PS 103-232
Spatial scale changes density dependence in the clonal plant Asclepias syriaca

Friday, August 14, 2015
Exhibit Hall, Baltimore Convention Center
Mary D. Seward, Biology, College of William and Mary, Williamsburg, VA
Harmony J. Dalgleish, Department of Biology, College of William and Mary, Williamsburg, VA
Abigail Kula, Mount St. Mary's University, Emmitsburg, MD

Density dependence is found in populations of organisms where density alters growth, survival and/or reproduction. While density usually decreases vital rates positive density dependence (i.e., Allee effects) can also occur. Aboveground ramets of clonal plants may not experience density dependence if genets are physiologically integrated. Asclepias syriaca, the common milkweed, is a clonal plant of particular interest because of its relationship with the IUCN near threatened Monarch butterfly.

The goal of this research project is to investigate density dependence in common milkweed ramets in four field sites across Virginia. Data on plant density, height, and number of viable seed pods was used to investigate the effects of density dependence. Density was compared at multiple scales ranging from a circle with 0.1 to 1.5 meter radius all the way up to the density at the entire patch scale. In addition, we examined differences in soil composition among patches and at varying densities to investigate for the effects of soil resources on plant vital rates. We hypothesized that there would be a signal of density dependence, with plants growing larger and having more viable seed pods in dense patches.


Initial results from the 2014 field season indicate that there are signs for density dependence at certain scales in both plant height and the number of viable seed pods produced. The number of viable pods was negatively density dependent at the small scale (0.41m), with a slope of .55 and a p-value of 0.013 and a positive relationship at the large scale (transect level) with a slope of -0.015 and a p-value less than 0.0001. Plant height also showed the anticipated positive density dependence relationship, which was significant at 5 scales, starting at 0.72m and again at the 1.19, 1.34, and transect level all with a slope less than 0.1, These results indicate that plant height is perhaps most driven by inter-ramet competition for sunlight. These results indicate that density dependence is scale-dependent in clonal plants, with different relationships at small spatial scales verses large spatial scales.