Dactylorhiza majalis s.l. (Orchidaceae) in acid habitats: variation patterns, taxonomy, and evolution

Morphometric and allozyme data were collected from 11 populations of Dactylorhiza majalis s.l. in the Ardennes (Belgium, France) and North Jutland (Denmark). All study populations were growing in obviously acid habitats. Genetic distances were calculated and illustrated by UPGMA dendrograms, while principal components analyses (PCAs) were used to summarize morphological variation patterns. Based on the PCAs, the probability of each character to distinguish correctly between the main groups of populations was estimated. The study populations from the Ardennes and from the Danish island of Læsø seem to agree with populations from Sweden previously studied by Hedrén—and to belong to D. majalis subsp. sphagnicola (comb. nov.). It should be noted that allozyme data suggest a polytopic origin of this taxon. The study populations from the Danish region of Thy differ morphologically from the others and exhibit much less genetic variation. They are described as a new subspecies, D. majalis subsp. calciñigiens. The evolution and biogeography of the two taxa are discussed, and brief taxonomic accounts are provided.