Fruit set in Styrax obassia (Styracaceae): The effect of light availability, display size, and local floral density

Maternal reproductive success was examined in Styrax obassia (Styracaceae), a bumble-bee pollinated mass-flowering tree in a cool-temperate deciduous forest in northern Japan. The effects of flower number on the success of in dividual flowers at three levels (inflorescence, individual, and population ) were considered. During 1995 and 1996, variations in size, light availabi lity to branches, floral display size, and fruit set were monitored in 37 o ut of 211 individual S. obassia trees in a 4-ha forest plot. In addition, t he locations of the 211 trees in this plot were mapped and the number of in florescences in each tree was counted. A multiple regression analysis showe d that flower number per inflorescence and inflorescence number per individ ual had negative effects on fruit set, and inflorescence number of aggregat ed clumps of flowering trees, tree size, and light resource had positive ef fects on fruit set although significant level were marginal. It is conclude d that pollinator attraction may occur not at the individual tree level, bu t at the level of a clump of flowering trees. It is also suggested that gei tonogamy increased with inflorescence number of tree and inflorescence size and that resource limitation was related to the light condition and variat ion of tree size.