Competition in Experimental Populations of Weeds with Emphasis on the Regulation of Population Size

Abstract

Several factors, singly or synergistically, may influence the behavior of weedy species under conditions of intraspecific competition. The species studied were Bromus inermis, B. tectorum, Capsella bursa—pastoris, Conyza canadensis, Plantago lanceolata, P. major, Senecio sylvaticus, S. viscosus, and Silene anglica. Some or all of the species were subjected to conditions of intraspecific competition involving variable regimes of: (1) seed—sowing density, (2) soil surface, (3) fertilizer, (4) moisture, and (5) habitat, i.e., greenhouse and non—greenhouse conditions. Initially the vegetative dry weight production per unit area increased as density increased for every species. After a certain density (relative to the species) was reached, production was independent of density. All species displayed considerable plasticity in that similar genotypes showed a great increases in seed production between high and low densities (e.g., over a 100—fold increase for Capsella bursa—pastoris). Mean seed weight was constant under variable conditions of density, fertilizer, soil surface, and moisture in al species except Bromus tectorum. In mixed stands, the population of each species is smaller than its potential maximum. The size of the population of seedlings may be determined by the number of microenvironments suitable for germination (safe—sites) in an area. The populations of the respective species may be further regulated by: (1) self—controlled germination (Bromus inermis, Conyza canadensis, Plantago lanceolata, and Silene anglica); (2) mortality (Plantago major); and (3) individual plasticity (Bromus tectorum, Capsella bursa—pastoris, Senecio sylvaticus, and S. viscosus).

Germination Biology and the Ecology of Annual Plants

“…Thus, the essential ingredients generating density dependence are variability in emergence time and the interaction between seedlings and buried seed. Density-dependent germination has been found in a number of studies (Palmblad 1968;Linhart 1976;Inouye 1980;Bergelson and Perry 1989). In Bergelson and Perry's study, the rate of germination was found to be dependent on the number of seeds in a microsite regardless of their specific identity.…”

The biology of Canadian weeds. 115. Conyza canadensis

“…In one of the seminal papers on self-thinning and the 3/2 power law, Yoda et al (1963) showed that density-dependant mortality of C. canadensis seedlings occurred only in populations of 1000 plants m -2 or more. Palmblad (1968) also suggested that population size of C. canadensis is regulated by phenotypic plasticity rather than mortality. As density increased, the number of flowering plants, individual plant size and the number of seeds per plant decreased, but seed production per unit area remained relatively constant.…”

THE BIOLOGY OF CANADIAN WEEDS.: 75. Bromus tectorum L.

“…Much effort has been expended in understanding the reasons for competitive success of downy brome (Stewart and Hull 1949;Harris 1967;Palmblad 1968;Rydrych and Muzik 1968;Rydrych 1974;Harvey et al 1975;Drawe and Palmblad 1977;Bookman 1980;Bookman andMack 1982, 1983;Thill et al 1984). Harris (1961) A.…”