Key message: Congeneric species showed similar stem and crown allometry, but differed in crown dimensions indicating that crown size is adaptive and variable despite mechanical restrictions.Abstract: Morphological adaptations favor differential use of the space in tropical trees, but the variability in stem and crown allometry can be constrained by phylogenetic and mechanical factors. In addition, dioecious species show marked differences in their energy requirements related to reproduction, but little information is available about the role of shape and allometry on differential acquisition of energy between the sexes. We studied the stem and crown dimensions of congeneric dioecious trees to determine if there are: (i) differences in the allometry between the sexes, (ii) different average sizes among sympatric species, and (iii) differences in stem and crown allometry between sympatric and allopatric species. Two pairs of sympatric Virola (Myristicaceae) in Brazil and Costa Rica were studied. SMA regression models were used to investigate allometric relationships between diameter at breast height (DBH) and tree height, and between DBH and crown volume (CV). No sexual dimorphism in stem and crown morphology was observed in this study, indicating that differences in resource allocation for reproduction between the sexes do not impact the stem and crown structure in these species. Overall, low variability among the species was observed. Only one species differed in stem allometry and none differed in crown allometry. CV differed between sympatric species. Stem and crown allometry are related to structural stability and our results support similar mechanical restriction for these species. The ecological significance of differences in CV among canopy species remains to be explored. © 2016 Springer-Verlag Berlin Heidelberg 19