Effects of Smooth Brome (Bromus inermis) Invasion on Tallgrass Prairie.

Data collected in 1984 and 2000 along a 65-m-long roadside-to-prairie gradient were compared to quantify brome (Bromus inermis) invasion into a native, tallgrass prairie in eastern Nebraska and to assess the effect of this expansion on prairie composition and diversity: Smooth brome expanded 15 meters further into tallgrass prairie during the 16 years of the study while also increasing cover an average of 8%. Overall, species diversity (H’) decreased from 1.04 to 0.95 along the entire toad-prairie gradient during this time although the decrease was significant (P ≤ 0.10) at only three of the five distances from the road that were sampled. Thirteen species declined significantly, including porcupine grass (Stipa spartea) (-23%), Indian grass (Soghastrum nutans) (-12%), and prairie phlox (Phlox pilosa) (-8%); sideoats grama (Bouteloua curtipendula), averaging 2% in 1984, was absent in 2000. Despite these decreases, there was a subset of species that increased, some native and some non-native, of which many were strongly rhizomatous. Four native species that increased significantly were stiff sunflower (Helianthus rigidus) (+25%; from 0% in 1984), prairie goldenrod (Solidago missouriensis) (+8%), false sunflower (Heliopsis helanthoides) (+7%) and clammy ground cherry (Physalus heterophylla) (+5%). Field bindweed (Convolculus arvensis), a non-native species also increased significantly (+5%). Canopy cover of New Jersey tea (Ceanothus americanus) (4% cover), a woody, prairie species, and gray-green wood sorrel (Oxalis dillenii) (\u3c0.5% cover), a non-native herb, were unchanged over time. In combination, these results suggest an overall decline in species diversity between 1984 and 2000, either in response to increases in smooth brome or coincident with conditions that favor its increase. The rate of decline varies among species. The net effect of these responses extended over time would be a tallgrass prairie characterized by a lower diversity than can be accounted for by fragmentation effects alone and one that supports a greater proportion of non-native species. While these results do not prove conclusive cause-effect relationships between smooth brome encroachment and tallgrass prairie diversity, they do provide sufficient cause for concern when considering both threats to native tallgrass prairie ecosystems and means by which to address these concerns