Sexual dimorphism in reproductive and vegetative allometry for two dioecious Rhamnus plants in north-eastern China

The dimorphism in reproductive allometry has received much attention, while rather little is known about the sexual dimorphism in the vegetative allometry of a dioecious species. This study presents vegetative and reproductive allometries for two dioecious tree species of the genus Rhamnus in north-eastern China. A power function and a linear model were found suitable for describing different components of the vegetative allometry. In addition, a linear regression was used to estimate the number of flowers and/or fruits based on observed stem size. R. davurica being the taller species of the two shows highly significant correlations for all selected combinations of different vegetative components. These correlations are much less pronounced and sometimes even nonsignificant in R. schneideri, which occupies a great variety of niches at the understory with diverse crown shapes. Component biomass was highly correlated with tree diameter for both sexes, but the allometries changed with sex and species. Both females and males show consistent patterns of reproductive size dependency. However, the slope of the linear relations differs between the sexes again indicating gender-related differences in reproductive size. According to theory, females allocate more biomass to reproduction than males, which negatively affects their vegetative growth. However, in this study, flowering females did not show less vegetative growth than males suggesting that the two Rhamnus species behave contrary to expectation. Further studies are required to test the assumption that photosynthesis by reproductive organs may substantially contribute to vegetative growth.