Population dynamics of herbaceous and shrub species were investigated in big sagebrush/Thurber needlegrass communities burned in wildfires. The sites burned are representative of extensive areas of degraded rangelands in the central Great Basin. Herbaceous succession after wildfires in these communities is dominated by the dynamics of populations of the alien annual grass downy brome. Downy brome caryopses are greatly reduced by most large wildfires, but the plants originating from surviving downy brome caryopses respond dynamically to the released environmental potential. The response may include hybridization and recombination. The result is a purge of native annual species from the community and the failure of seedlings of native perennial grass species to become established. Root and crown sprouting of the native shrubs, green rabbitbrush and horsebrush, occurs after the wildfire has killed the dominant big sagebrush. Green rabbitbrush sprouts produce abundant achenes, which readily germinate and grow. The reestablishment of downy brome dominance predisposes the vegetation to recurring wildfires and cyclic environmental degradation.

Plant litter that covers the soil surface acts as a layer of insulation moderating temperature and moisture, and creating favorable microsites for germination and the establishment of annual weed species in rangeland communities. Litter cover is an important factor in succession among annual species which culminates in dominance by downy brome (Bromus tectorumL.) and medusahead (Taeniatherum asperum(Sim.) Nevski) in these communities.

We investigated the periodicity of germination, longevity of caryopses viability, and reproductive capacity of downy brome (Bromus tectorumL.) populations in relation to chemical and mechanical fallow and fall tillage weed control treatments. The important factors in the dynamics of downy brome populations are: (a) large numbers of viable caryopses are carried from 1 year to the next in the litter and soil; (b) downy brome caryopsis production is density dependent; and (c) the simultaneous germination characteristics of freshly harvested downy brome caryopses can be conditioned environmentally to continuous germination.

The age, density, and fire history of western juniper (Juniperus occidentalis Hook.) trees growing on range sites of contrasting potentials were investigated. The l,OOO-ha study area consisted of 65% big sagebrush [Artemisia tridentata Nutt. subsp. wyomingensis (Rybd.) Beetle] and 30% low sagebrush (A. arbuscula Nutt.

The purpose of this study was to investigate seed dispersal and germination of seeds in the soil as factors in establishment of big sagebrush seedlings. Seed dispersal began at seed maturity in December. Under the shrub canopy the dispersal of seeds was greater to the east. Between shrubs the dispersal of seeds tended to be uniform. For 6 months of each year there were no detectable germinable seeds in the litter and surface soil at the sites sampled. Germinable seeds in the soil increased rapidly during seed dispersal but were not detected by June of the next season. Enrichment of bioassay samples with GA3or KNO3did not enhance emergence of big sagebrush seedlings. Big sagebrush seedlings were found at very low densities in areas previously burned in wildfires. This suggests the presence of viable big sagebrush seeds in the soil at a very low density.