Secondary sexual dimorphism in a dioecious tree: a matter of inter-plant variability?

Inter-plant variability (i.e., differences between individuals within population) may be a significant and rather neglected source of variation for addressing the occurrence of secondary sexual dimorphism in plants, especially in long-lived species. However, secondary sexual dimorphism (SSD) in plants is generally analyzed through the comparison of position metrics alone, mainly by the mean values of the male and female samples. Here we analyzed SSD in the dioecious tree Lithraea molleoides by a traditional comparison of position metrics and by taking into account inter-plant variability through a bootstrapping procedure i.e., by selecting randomly and one at a time, a female and a male from the sample and comparing them. This procedure was repeated several times and then the mean of those differences and the confidence intervals were calculated. Our results showed that inter-plant variability contribute to SSD, particularly in nutrient allocation to leaves and flowers. Some floral traits, on the other hand, were higher in males irrespective of inter-plant variability (i.e., flower number and flower mass). Accordingly, males produce more than twice as many flowers as females and their flowers had 22% more biomass than female's flowers. On the other side, females had 20% more floral N concentration per unit dry mass (floral Nmass) and had more leaf N concentration per unit dry mass (leaf Nmass) than males when inter-plant variability was considered. This differences in nutrient concentration could lead to a higher herbivory in females than in males given a higher leaf nutritional quality. Thus, the quantification of inter-plant variability seems crucial for detecting inter-sexual differences, which may have important ecological and evolutionary implications.