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

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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).