Indigenous species of Crataegus (Rosaceae-Maloideae) in Flanders (Belgium)  : an explorative morphometric study

The taxonomy of the five Crataegus taxa indigenous in Flanders was studied through a morphometric analysis. The applicability of the taxonomic treatments of Christensen, the botanical authority on Crataegus in Europe, was evaluated. Leaf and fruit characteristics were analysed for a total of 186 individuals. The results show that C. monogyna and C. laevigata can easily be differentiated. The putative hybrids C. ×media, C. ×subsphaerica and C. ×macrocarpa are more difficult to delineate. C. ×media, the putative hybrid between C. monogyna and C. laevigata, shows a remarkably large range and overlap with both parental species for most of the characters studied. This phenotypic variation could be due to a shift in morphological characters through several generations of backcrossing with the parental species.
Although C. rhipidophylla was not yet found in Flanders, populations of both putative hybrids with C. monogyna (C. ×subsphaerica) and C. laevigata (C. ×macrocarpa) were recently inventoried. The individuals identified as C. ×subsphaerica are hardly distinguishable from C. monogyna. Introgression with the C. monogyna parent may have obscured the diagnostic characters. Therefore, the presence of a typical population of C. ×subsphaerica in Flanders is questionable. On the contrary, the few individuals of C. ×macrocarpa form a distinct group.