Occurrence, Persistence, and Expansion of Saltcedar (Tamarix Spp.) Populations in the Great Plains of Montana

—Saltcedar (Tamarix spp.), a shrub native to Eurasia, is associated with major alterations to wetland and riparian systems in the southwestern United States. Since the 1960s saltcedar has been naturalized in northern states of the U.S. where its growth potential and impacts are not well known. Here, we describe the occurrence, age, size, and relative cover of saltcedar populations in several river basins in central eastern Montana, USA, to identify potential patterns of spread across the region and changes in individual populations as they age. Stands were aged according to the oldest saltcedar individuals and were sampled for dominant plant cover and soil properties. Multiple introductions appear to have occurred in Montana, with the oldest stands occurring on the Bighorn River in southern Montana. Saltcedar absolute and relative cover and stand area increased significantly with stand age, while native tree and shrub relative cover remained low across all stand ages. These results suggest that saltcedar stands establish where woody natives are not abundant and that they persist and expand over time. Although soil salinity remained constant, soil pH decreased with saltcedar stand age, indicating a possible effect of organic matter inputs. An analysis of annual wood increment of saltcedar and sandbar willow (a native with analogous growth form) stems along a latitudinal gradient showed that stem growth of both species did not differ significantly among regions. Stem growth decreased inversely with elevation for both species while growth responses to elevation did not differ between species. Our results show an increase in number of populations and continued viability of these populations. Mechanisms of saltcedar increases in this region are yet to be determined. Anthropogenic influences, such as saltcedar plantings, watershed alterations (e.g., river flow control), and habitat disturbances (e.g., cattle grazing or habitat clearing) may facilitate its spread in similar climates of the Great Plains.