Exotic Tamarisk on the Colorado Plateau

Author: Dr. Larry E. Stevens, Consulting Ecologist, P.O. Box 1315, Flagstaff, AZ, 86002

Saltcedar or Tamarisk (Tamaricaceae: Tamarix ramosissima Deneb)

Tamarisk trees grow in thick stands along the Little Colorado River near Cameron, AZ. Photo by John Grahame.

Deciduous, pentamerous saltcedar is a small, exotic tree introduced to the Southwest near the turn of the century from southern Eurasia (Horton 1977; Baum 1978). Saltcedar, now a dominant riparian shrubby tree in the Colorado River basin below 2,000 m elevation, spread rapidly throughout the system via wind-dispersed seeds (Graf 1978). Although saltcedar had reached the Grand Canyon by 1938 (Clover and Jotter 1944), the oldest trees found in the system thus far, date to about 1943 (Hereford personal communication; Stevens, in preparation). Saltcedar occupied pre-dam terraces and tributaries during the pre-dam era, and was the first species to invade the newly stabilized post-dam riparian zone in the Grand Canyon (Turner and Karpiscak 1980).

Mature saltcedar plants are capable of producing 2.5 x 10⁸ tiny, wind-dispersed seeds per year (Stevens, in press). Its seeds are short-lived (less than 2 months in summer), have no dormancy requirements, and germinate in less than 24 hr. Saltcedar seeds require a moist, fine-grained (silt or smaller particle size) substrate for eccesis, such as is found in southwestern riparian habitats after flood waters subside (Stevens 1989a, b). Saltcedar commonly co-occurs with Populus fremontii (Fremont cottonwood), Salix exigua (sandbar or coyote willow), Salix gooddinggii (Goodding's willow), and Tessaria sericea (Marks 1950; Stevens, in press), but the non-native species is more tolerant of harsh environmental extremes than are native species (Warren and Turner 1975; Stevens and Waring 1985; Stevens, in press).

Tamarisk in the lower Grand Canyon. These exotic trees are used by river runners for shade on river beaches. Photo by Bill Belknap, courtesy of Cline Library Special Collections, NAU.

Saltcedar’s success in riparian environments in the Southwest appears to be a function of its phenomenal reproductive output and its greater drought and flood tolerance, as compared to Salix exigua (Warren and Turner 1975; Stevens and Waring 1985). In an effort to understand the ecological success of saltcedar, experiments on its competitive ability, germination and nutritional requirements, and other aspects of its life history were conducted (Stevens 1986a). Competition experiments with Salix exigua, a common neighbor throughout the Colorado River system, failed to demonstrate competitive superiority of saltcedar over the willow. In fact, at the seedling stage, willow was competitively dominant.

Saltcedar was consumed by several introduced invertebrate herbivores, particularly the cicadellid leafhopper, Opsius stactogalus; however, invertebrate herbivore standing crop was equivalent with Salix exigua during normal (non-flooding) years in the Grand Canyon (Fig. 20; Stevens 1985). Saltcedar was more drought tolerant and inundation tolerant than any native species. Some saltcedar survived more than two years of root-crown inundation in the Grand Canyon during high water events from 1983-85 (Stevens and Waring 1988), a period far exceeding the 90 day record observed in warm, anoxic reservoir waters by Warren and Turner (1975). Saltcedar is extraordinary not only in its persistence, but also in its reproductive output, as mentioned, and seedling densities in excess of 16,000/m² have been observed in the southwest (Warren and Turner 1975). These life history characteristics make saltcedar highly successful in the harsh, unpredictable channels of unregulated southwestern rivers, but limit its recruitment along the more stabilized channels of regulated streams.

Baum, B.R. 1978. The genus Tamarix. Israel Acad. Sciences and Humanities, Jerusalem, 209 pp.

Clover, E.U. and Jotter, L. 1944. Floristic studies in the canyon of the Colorado and tributaries. American Midland Naturalist 32: 591-642.

Graf, W.F. 1978. Fluvial adjustment to the spread of tamarisk in the Colorado Plateau region. Geological Society of America Bulletin 89: 1491-1501.

Horton, J.S. 1977. The development and perpetuation of the permanent tamarisk type in the phreatophyte zone of the southwest. Pp. 124-127 In: Importance, preservation and management of riparian habitat: A symposium. General Technical Report RM-43. U.S. Forest Service, Washington, D.C..

Marks, J.B. 1950. Vegetation and soil relations in the lower Colorado desert. Ecology 31: 176-193.

Stevens, L.E. 1989a. Mechanisms of riparian plant community organization and succession in the Grand Canyon, Arizona. PhD Dissertation, Northern Arizona University, Flagstaff.

Stevens, L.E. 1989b. The status of ecological research on tamarisk (Tamaricaceae: Tamarix ramosissima) in Arizona. Pp. 99-105 In: Kunzman, M.R., R.R. Johnson and P.S. Bennett, editors. Tamarisk control in southwestern United States. Cooperative National Park Resources Study Unit Special Report Number 9, Tucson.

Stevens, L.E. and Waring, G.W. 1985. The effects of prolonged flooding on the riparian plant community in Grand Canyon. Pp. 81-86 In: Johnson, R.R., et al., editors. Riparian ecosystems and their management: Reconciling conflicting uses. General Technical Report RM-120. U.S. Forest Service, Tucson, AZ, 523 pp.

Stevens, L.E. and G. L. Waring. 1988. Effects of post-dam flooding on riparian substrates, vegetation, and invertebrate populations in the Colorado River corridor in Grand Canyon. Bureau of Reclamation Glen Canyon Environmental Studies Report 19, Flagstaff. NTIS PB88-183488/AS.

Turner, R.M. and Karpiscak, M.M. 1980. Recent vegetation changes along the Colorado River between Glen Canyon Dam and Lake Mead, Arizona. U.S. Geological Survey Professional Paper No. 1132. USGS, Washington, D.C., 125 pp.

Warren, D.K. and Turner, R.M. 1975. Saltcedar (Tamarix chinensis) seed production, seedling establishment and response to inundation. Journal of the Arizona Academy of Science 10: 135-144.