Saltcedar: Distribution, abundance, and dispersal mechanisms, northern Montana, USA

Introduced saltcedar (Tamarix ramosissima, T. chinensis, and their hybrids) reaches its northward distribution in the Great Plains in Montana, USA. We mapped the locations, patterns of abundance, and ages of saltcedar along the Musselshell River and Fort Peck Reservoir in northeastern Montana to identify concentrations of plants that could be used to infer introduction location, establishment year, and mechanisms of dispersal. We estimated the presence of 24,500 plants on the Musselshell River over a river distance of 240 km, with concentrations at three nodes close to Roundup (2,000 plants, seedlings to 24 years), Melstone' (6,000 plants, seedlings to 23 years), and the mouth of the river at Fort Peck Reservoir (10,000 plants, seedlings to at least 11 years). Concentrations at Roundup and Melstone probably originated from urban plantings in the 1960s. The third concentration may have established from seeds and plant pieces washed downriver during floods and deposited in the hydraulic backwater of the Musselshell River delta at Fort Peck Reservoir. We believe there may be one-half to one million plants on Fort Peck Reservoir, with concentration nodes at recreation areas on the south shore. We estimated 3,500 mature saltcedar to be present at the Devils Creek Recreation Area, more than 11,000 plants at Hell Creek Recreation Area, and more than 40,000 plants at 6 sites at the south end of Dry Arm close to the Nelson Creek Recreation Area and mouth of Big Dry Creek. The oldest plants on the reservoir were 21 to 33 years old in 2001. Based on these ages, we suggest that saltcedar arrived at the south shore of Fort Peck Reservoir in the mid- to late 1960s, which means that it must have dispersed from the Bighorn/Yellowstone River system soon after it became established in southern Montana. Although wind dispersal and ornamental plantings cannot be ruled out as primary transport mechanisms to the reservoir, the concentrations and ages of saltcedar at recreation areas suggest that seeds and other plant propagules were also transported to the reservoir by earth-moving equipment during site construction between 1966 and the mid-1980s and later by boats and their towing vehicles. Saltcedar was dispersed away from these nodes by wind and water. As Tamarix ramosissima and T. chinensis originated in the cold dry deserts of northeastern Asia, saltcedar may not be limited in its northward expansion by the cold winters, short growing seasons, and periodic droughts characteristic of the northern Great Plains in Canada.