During 2015 and 2016, in the Kampinos National Park (KNP), monitoring was conducted for the population of black alder (Alnus glutinosa (L.) Gaertn.), which occupies 12.5% of the tree stands in the park and, after the Scots pine (Pinus sylvestris L.), is the second most important species there. The aim of the observation was to obtain data about the current condition of alder in the National Park. Monitoring was carried out on eight plots designated throughout the park. Stands where alder was not the dominant species were omitted. On each plot, 20 trees were evaluated with a total of 160 plants assessed. The age of the tree stands analysed was 55–120 years. Monitoring was conducted by assessing the tree’s crowns, based on the methodology of forest monitoring implemented by the National Environmental Monitoring. Based on the analyses, it was found that the average defoliation of alder in 2015 reached 28.1%, and 34.4% in 2016, with the differences being statistically significant. Among 70% of the specimens, there was no discoloration of the assimilation apparatus. There were significant differences in defoliation between research plots located inside the park and near its borders, which can be associated with the influence of anthropogenic factors. Based on the monitoring, it was found that the state of alder crowns in KNP is a warning or average, depending on the year of observation. Therefore, it is necessary to continue monitoring the state of alder crowns in KNP. On the basis of the study results, it was also found that the biosocial structure of the researched tree stands is formed correctly. The dendrometric measurements conducted show that the average diameter of the trees analysed, at breast height was 30.5 cm with an average height of 25.5 m. To analyse the changes in height and breast height, dendrometric measurements must be repeated in 2020. Monitoring will be continued in the following years to observe the changes.
