DYNAMICS OF SYNTHETIC PHLOX DRUMMONDII POPULATIONS AT THE SPECIES MARGIN

We compare the dynamics of artificial populations of the annual Phlox drummondii introduced along a transect extending from within to beyond the species margin southwest of San Antonio, Texas. The margin is defined by a shift from sandy, nutrient poor soils to clay, nutrient-rich soils. Fourteen sites were chosen along the 6.5-mi transect. In each site seeds were planted into 1.5-m-square cleared plots and uncleared 10-m transects. Transplants also were introduced into cleared plots. Survival, flower production and seed set were monitored over the course of the growing season. Individual fitnesses and the reproductive rates of artificial populations generally declined as the distance from the margin increased. The results indicate soil type per se is not limiting Phlox and that plants can successfully grow and reproduce, albeit at a reduced level, in sites beyond their natural margin.