cover of a given species. The empirical material was collected in 53 forest districts. In method 1 and 2, data from an average of 740 sample plots within the forest district were used. In Method 3, data from the forest stand descriptions contained in the database of the State Forests Information System were used. For each forest district, species composition was calculated, including 6 tree species and 8 groups of tree species. For all forest districts, the average shares of individual species and groups of species differed, depending on the applied method (tab. 1). In case of methods 1 and 2, the biggest difference was found for pine – the most numerous species – but it was also big for less numerous ones: birch and oak (tab. 2). In case of methods 1 and 3, the biggest difference in share was also found for pine (tab. 3). The greater the species diversity of a forest district (determined according to Simpson’s index of diversity), the greater was the average difference in the estimated share of an individual species (figs. 1 and 2). The method of determining the species structure in a forest district used in forest management practice nowadays was considered insufficient. It was proposed to establish it in larger units (subclass, age class, forest district) only according to the volume of trees measured on the sample plots (without rounding and ‘switching’ of species), using the stratified sampling method.
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