Master of Science (M.Sc.)
M. Drew Lamar
The density of individuals in a population has the potential to affect growth and death rates of that population. This effect of density is called density dependence and can be negatively or positively related to a vital rate. In most populations, increasing density leads to lower growth rates, which is negative density dependence. Positive density dependence, or Allee effects, occurs when increasing density leads to an increase in a vital rate, such as increasing survival in large herds. However, in clonal plants, there is the potential for the effects of density to be ameliorated. Clonal plants produce genetically identical progeny and some maintain a physiological connection even after the progeny mature. Through this physical connection, resources may be shared through clonal integration and thus clonal plants ameliorate the effects of stressful environments by acting as one large plant, or a genet, rather than as many individuals. Asclepias syriaca, common milkweed, is a clonal species that maintains connections to its clonal progeny. Common milkweed has been a popular species of study because of its relationship with the monarch butterfly, Danaus plexippus, and other specialist herbivores and because it is a pest plant in agriculture. In spite of this research, little is known about how individuals, or ramets, interact with one another. Understanding if ramets are integrated is key to furthering our understanding of how this species functions. My research on milkweed integration was conducted in two parts; a greenhouse experiment and a field study. The greenhouse experiment used pairs of milkweed to first determine if the defense chemicals of milkweed, known as cardenolides, are shared between damaged and undamaged ramets. A second portion of this experiment studied foliar concentrations of nutrients such as nitrogen and carbon and how they varied between connected and unconnected ramets. The third portion of this study focused on integration in a stressful environment, shade. The field study used three years of data from 4 sites and 18 transects across the state of Virginia to determine how density changes across years and how that potentially changes the responses of the plants. Measures of survival, growth, reproduction, and herbivory were taken each year. These data were modeled with generalized mixed effects models to simultaneously take into account the effects of year, site, and transect on each plant response. These responses were predicted to follow patterns that would suggest integration, which could be seen as a lack of competition when it would normally be expected. In the cardenolide study, all ramets responded to the damage treatment, indicating a volatile signal of herbivory. The foliar trait study showed connected ramets may be better at taking up nitrogen than unconnected individuals. Biomass at the end of the shade experiment showed signs of competition in unconnected but not in connected ramets, indicating integration. In the field study density only affected survival, height, leaf area, growth, and the number of inflorescences. at the patch scale, increasing density decreased survival, height, leaf area, and the number of inflorescences. Growth was not affected by density at the patch scale, but at the small scale it decreased with increasing density. Survival and height also decreased with increasing density at the small scale. The number of inflorescences was not affected by density at the small scale.
© The Author
Seward, Mary Danielle, "Density Dependence and Clonal Integration in Common Milkweed, Asclepias Syriaca" (2016). Dissertations, Theses, and Masters Projects. Paper 1477068024.