Altitudinal variation of the sex ratio and segregation by gender in the dioecious plant Mercurialis perennis L. (Euphorbiaceae) in Serbia

The sex ratio is one of the most intriguing problems for evolutionary biologists. Spatio-temporal variation of male frequency in sexually dimorphic plant species, spatial segregation, and differential investment of the two sex functions have attracted much research interest. In this study, we examined altitudinal variation of the sex ratio and segregation by gender in Mercurialis perennis (dog?s mercury), a dioecious anemophilous species with wide distribution. The eight studied populations from Serbia represented an altitudinal range of 196 to 1480 m. Sex ratio was significantly biased in seven of the eight populations, with males outnumbering females 3.91:1 in the lowest-altitude population. Our results support the notion of spatial segregation of sexes along on altitudinal gradient: the frequency of males decreased with altitude, from 79.6% to 41.0%. The sex ratio was not significantly correlated with population density. We also examined intersexual differences of plant height in two populations from the extremes of the studied altitudinal range. Males were the larger sex in both populations, though the difference was not significant in the high-altitude population. .

Sexual demographics of riparian populations of Populus deltoides: Can mortality be predicted from a change in reproductive status?

Populus deltoides var. wislizenii (S. Wats.) Eckenwalder (cottonwood) forests along the Rio Grande drainage are predicted to disappear within this century. We evaluated stand health over 3 years by examining the sex ratio, size, and spatial distribution of male, female, and nonreproductive trees in six even-aged stands of P. deltoides spanning 280 km along the Rio Grande drainage. There was no evidence of biased sex ratios or spatial segregation of sexes; however, tree mortality was strongly related to reproductive status. In the most stressed population, over half of the mature trees remained nonreproductive and a substantial number switched from reproductive to nonreproductive status. Logistic regression revealed that over all populations, tree size, reproductive status in previous years, and population were significant factors affecting reproduction. By considering physiological and environmental differences, we determined two groups of nonreproductive individuals: those previously reproductive that became nonreproductive and those that remained nonreproductive over the 3 years. The former group appeared to be under greater physiological and environmental stress. Monitoring the rate at which individuals change reproductive status provides an effective and simple measurement that can predict survival of riparian cottonwood populations.Key Words: Populus deltoides, sex ratio, Rio Grande basin, reproductive status, nonreproductive.