Syntheses of microenvironments based on monitoring are needed for understanding complex phenomena of an ecosystem and for establishing benchmark or standard regimes of temperature, light, and soil moisture for laboratory experiments involving seed germination, seedling growth, and other plant responses. These studies, in turn, answer questions about the dynamics of a plant community.

The microenvironment was monitored and vegetation was intensively sampled to interpret complex responses of an annual rangeland community to chemical weed control and reseeding.

Yield and density of plants varied among years, with aspect, within seasons, and in response to paraquat (1,1?-dimethyl-4,4?-bipyridinium ion). Establishment of hardinggrass and perlagrass (Phalaris spp.) was related directly to paraquat treatment and reduction of competition from resident annuals. Species and aspect (north and south slope) differences were more important in establishment of annual clovers (Trifolium spp.), with reduction of competition not the overriding factor.

Available soil moisture permitted seed germination, seedling emergence, and growth of plants in this community. Temperature controlled the general rate of growth, and also created detectable differences in plant response when it dropped low enough to affect germination of seeds or caryopses or rose high enough to stress plants directly, or indirectly by more rapid depletion of soil moisture. The overriding factor of competition which prevented establishment of hardinggrass and perlagrass was interception of light by the canopy of annual plants during the growing season. The shading effect was most severe at ground level. During crucial periods of emergence and growth of the seeded species, this shading was virtually restricted to 1 or 2 cm above the surface. Soil-moisture depletion mattered primarily at the end of the growing season, but in some restricted periods during the season it became an important factor.