Predicting spread of cheatgrass, Bromus tectorum, in the Owyhee Uplands of southeastern Oregon

Spread of invasive exotic organisms is expected to be affected by site suitability, but assessing suitability is complicated by the fact that invasive exotics are not yet at equilibrium with their environment. Cheatgrass (Bromus tectorum) is an important invasive exotic plant in shrublands of the Great Basin, and has become a problem in the Owyhee Uplands of southeastern Oregon. Predicting the spread of cheatgrass would help managers in this region plan monitoring and control efforts. This project is aimed at combining information about the contagious pattern of spread of cheatgrass with information about site suitability, so that areas with a high risk of invasion can be identified; preliminary analysis of site suitability characteristics are presented here. Field studies of cheatgrass distributions were conducted in the summer and fall of 2006 in the Owyhee Uplands. Cheatgrass cover declined with elevation, and was nearly absent above 1700 m. Although cheatgrass was found on varied topography, the greatest cover was found on steeper, south-facing slopes. Together, elevation and "southness" (an index that combined slope and azimuth) explained 29% of the variation in cheatgrass cover (F = 22.56; d.f. = 2,111; p < 0.001). Measures of site suitability based on field studies will next be combined with measures of proximity to existing cheatgrass populations so that the path of spread can be predicted.