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.