Population Dynamics of Two Competing Annual Plant Species

Population attributes including density, distribution, biomass, seed production, and survival were measured at significant stages throughout the life cycles of natural and experimental populations of Sedum smallii (Britt.) Ahles (Crassulaceae) and Minuaritia uniflora (Walt.) Matif. (Caryophyllaceae). These two species along with Viguiera porteri (A. Gray) Blake (Asteraceae) are dominant in island communities on granite outcrops in the southeastern United states. Actuarial life tables and survivorship curves were constructed for natural populations of both Sedum and Minuarita. On the basis of field studies the hypothesis was proposed that the population dynamics of dominant outcrop species are primarily influenced by interspecific competition for soil moisture in habitats of varying soil depth. This hypothesis was tested by growing both species under varying levels of soil moisture, soil depth, and competition in a programmed environment. Final verification of cause—and—effect relationships was obtained through experiments under modified field conditions. Strict zonation of Sedum smallii and Minuartia uniflora along intensity gradients of soil depth and soil moisture in the island communities is due to (1) high tolerance of Sedum to low moisture levels in soils less than 4 cm deep; (2) the competitive superiority of Minuartia over Sedum under more favorable moisture levels prevailing at soil depths from 4 cm to 10 cm; and (3) the competitive disadvantage of Minuartia at high moisture levels in deeper soils that support larger annual and perennial species. Sedum smallii is a primary invader in granite outcrop succession because of adaptations which allow it to exist under low moisture conditions in shallow soil unfavorable for growth of other outcrop species. Minuartia uniflora is less resistant to abiotic stress and invades later. Both Sedum and Minuartia exhibit high reproductive potentials typical of colonizing species. Early juvenile stages are the most vulnerable periods in the life cycles of both species. Washout of seeds and seedlings from the communities and the sensitivity of these juvenile stages to moisture stress are the major causes of early mortality. Sedum plants are stunted by increased abiotic stress although population densities may remain high. On the other hand, interspecific competition causes increased mortality. In Minauritia stunting is largely a response to severe environment, whereas mortality is caused by both abiotic stress and interspecific competition. Species zonation and outcrop community structure are therefore a consequence of shifting competitive superiority as physical conditions shift.