Studies in Crataegus L. (Rosaceae: Maloideae). XIII. Degree and pattern of Phenotypic variation in Crataegus sect. Crus-galli in Ontario

Multivariate studies of flower, fruit, and leaf variation in Crataegus crus-galli L. s. l. have largely confirmed a priori notions of the structure of this species complex in Ontario. Four morphotypes are recognized and these are assigned to taxa published by earlier workers. Intensive sampling of local stands of the two most common morphotypes indicates a high degree of between-site heterogeneity and within-site homogeneity It appears that the partitioning of variability observed in C. crus-galli s. l. may be explained by features of dispersal, habitat preference, and the occurrence of uniparental reproduction. These results suggest possible explanations for the very narrow circumscriptions and limited distributions of many North American Crataegus taxa described at the beginning of the twentieth century.