The paper describes changes, which took place over the last 30 years in old−growth forest stands occupying two permanent research plots, established in the Kaliszki and Sieraków strict protection areas in the Kampinos National Park (central Poland). Both plots have a form of ecological transects. The Kaliszki plot is 20 m wide and 700 m long (1.40 ha), while the Sieraków plot (total area of 2.56 ha) consists of two parts: main (40×460 m) and side (40×180 m) transect. In the Kaliszki plot, the stand measurements were conducted in 1993, 2007, and 2017, while in the Sieraków plot – in 1989, 1994, 2006, and 2017. For every tree with breast height diameter (DBH) ≥5 cm, species identity, DBH and spatial coordinates were determined. Starting from the second census, all trees which had died (‘losses’), as well as trees which exceeded the DBH threshold (‘gains’) since the previous record were noted, as well. The major tree species occurring on sample plots are Scots pine (Pinus sylvestris), pedunculate oak (Quercus robur), silver and downy birch (Betula sp.), and black alder (Alnus glutinosa). During the study period, the overall tree density declined by ca. 50%. The diminishing trends occurred for all dominant tree species. In contrast to major, dominant tree species, the demographic status of a group of minor tree species (lime, hornbeam and maple) was much more stable. One may even speak about some expansion in this case. Particularly, hornbeam density increased rapidly in the period 2006−2017 in some areas of the main transect located in the Sieraków plot. The future studies will show if this tendency will be maintained and will include other regions of sample plots. Assuming a lack of significant disturbances, one may expect that the stands growing on research plots will rather slowly change in the nearest future. Most dynamics will have a quantitative and not qualitative character. One may anticipate, for example, a growing role of pedunculate oak at the expense of Scots pine, however, considering that the two species are rather long−living, they will probably for a long time keep their dominant roles in the stand canopies.
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