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14 <br />number of shrubs decreased (Figure 4). The 15 and 30 cm treatments <br />• supported significantly more shrubs than the 45 and 60 cm <br />treatments. The shrub response is similar to the pattern for <br />perennial forbs that was discussed earlier. The most abundant <br />shrubs were antelope bitterbrush (Purshia tridentata), big <br />sagebrush (Artemisia tridentata) and big rabbitbrush (Chrysothamnus <br />nauseosus). The densities of antelope bitterbrush and big <br />sagebrush were significantly greater on the 15 cm treatment <br />compared to 60 cm (Table 5). Three other shrub species were <br />encountered during the 1990 sampling but were present in only trace <br />amounts. These species included Douglas rabbitbrush (C. <br />viscidiflorus), winterfat (Ceratoides Janata) and mountain <br />snowberry (Symohoricarnos oreophilus). Antelope bitterbrush and <br />• winterfat were the only two species encountered in 1990 that were <br />originally seeded in 1980; the remaining species reached the site <br />as part of the topsoil seed bank or through a natural migration <br />process. <br />The total number of species was inversely related to topsoil <br />depth with significantly more species occurring on the 15 cm <br />treatment compared to 60 cm (Figure 5). This phenomenon is related <br />to the hypothesis proposed by Huston (1979) where species diversity <br />in developing plant communities is not determined by the absolute <br />competitive ability of the species, but by the rate at which they <br />are expressed in the system. As such, conditions that increase <br />growth rate of competitive species should result in lower diversity <br />due to competitive exclusion and conditions that reduce growth rate <br />should result in higher diversity because the rate at which <br />