Alder (Alnus glutinosa) carr in Poland.

The paper aims at the analysis of ecological and regional variation of forest communities from the class Alnetea glutinosae in Poland. A previous syntaxonomical position of the association Carici elongatae-Alnetum has been revised. Instead, two new alder-carr associations have been described. They differ in character and differential species, with dissimilar trophic requirements. The group of species with Sphagnum squarrosum is differential for acidophilous bog moss alder carrs (Sphagno squarrosi-Alnetum), while the group of species with Ribes nigrum differentiates mesotrophic black-currant alder carr (Ribo nigri-Alnetum). Both alder-carr associations occur throughout lowland Poland. Their floristic compositions differ regionally. Suboceanic and mid-European species grow in alder carr of western Poland, while boreal and boreocontinental species enter from the northern-east. The studies on the regional variation are based on the analysis of distribution of differential species groups. These have resulted in the delimitation of geographical races of alder carr. Both alder-carr associations differ from one another in their internal variation. Sphagno squarrosi-Alnetum varies greatly from region to region. For this association four geographical races have been distinguished. In the Ribo nigri-Alnetum internal trophic variation plays a more important role. Therefore, it has been divided into four subassociations and two geographical races. In central Poland black-currant alder carr occurs in a form with both geographical races but with different subassociations.