Invasion of Medusahead into the Great Basin

We characterized soil and vegetation assemblages, many of which are infested with medusahead (Taeniatherum asperum (Sim.) Nevski), on the margin of the Great Basin. Interpretations of these assemblages provide an index of the validity of the basic environmental unit of this ecosystem which can be manipulated through weed control and revegetation techniques. Vertisol (churn- ing clay soils) sites with sparse native plant communities are more susceptible to medusahead invasion than more complex low sage- brush (Artemisia arbuscula Nutt.) or low sagebrush-woodland communities on related clay soils. If the more complex communities are degraded to a low seral state, medusahead can invade and occupy the site. Wet meadows and burned coniferous forest sites at high elevations were the only sites where medusahead occurred on soils with textures other than clay. Big sagebrush (Artemisia tri- dentata Nutt.) communities on medium to coarse textured soils were very resistant to medusahead invasion. The restriction of medusahead to certairl sites controls the mechanism of invasion and interacts with the bree(ling system of the species to influence its evolution.