Multivariate analyses of hybridization between Betula cordifolia and B. populifolia (Betulaceae)

Abstract

Multivariate analyses of Betula cordifolia Regel, B. populifolia Marsh., and B. × caerulea Blanchard in a mixed population in northern Maine show that the latter is a hybrid between the former two species. Hybrid status of B. × caerulea is further supported by the fact that two putative hybrid individuals have the same chromosome number as reports for the two parental species and that all three taxa have overlapping flowering times at this site. Studies of pollen germination and stainability and seed germination show that the putative hybrids are fertile. Betula papyrifera Marsh., which is also present in this mixed population and was included in the analyses, flowers earlier than B. cordifolia and B. populifolia, and it apparently does not hybridize with the latter species at this site. Characters that have not been used prior to this study and that are among the best discriminators between B. cordifolia, B. papyrifera, and B. populifolia are number and width of preformed staminate catkins and length and diameter of overwintering vegetative buds.