The greater growth rate of male over female of the dioecious tree Juniperus thurifera only in worse habitat conditions

Dioecy is a mating system in which a greater reproductive effort has been observed in one sex than the other. This is expressed in differences between the sexes in growth rate and spatial segregation. The spatial structure of Juniperus thurifera subsp. thurifera and Juniperus communis subsp. communis and growth rate of tree-ring width of J. thurifera were tested in different habitat conditions in Santo Domingo de Silos in northern Spain. We analyzed junipers growing on the eastern slope, with better habitat conditions, and the western, with poorer habitat conditions. The studies found that males of J. communis were taller when compared to the females, both on the eastern and the western slope. J. thurifera females were taller on the eastern slope (the better habitat), but shorter on the western slope as compared to males. J. thurifera males and females on the eastern slope had the greater tree-ring width in comparison with those on the western slope. The annual tree–ring width of males dominated over females on both slopes at first, probably from the time of sexual maturation. However, these differences persisted to the end of the analyzed period only on the poorer western slope. The growth rate of females and males after several years was equal on the eastern slope. Spatial segregation has not been shown, probably due to the homogeneity of the habitat on both slopes. Studies have shown that the decrease of growth rate in J. thurifera females can occur only in the poorer habitat conditions.