Different cost of reproduction for the males and females of the rare dioecious shrub Corema conradii (Empetraceae)

Males and females of dioecious plant species often differ in their reproduc tive investment. Such differences frequently result in differential demogra phic costs represented by lower growth, survival, and/or frequency of repro duction, and/or by more Variable reproductive effort through time for femal es. We present the results of a study on Corema conradii, a rare dioecious shrub of the coastal dune heathlands of northeastern North America. We esti mated the reproductive investment of both males and females, determined the ir age structure, and compared their spatial patterns in a population at li es-de-la-Madeleine, Quebec. We also determined the sex ratio of the foul po pulations known to occur on the islands. Males invested more in reproductio n at flowering. but when fruit production was considered, female reproducti ve investment was higher in terms of biomass, Mg, and Ca, but not in terms of N. P, and K. The age frequency distribution of males and females did not differ significantly from one another. The population dispersion pattern w as contagious. with patches of similar-age individuals. There was no spatia l segregation between males and females, although the sex ratio varied some what spatially. Females did not start reproducing at a later age than males and did not appear to have a shorter longevity. However. the crown and rad ial growth rates of females were lower than those of males. When estimated by the crown intercept method, the sex ratio of all four populations was ma le biased. However, because males had a higher crown growth rate, genet sex ratio was in fact balanced. Higher investment in reproduction was associat ed with a lower growth rate, which represents a differential cost of reprod uction according to sex in this species.