Seed banks of Bromus tectorum-dominated communities in the Great Basin

Many shrub-steppe communities of the Great Basin have been converted to Bro mus tectorum-dominated communities. Seed production and seed bank traits of native perennials may be poorly suited to conditions of communities domina ted by this introduced annual, and native perennials may be lost from the s eed banks. Seed banks of former shrub-steppe communities now dominated by a nnuals were quantified on 3 sites in western Utah to determine if seeds of native perennials were present and to track changes in Bromus tectorum seed densities and species composition of seed banks after fire. Burned and unb urned plots on 1 site were sampled for 3 years after a wildfire. Plots cons isted of grids of 5.2-cm-diameter soil cores. Seeds were quantified by moni toring seedling emergence from these cores over an extended period of time in the greenhouse. On unburned,plots introduced annuals, mainly Bromus tect orum, constituted >99% of the seed bank, with Bromus densities of 4800-12,8 00 seeds m(-2). Immediately after the fire, Bromus seed density was <3% of unburned plots, but its seed bank density recovered in 2 years. The major c hange in species composition of the seed bank following fire was a shift in proportional abundance between Bromus and 2 other introduced annuals immed iately after the fire. One native annual and a native annual/perennial (Oen othera pallida) increased in the seed bank the 1st year after the fire. Of all samples, only 4 perennial-plant seeds representing 3 species (excluding Oenothera) were found, for a total perennial-plant seed bank of 2-3 seeds m(-2). Lack of perennial-plant seeds in annual-dominated communities impair s the reestablishment of native perennials. Because perennial-plant seeds a re so few, the reduction of Bromus seed banks by fire provides no opportuni ty for reestablishment of native species.