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

Abstract

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 ovule‐ and 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.