Vegetatio 117: 81-94, 1995. 81 t~) 1995 KluwerAcademic Publishers. Printed in Belgium. The structure and dynamics of old-growth Pinus sylvestris (L.) stands in the Wigry National Park, north-eastern Poland Tadeusz Andrzejczyk 1,3 & Bogdan Brzeziecki 2,3 1Institute for Forestry Research, Field Experimental Station, Krzywe 82, PL-16-400 Suwalki, Poland; 2Swiss Federal Institute for Forest, Snow and Landscape Research, CH-8903 Birmensdo~ Switzerland; 3Department of Silviculture, Warsaw Agricultural University, 26/30 Rakowiecka, PL-02-528 Warsaw, Poland (address for correspondence) Accepted 14 December1994 Key words: Age analysis, Canopy-understory interaction, Compositional instability, Natural regeneration, Subbo- real forests Abstract The paper describes the structure and the developmental trends of old-growth Pinus sylvestris stands in the Wigry National Park, in north-eastern Poland. The stands represent a transitional zone between deciduous forests of Central Europe and boreal, coniferous forests of north-eastern Europe. Besides P sylvestris, the most important tree species are Picea abies and Quercus robur. Among the subcanopy species, Corylus avellana and Sorbus aucuparia occur most frequently. On the basis of the data from 6 permanent sample plots (total size: 1.90 ha), several parameters and stand indices are analysed including species composition of the canopy and the regeneration, diameter distribution, age structure of main tree species, and the relationship between canopy and spatial dispersion of woody regeneration. The most striking feature of the stands studied is the almost complete absence of natural regeneration ofP sylvestris. This seems to be in contradiction with the apparently natural origin of this species in the stands, and a common occurrence of natural disturbances resulting in openings and gaps in forest canopy. The main tree species replacing P sylvestris in the canopy are P abies and Q. robur. Also increasing are some broad-leaved species typical of high fertility sites: Acer platanoides, 1ilia cordata, Ulmus glabra, and Fraxinus excelsior. A shrub C. avellana occurs extensively competing with tree species and delaying tree replacement processes. While no direct data on the changes in the site conditions can be provided and the 'recovery' hypothesis appears to be the most straightforward explanation of the changes in P sylvestris stands, the possible role of the allogenic changes in environmental conditions (climate warming, nitrogen deposition) is also discussed. Introduction Numerous studies on structure and dynamics of old- growth forest stands throughout the world show that they are seldom compositionally stable (Parker et al. 1985; Peterken & Jones 1987; Ward & Parker 1989; Glitzenstein et aL 1990; Cho & Boerner 1991; Masaki et al. 1992; Roloff 1992). The situation that the tree species occupying dominant position in forest canopies do not regenerate and are replaced by other species, is indeed very common. For Polish forests, examples of this phenomenon are given by many authors. For example, Kowalski (1992) found significant changes in the tree species composition on permanent sample plots established in 1936 in Bialowieza National Park (under protection since 1920). Their most essential feature was the estab- lishment and further development of new generations of Tilia cordata and Carpinus betulus, as well as Frax- inus excelsior. At the same time, such species as Picea abies and Betula sp. lost their importance. Similarly, Sokolowski (1991a), in the studies conducted in the period 1973-1985 in several for- est reserves within Bialowieza Forest (but outside Bialowieza NP), observed a decline of P. abies on more eutrophic sites, an expansion of C. betulus in