Geobotanical distinction using the floristic method: example of the Polish Pomerania

In the geobotanical division of Poland, Pomerania was treated as a separate subdivision, divided into "coastal plains" and the "Pomeranian uplands" or, in the newest regionalization, the "Pomeranian division". The goal of this paper is to determine how well the distinctiveness of Pomerania is floristically justified. A total of 1,467 native species occur within the area of Pomerania, representing 57.5% of the native flora of Poland. Fifty-seven species distinguishing Pomerania were selected and their phytogeographic features were discussed and compared to the Polish vascular flora. The distribution of 32 species (56.1%) is characterized as (sub) Atlantic/oceanic. Of the taxa that characterize Pomerania, 47.45% represent the northern element of the Polish flora and 7.01% the western element; species of the eastern element do not exist in Pomerania. The maximum entropy (MaxEnt) method was used for collective distribution analysis of the 57 selected taxa. The analyzes were performed using 41 environmental variables. All the species analyzed are strictly linked to the western and the North Atlantic part of Europe. Statistical evaluation of the MaxEnt model yielded an AUC value of 0.75 for both training and test. The most important environmental variables are minimum high (min_h), temperature annual range (bio07), and mean temperature of driest quarter (bio09). The results of the MaxEnt analysis allowed us also to verify the boundaries of the region and suggest new criteria for them. The southern limit of Pomerania should run along the northern boundary of the terminal moraine belt. Also, the area east of the Vistula does not contain abundant representatives of the flora characteristic for Pomerania. The results of the present study may be used for a broader discussion on the revision of the geobotanical division of Poland, at least in its northern part.