Tamarisk growth at the northern margin of its naturalized range in Montana, USA

Tamarisk (Tamarix spp.), an introduced shrub or small tree, has invaded rip arian areas throughout the western United States. Tamarisk invasion has bee n studied extensively in the Southwest, but there is little information on its performance at the northern margin of its naturalized range. We measure d the canopy cover, density, height and age of tamarisk and plains cottonwo od (Populus deltoides) in 50 plots at 25 sites along the Bighorn, Powder, a nd Yellowstone rivers in southeast Montana near the northern edge of tamari sk's western North American range. Tamarisk commonly formed thickets on ope n, low terraces and along overflow channels but was less dense beneath a co ttonwood canopy. Tamarisk stems routinely died back to the ground, and the oldest live stems were generally much younger than the plants. Tamarisk 30 to 40 years old in our study area usually attained heights of only 4 m or l ess. Height and number of live sterns of tamarisk plants were 16% and 44% l ower respectively under a tall cottonwood canopy. Cottonwood grows faster t han tamarisk, eventually shading it and causing its decline. We believe tha t tamarisk will be only an understory shrub in most eastern Montana riparia n forests, declining as cottonwoods form a closed canopy. Minimizing the sp read of tamarisk in riparian areas in Montana can best be accomplished by m anaging for cottonwood.