The reproductive ecology of the dioecious shrub Lindera benzoin L. Blume (Lauraceae) in sun and shade habitats

Abstract

Understory plants can occur in a wide range of light conditions. The effects of light environment on the pre- and post-pollination reproductive ecology of the dioecious understory shrub Lindera benzoin L. Blume (Lauraceae) were examined in a variety of field studies. Pre-pollination studies revealed that sex ratio was always 1:1; but flower production was greater in the sun than in the shade, and greater for male plants than for females in both habitat types. The population level ratio of pollen to ovules (P/O) was greater in the shade than in the sun, but pollinator abundance was a more important determinant of pollen deposition than P/O. Fruit set was significantly greater in the sun than in the shade, and was reduced in both habitats by artificially shading branches. Supplemental pollination added significantly more viable pollen to stigmas, but had no effect on fruit set, even under conditions of high light availability. Treatments did result in a significant reduction in per branch flower production, especially on fruit bearing branches in shade sites and on artificially shaded branches. Defoliations at the level of the branch significantly reduced fruit maturation. The gametophytic competition hypothesis which predicts that a plant should selectively mature fruits from flowers experiencing the highest levels of pollen tube competition was tested by examining the probability of fruit maturation as a function of pollen tube number. Styles were removed soon after the period of stigma receptivity, and the subsequent fate of the flowers was monitored. Flowers that eventually matured fruits had significantly more pollen tubes per style than those that initiated but then aborted fruits, supporting the hypothesis. The effects of gender and light environment on rates of herbivory and growth were explored. Herbivory was greater in shade habitats than in sun habitats, and vegetative plants suffered greater herbivory per leaf area than males or females which did not differ in the amount of leaf material eaten. Branch growth rates were higher in the sun than in the shade, and females grew less than male or vegetative plants