"Two Bob Each Way": The Pollination and Breeding System of the Australian Rain Forest Tree Syzygium cormiflorum (Myrtaceae)

Abstract

The breeding system of a cauliflorous rainforest tree Syzygium cormiflorum was examined to assess the degree of self-compatibility, the type and effectiveness of its pollinators, and the postfertilization seed loss and net seed production. The species is xenogamous, but shows a degree of self-compatibility, with successful pollination via autogamy ranging from 10 to 32 percent, via geitonogamy 16-60 percent, via intrapopulation xenogamy 58-82 percent, and via interpopulation xenogamy 80-92 percent. The difference in fertilization rates between autogamy and geitonogamy is discussed. Bats, birds, and insects were successful pollinators, with bats (and possibly large moths) being the most effective despite visiting the flowers less frequently than birds. Bats, and possibly large moths, accounted for 55.4 percent of successful pollinations, compared to 19.6 percent for birds and 25 percent for small insects. Postfertilization seed loss due to the predatory activity of a weevil was very high, up to 70 percent. The pattern of average nectar flow favors birds, but individual flowers produce nectar at varying times suitable for nocturnal or diurnal visitors or both. There is some evidence that breeding is restricted to crosses between individuals in a local population. The pollination system may be adaptive in maximizing fertilization in situations lacking the major pollinators, but the difficulty of distinguishing this from a system evolving from night to day pollination or vice versa is recognized. THE RAINFOREST TREE FLORA OF AUSTRALIA totals over 1200 species, yet there has been no quantitative study made of the pollination biology and breeding system of any of them. This is in marked contrast to the Neotropics and southeast Asia where a few studies of controlled pollination and pollinator biology have been done, resulting in a rapid reassessment of the general view of breeding systems in tropical forest trees. The earlier view (Corner 1954, Baker 1959, Fedorov 1966) was that rainforest trees were mostly self-compatible and self-pollinated, allowing maximum seed set in an environment where pollinator movements were supposedly limited. It is now held that outcrossing is the more important phenomenon encountered (e.g., Ashton 1969, Bawa 1974, Frankie & Baker 1974, Frankie et al. 1976). The Myrtaceae is an important and conspicuous element in the Australian flora and is well represented in our rainforests. The largest genus in these forests is the widely distributed Syzygium, which has over 500 species world wide with 52 in Australia of which 41 are endemic (Hyland 1983). Studies of two species of Syzygium have already been reported. Lack and Kevan (1984) examined the breeding system of Syzygium lineatum on Sulawesi, and Hopper (1980) recorded floral visitors to Syzygium tierneyanum in north Queensland. I Received 31 December 1984, revision accepted 20 April 1985. The subject of the present study, the first quantitative assessment of breeding of an Australian rainforest tree, was S. cormiflorum (F. Muell.) B. Hyland, a small subcanopy tree which commonly occurs in a variety of rainforest types between Townsville and Cooktown in north Queensland and again further north in the forests of the Mcllwraith and Iron ranges. It exists as two forms, a small (up to 60 cm DBH) cauliflorous tree or a larger (up to 100 cm DBH) ramiflorous tree; the former occurring from sea level to 1200 m altitude but commoner at higher elevations, and the latter from sea level to 900 m and commoner at lower elevations. On a small scale it appears to be patchily distributed. All further references are to the cauliflorous form. The flowers are not numerous and are borne in small cymes on cauline excrescences that increase in size with age. They are large (calyx tube 7-20 mm long x 9-16 mm diameter, filaments 15-30 mm long) and creamy white with four small white petals and numerous stamens (a shaving brush type) and have a faint unpleasant smell. The majority project downward, but in big clusters they are forced to point in all directions. There are 3050 ovules in the ovary of the cauliflorous form and 90100 in the ramiflorous form. The fruits are large (30-60 mm diameter), red or white, and fleshy. The only recorded floral visitor is the nocturnal pygmy possum (Cercatetus caudatus) (Winter 1979), but the BIOTROPICA 18(2): 115-125 1986 115 This content downloaded from 157.55.39.17 on Fri, 02 Sep 2016 05:36:59 UTC All use subject to http://about.jstor.org/terms floral attributes of this tree are those that characterize plants with a bat-pollination syndrome (Faegri & van der Pijl 1971). The aims of this study were to determine by controlled pollination the degree of self-compatibility in S. cormiflorum, to assess the effectiveness of its floral visitors as pollinators and to relate this to floral biology, and to measure postfertilization fruit loss and net seed produc-