Dioecy is a relatively uncommon but widely distributed breeding system among angiosperm families. Much attention has been paid to factors influencing the evolution of dioecy and its correlates. For example, the association between dioecy and woody growth form appears to be a mechanism mitigating reduced mate assurance that accompanies possessing separate sexes. Conversely, relatively short shrift has been given to factors affecting the present-day abundance and distribution of dioecious species. The relative incidence of dioecy is generally highest near the equator, and declines towards the poles. Based on largely anecdotal observations, it has been suggested that dioecy may also generally increase with altitude. Here, we used data from 15 1-ha plots along a 2600m altitudinal gradient in Costa Rica to examine patterns in species richness (SR), phylogenetic diversity (PD) and incidence of dioecy. As a control, we also analyzed the incidence of monoecy. For all woody species in plots, we collated data on their breeding system, inferred pollination system, total number of stems, and total PD. Based on the weight of limited empirical investigations and anecdotal observations, we predicted that relative incidence of dioecy should be positively associated with altitude, SR, and PD. Conversely, we predicted weak to nonexistent associations between relative incidence of monoecy and these variables.

Results/Conclusions

Species richness and PD displayed strongly humped relationships with altitude, both reaching their highest peak at 750m. Relative incidence of dioecy displayed a weakly humped relationship with altitude, and strong positive correlations with SR and PD of plots. Conversely, relative incidence of monoecy was negatively associated with altitude, and not correlated with SR or PD. A post hoc investigation of pollination systems revealed no marked differences between dioecious and monoecious species, nor a signal of a consistent shift from one pollinator type to another among dioecious species, along the gradient. Insofar as altitudinal gradients may be microcosms of latitudinal gradients, it is interesting to note that the relative proportion of dioecy is often markedly elevated in tropical forests, which have high SR and PD. Time permitting, we will briefly discuss preliminary results from ongoing analyses of other altitudinal gradients. Further investigation will be required to shed light on the generality of, and mechanisms underlying, such patterns.