Population structure and breeding biology in relation to conservation in the dioecious Gardenia actinocarpa (Rubiaceae) – a rare shrub of North Queensland rainforest

Abstract

Knowledge of breeding systems of endangered plants is crucial in formulating effective management plans for their conservation. Consequently, spatial distribution, flowering and fruiting patterns of Gardenia actinocarpa Puttock (Rubiaceae), a rare and endemic dioecious shrub of lowland tropical rainforest of Cape Tribulation, North Queensland, Australia were documented in four sub-populations over a two year period. Sex ratio was unity in two of the sites, and significantly biased in favour of male plants in the other two sites. At most sites, analysis of spatial distribution of individuals indicated aggregated dispersion patterns irrespective of quadrat size and plant size/reproductive status. There was no evidence of niche differentiation nor switching of the sexes. The species exhibited an extended flowering pattern of almost six months. However, male trees flowered earlier, longer, and produced more flowers than female trees. Intensity of flowering was significantly correlated with plant size in both sexes. Fruit set was very low in each of the two years of observation, but was significantly predicted by female habitat light environment, flower production and plant size. The lack of neighbourhood effects such as male tree density, male tree flower production around a focal female tree and distance from a female tree to the nearest male tree on fruit set is an indication that G. actinocarpa pollinators are capable of carrying pollen over long distances. Conservation implications of these findings are discussed.