Individual and Population Responses of Sexual and Apomictic Plants to Environmental Gradients

The responses of populations and of individual genotypes of the perennial herb Antennaria parlinii to gradients of light and nutrients were measured. Sexual populations generally produced more total biomass than did apomictic populations. However, the reproductive biomass of apomictic populations was consistently greater than that of sexual populations at all resource levels. In addition, the percent allocation to reproduction by apomictic plants increased as light or nutrients increased, whereas that of the sexual plants was comparatively stable. No consistent differences in total or reproductive mass between male and female sexual plants were evident across the gradients. In general, the growth of apomictic plants was low in magnitude and more even across the range of resources available on the two gradients, whereas sexual populations were usually more productive but narrower than average. However, reproduction in apomictic populations was higher in magnitude and lower than average in breadth, whereas sexual populations were low in magnitude but more even in response breadth. At a within-population level, the responses of individual sexual genotypes were quite variable, but populations of apomictic plants were relatively homogeneous. Populations of sexual plants also had a greater between-genotype component of response breadth than did apomictic populations. These results suggest that A. parlinii apomicts are well suited for the colonization of disturbed sites. These populations are highly plastic in their ability to grow across a wide range of resources, and they are opportunistic in reproduction, increasing their percent allocation to reproduction as resources increase. In contrast, sexual populations showed characteristics that should promote competitive ability: a relatively broad response for reproduction; a conservative allocation pattern that emphasizes vegetative growth over seed production; and a diversity of responses to resource gradients among individuals within populations.