Trees and Heterosis

This chapter begins with selected major groups of trees, such as poplars, eucalypts and conifers, and deals with discussions of the roles of complementarity and genetic distance in hybrid superiority. The genus Populus (Salicaceae) includes the poplars, aspens, and cotton‐woods, trees that provided some of the earliest evidence of heterosis in plants. The three primary subgroups of the genus Populus are Aegiros (black poplars), Populus (aspens and white poplars) and Tacamaca (balsam poplars). The genus Populus, like that of several to be discussed, provides products such as pulpwood for which the breeder has much less constraint on genetic options. Tree breeders must emphasize strategies that improve their predictability of performance, working as they do with long‐rotation crops. A general trend of improved heterosis with geographic distance was encountered, but complicated by localized inbreeding and by problems of adaptation to local environments.