Reproductive Success of Two Eucalyptus Hybrid Populations. II. Comparison of Predispersal Seed Parameters

Abstract

The reproductive success of E. melanophloia × E. crebra and E. populnea × E. crebra hybrid populations is examined in terms of seed output. Predispersal seed losses and outputs from fruits and trees of hybrids and species in mixed- and single-species stands are presented. The contribution of the hybrid population to the total eucalypt seed output is considered by including in the estimates the numbers of hybrid and species trees per unit area of ecotone. Losses of E. melanophloia × E. crebra hybrid seeds through abortion, insect predation and fungal infection are significantly greater than losses of parent seeds and seeds of the other hybrid, and average 70% overall compared with 22% for each parent. If losses through bud maturation failure are included, 93% of the hybrid potential is lost. Hybrid outputs of 7 × 10³ whole seeds per tree and 22 × 10⁴ per ha of ecotone are respectively 1.5%, and 0.3% of the total seed output of the three taxa. Overall losses in the E. populnea × E. crebra hybrid system are much lower and less important to survival. The effect on seed number is dependent upon seed potential per fruit. E. populnea is the least affected (9% loss, very high potential), E. crebra the worst (21%, very low) and the hybrids are intermediate and in a relatively strong competitive position (20%, intermediate). Hybrid outputs of 145 × 10³ seeds per tree and 23 × 10³ per ha are respectively 21% and 17% of the total output. The significance of these results is discussed in relation to the reliance of eucalypts upon a high seed output strategy for survival and to the hybridization success of the two systems.