Tamarisk, a shrub or small tree native to Eurasia, was introduced to North America in the early 1800s and is now naturalized throughout many riparian areas of the southwestern United States, where extensive research has been conducted. It is a more recent invader to the northern Great Plains, and fewer studies have been conducted on tamarisk ecology and management in this area. The objectives of this research were to investigate the overwintering potential of tamarisk seeds in Montana and the relationship between hydrologic conditions and historic tamarisk establishment. Emergence of seedlings from seeds stored for different time periods at a range of temperatures was evaluated in a greenhouse study. Emergence rates declined after a 7-d storage period, but storage time had no effect on subsequent emergence rates, and seeds stored at −14 C and 5 C had greater emergence rates than those stored at 20 C and 35 C. Patterns in tamarisk establishment were assessed through age and hydrologic data collected from a reservoir (Fort Peck), a regulated river (Bighorn), and an unregulated river (Yellowstone) in Montana. These data indicated that tamarisk establishment at the reservoir was closely related to historic water levels, whereas establishment on rivers was not related to flow. However, data from the rivers indicated that recruitment differed between regulated and unregulated rivers, with the regulated river having less recruitment after the period of initial colonization than the unregulated river. Our results show that tamarisk seeds have the ability to overwinter in Montana and can establish under a range of flow conditions, indicating potential recolonization of sites after tamarisk removal.

Nomenclature: Tamarisk, species in the genus Tamarix L., primarily Chinese tamarisk, Tamarix chinensis Lour., saltcedar, Tamarix ramosissima Ledeb., and their hybrids

Interpretive Summary: Tamarisk is a shrub or small tree that has invaded riparian areas of the southwestern United States, and is becoming more of a concern in the northern Great Plains. Although considerable ecological research has been conducted on tamarisk in the Southwest, much less information exists to guide its management in its northern range. This research investigated tamarisk seed overwintering potential in Montana and the effects of reservoir (Fort Peck) level and river (Bighorn and Yellowstone) flows on tamarisk establishment. Results showed that tamarisk seeds have the ability to survive cold and cool temperatures (−14 and 5 C) for at least 6 mo, but that warm and hot temperatures (20 and 35 C) led to declines in seedling emergence, with no seeds surviving more than 90 d at 35 C. Results from rivers showed that tamarisk trees can establish under various flows, including very high and very low peak flows. Together, these results suggest that sites where tamarisk has been removed could be subjected to recolonization from a short lived seedbank or from other seed sources, regardless of flow conditions. Therefore, managers should implement a monitoring program to detect new colonization after treatment. Results from the reservoir showed that tamarisk established as the water level receded, suggesting that managers should monitor around the shoreline as water levels decline to enhance early detection of new tamarisk populations. Finally, tamarisk populations decline subsequent to submergence by rising water, indicating that they can be controlled by raising the water level in reservoirs.