HYBRIDIZATION AND CLASSIFICATION OF THE WHITE PINES ( PINUS SECTION STROBUS )

Summary Many North American and Eurasian white pines retain their ability to hybridize even after long isolation, and about half of all white pine hybrids from controlled pollinations are inter‐hemisphere crosses. Within the morphologically homogeneous and otherwise highly crossable core group of white pines, an exception in crossing behavior is Pinus lambertiana of western North America. Although it is an archetypal white pine in its winged seeds and long, unarmed cones that open at maturity, P. lambertiana has not been successfully crossed with any other species in the core group. It has been hybridized only with P. armandii and P. koraiensis , two natives of eastern Asia that do not resemble P. lambertiana or each other in the characteristics used to classify white pines. Both Asian species have seeds without wings, and P. koraiensis has indehiscent cones that retain the seed at maturity. Wingless seeds and seed‐retaining cones may have evolved more than once as adaptations to seed dispersal by birds, and their taxonomic value is questionable. Lack of agreement among the kinds of data that bear on relationships of the white pines emphasizes the need for new approaches to their classification.