Invasive Saltcedar (Tamarix): Its Spread from the American Southwest to the Northern Great Plains

Abstract

The advance of saltcedar from the American southwest to Montana between the mid-19th and mid-20th centuries crossed the Continental Divide—a major topographic and climatic barrier to natural dispersal by southern plants. Interviews, archival information, and field observations are used to explain this advance. According to archival documents, saltcedar was planted in communities adjacent to the Bighorn River in central Wyoming as part of urban beautification projects in 1936. Bureau of Reclamation reports describe tree plantings for erosion control between 1940 and 1953 within the Riverton Irrigation Project in the Wind-Bighorn River watershed. These introductions were followed by rapid natural dispersal northward by water and wind through the Wind/Bighorn system and into the Yellowstone and Missouri rivers in Montana before closure of Boysen and Bighorn dams in 1951 and 1967. Construction equipment and ornamental plantings further transported saltcedar to the Fort Peck Reservoir and the Musselshell River. We conclude that, without control at its advancing fronts, a complete ban of its sale, and removal of existing ornamental and erosion control trees, saltcedar will invade most suitable sites in the Missouri watershed and disperse northwards into western Canada.