Testing predictions of spatial segregation of the sexes and life-history theory in three species of the dioecious genus Baccharis L. from southeastern Brazil.

I report results of a field study testing predictions of life history theory and spatial segregation of the sexes in three dioecious shrub species of Baccharis (L.) that co-occur in the eastern Brazilian Highlands. Baccharis is one of the most speciose genera of dioecious plants, yet the ecological implications of dioecy are poorly understood in the genus. I conducted statistical analyses to evaluate three hypotheses related to: (1) sex ratios and their relationship to spatial environmental gradients, (2) differences in reproductive and vegetative allocation among the sexes and their relationship to environmental gradients, and (3) the potential for notable divergence in response, based on Baccharis biology. I found that one species, B. opuntioides, met almost all theoretical expectations of life history theory and spatial segregation of the sexes, while another, B. platypoda, met some of them but did not meet others. The third species, B. myriocephala, met none of the theoretical expectations. For both B. myriocephala and B. platypoda, relationships between plant size and allocation to reproductive or vegetative biomass in males or females sometimes ran counter to life history predictions. Earlier studies suggested inconsistency in Baccharis' adherence to predictions of dioecy-driven resource allocation and spatial segregation patterns, perhaps driven by idiosyncrasies of genus biology. My study broadly supports that notion, but more robust testing of theoretical expectations in Baccharis will require more attention to the nature and length of resource gradients that are studied, and other complicating factors like intraspecific variance in peak flowering times.