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

Several factors, singly or synergistically, may influence the behavior of weedy species under conditions of intraspecific competition. The species studied were Brownals inermis, B. tectorum, Capsella bursa-pastoris, Conysa canadensis, Plantago lanceolata, P. major, Se7tecio sylvaticuts, S. viscosus, and Silene acnglica. Some or all of the species were subjected to condi- tions 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-greenlhouse (onditions. Initially the vegetative dry weight production per unit area increased as density in- creased for every species. After a certain density (relative to the species) was reached, produc- tion was independent of density. All species displayed considerable plasticity in that similar geno- types showed a great increase 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 con(litions of density, fertilizer, soil surface, and moisture in all species except Bromus tectorum. In nmixed stands, the population of each species is smaller than its potential maximum. The size of a 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 bc further regulated by: (1) self-controlled germination (Bromus inlermis, ConyZa cana- densis, Plantago lanzceolata, and Silene anglica) ; (2) mortality (Plantago major); and (3) individual plasticity (Bromus tectorum, Capsella bursa-pastoris, Senecio sylvaticus, and S. viscosus).