ANDROMONOECY IN THE NEW ZEALAND MONTANE SHRUB MANUKA, LEPTOSPERMUM SCOPARIUM (MYRTACEAE)

The ecological and evolutionary advantages of andromonoecy are considered by examining populations of a montane grasslands shrub. Manuka plants (Leptospermum scoparium, Myrtaceae) produce two kinds of flowers: male flowers having functional stamens and a nonfunctional pistil, and hermaphrodite flowers in which both the stamens and pistil are functional. Factors affecting the ratio of the two flower types were investigated at Cass, Canterbury, New Zealand. Stamens mature at the same rate in the two flower types. Individual plants within one population vary from having predominantly hermaphrodite flowers to having no hermaphrodite flowers. Individual variation within a population in the percentage of hermaphrodite flowers is correlated between seasons, indicating some stability in this character. In a particular season, variation among plants within a population in the percentage of hermaphrodite flowers is negatively correlated with date of flowering, but it is not correlated with either the number of flowers or vegetative growth per plant. The hermaphrodite flowers tend to open in the first flush of flowering, and the proportion of hermaphrodite flowers subsequently declines in individual plants and in the population as a whole. The earlier anthesis of hermaphrodite flowers probably increases the frequency of outcrossing; it also provides more time for fruit to mature. The percentage of hermaphrodite flowers is greater toward the top of the plant and among terminal flowers on branchlets than among proximal flowers. The percentage of hermaphrodite flowers on a plant was progressively increased with increasing nutrient treatments. The flowers are visited by a wide range of insects. Fruit set was not increased by hand pollinations of flowers exposed to natural insect visits, indicating that pollinator activity does not limit natural fruit production. A considerable fraction of the variation between individuals in the proportions of male and hermaphrodite flowers is environmentally induced. Andromonoecy may have been selected for in manuka because it is advantageous for pollen to be presented in more flowers than the number of ovuleand seed-bearing flowers. Alternatively, andromonoecy may allow an individual plant to adjust its reproductive output to its immediate physiological condition. However, andromonoecy by itself is not an out-breeding mechanism. IN MONOECIOUS plants, male and female flowers occur on the same individuals, so their distributions on the plant must be controlled in part by physiological factors. Temperature, mineral nutrition, light, edaphic factors, and plant hormones have all been suggested as controlling the proportions of the two flower types, but there have been few conclusive experiments (reviewed by Frankel and Galun, 1977). In addition, genetic factors may modify the control of flower types, and genotypes may vary in the proportions of the two flower types and in their responsiveness to environmental 1 Received for publication 4 June 1979; revision accepted 21 November 1979. A post-doctoral fellowship to one of us (RBP) was provided by the Miss E. L. Hellaby Indigenous Grasslands Research Trust and the National Science Foundation (Grant INT 76-24110) under the United States-New Zealand Science and Technology Agreement. Drs. K. Bawa, E. Godley, and C. Webb provided helpful comments on the manuscript. Simon Pollard assisted with the field work. 2 Present address: Biology Department, Boston University, Boston, MA 02215. conditions affecting flower types (Braak and Kho, 1958; Singh and Ramanujam, 1973; Frankel and Galun, 1977). Previous studies on andromonoecious species (plants which produce male and hermaphrodite flowers) have emphasized herbaceous species grown under controlled conditions. The purpose of the present study is to determine what factors influence the proportions of the two flower types in the andromonoecious shrub manuka, Leptospermum scoparium J.R. et G. Forst. (Myrtaceae), in the New Zealand montane grasslands. The evolutionary and ecological advantages of andromonoecy are not understood at present. Although it is generally assumed that andromonoecy is a mechanism for out-crossing, this has never been examined critically. By providing a detailed field study of this mating system, the advantages of andromonoecy may become apparent. Manuka is an important species in the woody vegetation which invades ground disturbed by fire and grazing in many parts of New Zealand. Manuka often forms a dense thicket, reducing