Submitted to: Canadian Journal of Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/1/2001
Publication Date: 11/20/2001
Citation: - Interpretive Summary: The survival of southwestern United States riparian forests is threatened due to decreases in available water with the construction of dams in the past century. We tried to determine some relatively easy measures of stress in cottonwood trees (Populus deltoides) along the Rio Grande in New Mexico. Over a three-year time period, we counted the number of male, female, and non-reproductive trees and measured tree size and distance to the river at six populations along the Rio Grande. An uneven sex ratio (the ratio of males to females) can often be caused by stress in a population, so the measurement of sex ratio could be a useful indicator of population stress. However, we found no biased sex ratios in any of these six populations. We did find that, even though these were adult age trees, a very high percentage of trees were non-reproductive. In addition, in populations where stress was the highest, many reproductive trees were becoming non-reproductive. There were also two types of non-reproductive trees: those that stayed non-reproductive over several years, and those that were previously reproductive. The former group appeared to be better adapted to the water stress at each population and was genetically different from the other trees in the population. Therefore, it appears that monitoring the percentage of non-reproductive trees and the number of reproductive trees becoming non-reproductive are useful indicators of stress in these six populations, and are relatively easy and inexpensive ways to predict the future survival of these forests.
Technical Abstract: Populus deltoides forests along the Rio Grande river drainage are predicted to disappear within this century. We evaluated stand health over three years by examining the sex ratio, size, and spatial distribution of male, female, and non-reproductive trees in six even-aged stands of Populus 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 non-reproductive, and a substantial number switched from reproductive to non-reproductive 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 non-reproductive individuals: those previously reproductive that became non-reproductive, and those that remained non-reproductive over the three years. The two groups appeared to be genetically different, and the former group may 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.