Reproductive Success of Two Eucalyptus Hybrid Populations. I. Generalized Seed Output Model and Comparison of Fruit Parameters

Abstract

The dynamics of the reproductive success of two interspecific Eucalyptus hybrid populations are examined here and in a subsequent paper in relation to a generalized model of seed output of a single plant. The model describes a structural system of seed output per plant under both physiological and physical regulation. It can readily be extended to a population model and to postdispersal reproductive stages. Fruit loss, output and damage in the two populations are presented. The E. melanophloia × E. crebra hybrids are considerably less fertile than their parent species. Significant loss of reproductive structures during maturation and low reproductive effort, together with variable canopy size and proportion of canopy bearing fruits, result in the fruit output per average hybrid tree being 10% of either of the average species trees. Biotic damage to surviving hybrid fruits by an insect and a fungus is individually significantly greater than levels suffered by the species and together account for an average of 56% of fruits compared with 16% for either species. Damage and loss levels to these hybrids are more or less constant in space and time and constitute a strong selective force against these hybrids. In contrast, fruit loss parameters are low and differences between the taxa are non-significant throughout the E.populnea × E. crebra hybrid system, resulting in hybrid fertility about equal to the fertilities of the parents. These contrasting results are discussed in relation to hybrid frequencies in the ecotone and levels of hybrid population development.