Red fescue (Festuca rubraL.), yarrow (Achillea millefoliumL.), and mouse-eared chickweed (Cerastium vulgatumL.) were seeded at six densities under greenhouse conditions. Total dry matter production per pot (shoots and roots) increased as density increased for every species until a point was reached where a further increase of the density did not result in a higher production per pot. All species produced about the same shoot yield per pot. Root production per pot, however, was different among the species. The shootndash;root ratio was the highest for mouse-eared chickweed, the lowest for red fescue, and intermediate for yarrow. All species exhibited fairly high germination and very low mortality, except yarrow. The three species displayed considerable plasticity in yield and in seed production per pot from low to high densities. The reproductive behavior varied between the species. Red fescue remained vegetative. Sexual reproduction of yarrow decreased drastically with increasing density. Mouse-eared chickweek maintained a large seed production over the whole range of densities. The population of the respective species seemed regulated by (1) individual plasticity and vegetative reproduction (red fescue), (2) individual plasticity, mortality, and limited seed production (yarrow), and (3) individual plasticity and high seed production (mouse-eared chickweed).
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