Long-term changes of tree species composition in high protective regime stands in the Augustow and Knyszyn Forests

In the last decades, we observed significant changes in the tree species composition in all layers of close−to−natural stands. Since the 1970s, the Forest Research Institute has collected and stored dataset from stand measurements, phytosociological and other research that are the basis for the long−term monitoring of vegetation. The aim of the research was to determine the directions of changes in the species composition and structure of tree stands, and to analyze the changes in the species composition of natural regeneration growing without human interference in various forest communities in the period of about 40 years. From 2014 to 2017, 40 permanent plots of 0.25 ha were re−measured in the nature reserves of the Knyszyn and Augustów Forests (NE Poland). The diameter of all trees was measured and the trees of the natural regeneration and bushes up to 1.3 m high were counted. The stand similarity index (S) between successive measurements was calculated. The analysis of differences in the composition of tree stands in subsequent measurements was carried out using Student t and Kruskal−Wallis tests. In the species composition of fresh coniferous forest stands and fresh mixed coniferous forest, there were significant changes in the number and share of Scots pine and Norway spruce. In the Knyszyn Forest a significant decrease in the share of spruce was observed. However, an increase in the share of deciduous species was remarkable. An inhibition of the natural pine renewal process and increase in density of spruce regeneration was observed. It is necessary to emphasize the significant expansion of hornbeam in a fresh mixed deciduous forest in the Knyszyn Forest and expansion of hazel in the Augustów Forest. Mean basal area in the forest stands has increased. The similarity index between the beginning and the end of the research reached the value of 0.73−0.94. The species composition and density of natural regenerations created without human intervention are not satisfactory from the point of view of forest sustainability. The most important factor affecting the direction and rate of changes in the species composition of tree stands is the expansion of hornbeam and/or hazel. These species cause changes in the ecological conditions of the lowest forest layers, leading to the unification of the species composition of forest communities