Changes in the structure of tree stands on bog habitats in the Bialowieza forest

The analyses of 4 permanent study plots located in the Białowieża forest (NE Poland, at 52º43´ N, 23º50´ E) were carried out 4 times, first time in 1973 or 1974 and next in 1985, 1998, 2008. The paper presents the results of a long–term study on natural forest dynamics in two forest communities: raised-bog pine forest of the association Ledo-Sphagnetum magellanici Sukopp 1959 em. Neuhäusl 1969 and transitional bogs represented by the boreal spruce forest Sphagno girgensohnii-Piceetum Polak. 1962. On each measurement date the DBH of all trees were recognized. At the same time, stems of shrub species as well as tree species with the height lower than 1.3 m were counted within the experimental areas. During the study period the structure of stands changed significantly. The major change observed on raised bog sites was an increase in numbers of Betula spp., Picea abies (L.) H. Karst. and Quercus robur L. After 34th-year of observations the boreal spruce forest stand partly passed the terminal phase and regenerated. New species occurred in the stands, especially in their lower layer, are typical for oak-lime-hornbeam forest, such as e.g. Carpinus betulus L., Q. robur, Acer platanoides L. Rapid development of Corylus avellana L. was also observed. During last decades, the decrease in the number of Pinus sylvestris L. trees has been observed. The results of analyses of stand data in four research periods show that the wetland habitat of the Białowieża forest has been a subject to the processes of succession conditioned by environmental changes such as desiccation through lowering of the water table as well as climatic changes entailing inter alia an increase of the air temperature and a decrease of precipitation