The Effects of Introduced Annuals on Secondary Succession in Sagebrush-Grassland, Wyoming

A field study was conducted in Wyoming sagebrush-grassland to determine the changes in native early successional communities after introductions of annual weeds. Three sites were disced and quadrats with differing densities and percent cover of introduced weeds were created by planting or removing them in some quadrats at each site, depending on colonization rates. The dominant weeds colonizing over a three-year period were Salsola kali, S. collina, Sisymbrium altissimum, and Bromus tectorum, with cover ranging from 0 to 94%. At the higher weed cover and densities, native species had reduced cover, density, and richness per unit area in the first two years. Compared with native communities, those with introduced species had higher total cover, greater total richness, and lower evenness (J') due to the great abundance of a few introduced species. By year three the introduced weeds were much less common. Some native annuals had greater cover in quadrats littered with weed debris from the previous year, but the native perennials which dominate later successional stages still had less cover in quadrats with weeds, suggesting a slowed rate of succession with high weed colonization. Success of the introduced species is attributed to (1) effective dispersal due to the tumbleweed habit, (2) deeper roots which may deplete soil moisture to the detriment of associated native species, and (3) temporal resource partitioning, where the introduced species maximize growth either earlier (C3 plants) or later (C4 plants) in the growing season than most of the native C3 species. A significant effect of man on vegetation has been the introduction, both planned and inadvertent, of new species to native floras around the world (Ridley, 1930; Elton, 1958). Such introductions are still occurring and have the potential for altering community composition (Mulligan, 1965, Raven and Thomas, 1970; Lhotska and Voitenko, 1975; Medd and Smith, 1978). Many of these introduced species are weeds which grow primarily in habitats disturbed by man (Baker, 1965, 1974; Stebbins, 1965). Native plants have evolved for colonizing natural disturbances, but they often appear to be less aggressive than those from areas with a long history of human