Postdispersal selection following mixed mating in Eucalyptus Regnans

Eucalyptus regnans is a mass flowering, tall forest tree of southeastern Australia with a mixed mating system. A field trial containing randomized single tree plots of self, outcross, and naturally open-pollinated (OF) progenies of 13 parents from two natural populations was surveyed over 15 yr. Inbreeding depression in survival at 15 years was 67% for selfs, one of the highest levels reported for a tree species, and differed little between populations. OP progenies were intermediate and the difference in fitness among the three cross types indicated that at planting, 59% were derived from outcrossing. However, with selection against the inbred progenies, this increased to 83% by 15 yr of age. Most selection against selfs occurred after four years, coincident with canopy closure and the apparent onset of intense competition. Almost twice the amount of phenotypic variability occurred among self-progenies than occurred among outcrosses with OPs again intermediate. Phenotypic variation was much greater within OP families than among families. Mortality removed the smaller trees, reducing inbreeding depression and variation both among and within OP families. In contrast, inbreeding depression and variation within OP families increased during stand development. These processes redistributed variation from among to within OP families, resulting in little overall change in phenotypic variance. Under intense competition in naturally regenerating forests, selfs are unlikely to survive to reproductive maturity. Although there was no additive variation among parents in the survival of their outcross progenies, the survival of both OP and self-progenies differed markedly among parents. However, variation in parental fitness under natural open-pollination was unrelated to estimates of fitness under outcrossing or selfing. Because parents each contributed the same number of offspring in this experiment, variation in OP fitness may instead reflect differences in outcrossing rate among families. Mixed mating may offer reproductive assurance and be advantageous in native forests if the availability of outcross pollen limits overall reproductive output.