Résumé
Variation in plant sex ratios is often attributable to sex-specific
mortality in heterogeneous environments that differentially limit male and
female plant reproduction. Yet sexual dimorphism and plastic responses to
environmental heterogeneity are common and may co-vary with variation in
sex ratios. Here, we show that the sex ratio and the degree of sexual
dimorphism for a number of plant traits varied along climatic and
elevation gradients in three wind-pollinated dioecious species, Rumex
lunaria, Urtica dioica and Salix helvetica. Some of the observed
sex-specific responses to climatic variation are consistent with greater
sensitivity of females to water scarcity, but most responses rather point
to the greater sensitivity of males to ecological stress, consistent with
larger male reproductive effort, as has been commonly reported for
wind-pollinated plants. In contrast, we found no evidence for variation in
either sex ratios or sexual dimorphism expected under sexual selection.
Interestingly, sex ratios and sexual dimorphism varied both along distinct
and the same ecological axes of variation, suggesting that the evolution
of sexual dimorphism in the measured traits was not sufficient to prevent
sex-specific mortality.