Scourge of the West:
The Natural History of Tamarisk in the Grand Canyon

   Exotic tamarisk, or saltcedar, is ubiquitous throughout the Colorado River drainage below elevations of about 6,000 feet. This small Eurasian tree is now the dominant phreatophyte (streamside) species along river banks in the American Southwest and in central Australia. Tamarisk was brought to the U.S. as early as 1805, and it was widely available as an ornamental on the West coast by the 1870’s. Used for ornamental and bank stabilization purposes by 1900 in the lower Colorado River and Rio Grande rivers, tamarisk appeared in the Grand Canyon between 1922 and 1938. Its broad tolerance of drought and inundation, its enormous fecundity and wind-dispersal of seeds quickly allowed tamarisk to spread. It now occupies much of the new high water zone and pre-dam terraces, and it has invaded most of the river’s tributaries as well. Today, tamarisk occupies more than a million acres of riparian habitat in the West where it is widely reviled by habitat managers.

   Significant research has been conducted on water consumption and methods of control of tamarisk, but surprisingly little attention has been published on its natural history in the West. Over the past twenty years, I’ve spent a fair amount of time trying to understand how and why this exotic plant species has been so successful, and how its distribution may change in the future.

Taxonomy

   The common, weedy, deciduous, five-stamen, white to red-flowering, small tree we call tamarisk in the Grand Canyon is one of 54 described species in the genus Tamarix in the family Tamaricaceae (Baum 1978). The Tamaricaceae are native to southern Eurasia and Africa, and contain three other genera: Holoachne, Myricaria and Reaumuria. The family history parallels our own, dating back to the Oligocene epoch along the waterways of Africa. The Greeks knew tamarisk as Myrica, and Linnaeus formalized the genus name in 1753. Our common tamarisk has been variably referred to as T. chinensis, T gallica, T. pallasii, T. pentandra and T. ramosissima; the confusion arising from obscure details of floral structure and the resolution of which will require studies of genetic relatedness to the Eurasian taxa. Revision of the Arizona Flora next year (we hope) will supposedly reinstate our taxon’s identity as Tamarix pentandra.

   The anthel (Tamarix aphylla) is the only other common tamarisk in the Southwest. It is a large (to 100 feet high), ornamental shade tree that rarely escapes cultivation and occurs at elevations below 3,500 feet in Arizona.

Reproduction

   In the realm of reproduction, tamarisk is outstandingly successful. Tamarisk flowers are almost wholly insect-pollinated and it is not a self-compatible species. Tamarisk on higher terraces bloom from mid-April to mid-June in the Grand Canyon, and some plants along the river bloom throughout the growing season. The tiny flowers range in color from red to white between plants, and produce up to 32 seeds each. Consequently, a large, mature plant can produce more than 250 million seeds per year. These minute seeds are short-lived, surviving an average of about 45 days during the summer. Because an individual plant may live more than a century, 50 plants may produce a trillion seeds. Seedling densities of 15,000 seeds per square yard have been reported in the “green lawns” of tamarisk seedlings that line the Green and upper Colorado rivers, and the headwaters of southwestern reservoirs in mid-summer.

   But seed production is remarkably variable, even between individual tamarisk growing in close proximity. Approximately 10 percent of the plants around Lees Ferry are “duds", producing few or no flowers over a ten-year observation period. This may be a result of alteration of allocational strategies related to competitive interactions early in life, or to poor linkage between intra-plant maintenance (survival and growth) and reproduction functions in some as-yet-unexplained permutation of Darwinian logic.

Ecological Importance

   Although tamarisk is fed on by more than 250 species of invertebrates in the Old World, as well as camels and livestock, few North American organisms use it as a food source. The only two common insects that feed on tamarisk are host-specific exotic species from the Old World: an eighth-inch green, brown-tipped cicadellid leaf-hopper, Opsius stactogalus; and a sixteenth-inch, white diaspidid scale, Chinonaspis etrusca. The leaf-hopper is readily visible on the plants in the summer months. It feeds on sap, “hoping” for amino acids but getting mostly sugar water, which it excretes in tiny droplets. Leaf-hopper densities are often so great that the ground (and often the shade-craving river recreationists) beneath tamarisk, experience a gentle rain of leaf-hopper exudate.

   This genus of leaf-hopper has a biblical history. When the Israelites ran out of food on their desert trek, Moses prayed for food and the Lord sent them manna. Manna is the sugary coating of the ground exuded by Opsius leafhoppers feeding in tamarisk. (An aside to the adventurous, it tastes reasonably good.)

   Although our large, green, native Shoshone grasshopper Schistocerca shosoni occasionally feeds on tamarisk foliage, its depredations are erratic. Hippomelas sp., a beautiful metallic green (and usually golden-pollen coated) metallic wood-boring beetle is attracted to the sweet sap of new tamarisk foliage in mid-summer, but no wood-boring beetles feed on live tamarisk wood. Native termites avail themselves of some fallen tamarisk wood, and rare web-spinners (Embioptera) occur in tamarisk litter.

   Several vertebrates feed on tamarisk. Beaver (Castor canadensis) commonly gnaw lateral branches that are about one inch in diameter, but rarely take down large tamarisk trees. Additionally, deer and bighorn sheep occasionally browse tamarisk. Red-naped sapsucker (Sphyrapicus varius) feed extensively on tamarisk sap during the winter months.

A Strategy of Tolerance

   The success of tamarisk in the American Southwest is attributable to its wide range of physiological tolerance to moisture availability. Although tamarisk attains the stature of a small tree (up to about 35 feet high), it has a deep tap-root that may extend 100 feet or more in depth, with lateral roots that reach out 150 feet. Its roots anchor the plant firmly, and its dense, hard wood is resistant to battering during floods. As a phreatophyte, its roots are in contact with the water table or in the adjacent capillary fringe. Tamarisk can survive prolonged flooding better than any other woody riparian species in the Colorado River corridor. For example, a few plants lived more than 500 days with their root crowns inundated by the cold Colorado River water from 1983 to 1986.

   Tamarisk is also an extremely drought tolerant phreatophyte, living in a range of water stress comparable with that of catclaw (Acacia greggii). In addition, tamarisk is a halophyte and is exceptionally tolerant of high levels of soil salinity. Tamarisk germinate readily along the Little Colorado River (LCR), and there are tamarisk saplings growing on the halite seeps above and below the LCR confluence. Tamarisk has a wider range of tolerance to moisture availability than other native riparian species.

   If tamarisk is so successful, why are seedlings so rare in Marble Canyon? By limiting sediment transport, Glen Canyon Dam has created a stable, rather coarse-grained environment. Plants like tamarisk that reproduce virtually only by seed simply can’t germinate on Canyon sand bars, particularly those above the Little Colorado River. Species like coyote willow (Salix exigua) and arrowweed (Pluchea sericea) that sprout up from running roots can quickly dominate these habitats. Because tamarisk seedling growth and reproduction is strongly reduced by shading and chemical interactions with willow, it simply cannot become established. Even though a billion seeds many reach a beach in a year, no establishment is possible. Therefore, tamarisk remains dominant through a strategy of tolerance: although it is a good colonizer if conditions are right, it is not much of a competitor.

Management

   By 1970 tamarisk occupied more than one million acres of riparian habitat in the West, attracting the disgruntlement of land managers on three counts. First, tamarisk transpires the weight of its foliage every hour at 80o F. This is a lot of water to lose to an exotic species. Second, tamarisk provides poor wildlife habitat in the lower Colorado and Rio Grande basins. In contrast, many of our Neotropical migrant birds along the Colorado River in the Grand Canyon nest preferentially in tamarisk. Tamarisk in the hot, lower basin elevations typically exist in a water-stressed condition that apparently prohibits it from supporting leafhoppers. Leafhopper populations provide food for reptiles and obligate riparian birds during the middle and late summer months in the Grand Canyon, but apparently not downstream in the lower Colorado River basin. Third, tamarisk takes up space that native plants might otherwise occupy. Cottonwood has been largely displaced by the drought- and disturbance-tolerant tamarisk in the lower Colorado River and Rio Grande basins.

   Tamarisk is used as a nectar source for honey producers; however, the dark, overly aromatic honey is used mostly as a food source for overwintering hives. The sugar potential of the sap has not been explored. Good chop sticks but poor fuel.

Larry Stevens