Invasive Species in the Southwest:
Tamarix sp. (Salt Cedar) 
One of the most significant threats to global biodiversity is the invasion of plants into foreign areas. Replacement of native species, disruptions of nutrient and fire cycles, and changes in plant succession are some effects exotic species can have on an ecosystem (Lovich 1996). However, some exotic species have little effect on natural habitats. Currently, there are thousands of exotic species in the United States, many of which were introduced intentionally, and others unintentionally. Some of these species include red brome, wild barley, wild oats, Mediterranean grass, wild mustards, tumbleweed, buffel grass, fountain grass, and lonegrass (James 1996). This section give an in-depth look at Tamarix sp., or salt cedar, one of the most successful and common invasive species in the United States.

History of Tamarix in America
Tamarix was introduced to the United States in the 1800's. It was originally thought that Spaniards were responsible for the introduction of salt cedar, but it is now believed that nurserymen sold the plant to Old American Nursery in Philadelphia (Di Tomaso 1998). However, the genus was named after the Tamaris River in Spain which contains many shrubs and small trees native to Western Europe (Di Tomaso 1996). At least three species of Tamarix were sold in 1854 to perform a variety of functions such as setting up wind breaks, creating shade, stabilizing eroding stream beds, or as ornamental shrubs (see figure).

Salt cedar was first reported outside of cultivation in the 1870's (Di Tomaso 1998) and between 1935 and 1955, the greatest degree of invasion occurred (Christensen 1962). Today, Tamarix, commonly known as "tammies" (Kauffman, personal communication) are some of the most widespread species in riparian areas throughout the Southwest.

General Biology

Salt cedar is in the Tamaricaceae family, consisting of 90 different species. Of these 90 species only 8 have been introduced into the United States and furthermore, only 2 of these 8 species pose a significant threat to the natural ecosystems of the Southwest: Tamarix parviflora and Tamarix ramosissma. It can be found primarily in Colorado, Utah, Kansas, Texas, New Mexico, Wyoming, and Arizona (see figure below). However, Tamarix has been a difficult species to classify according to some botanists because of it morphology and requires further study before classification can be completed (Brock 1994).

Salt cedar is a deciduous shrub/small tree that grows most successfully along riparian zones (see figure below), that is, along streams and/or lake edges (Brock 1994). It is a facultative phraeophyte which means that its roots extend deeply into the soil and depend on ground water for water supply. Because of its facultative nature, however, salt cedar can potentially obtain water from other sources, i.e. through sending out advantitious roots. This enables salt cedar to inhabit areas where no ground water is present (Di Tomaso 1996).

Multiple stems and slender branches characterize the general morphology of salt cedar. These young branches are typically reddish-brown and are clearly marked in dormancy by the presence of light-colored leaf scars (Brock 1994). Up to 3-4 meters of growth have been recorded in a single growing season by species of Tamarix. Adult salt cedars are remarkably tolerant to a variety of stress conditions such as heat, cold, drought, flood, and high concentrations of dissolved solids.  Also, salt cedar removes salt crystals from openings in its scale-like leaves (Di Tomaso 1996).

Tamarix produces small, pink flowers in catkin-like racemes.  The flowers have five stamens with distinct sepals and flowers (see figure below).  In congruence, salt cedar also has the ability to propogate vegetatively, especially in after the top layer of vegetation has been burned (Brock 1994).


Tamarix Ecology and Impacts on the Physical Environment

Tamarix has four main impacts on the local environment once it becomes established: (1) increased soil salinity, (2) increased water consumption, (3) increased wildfire frequency, and (4) increased frequency and intensity of flooding (Wiesenborn 1996).  In general, as floodplains become more dessicated with age, salt cedar assumes a greater dominance due to its high drought tolerance relative to native phraetophytes.  This results in an ability to produce high density, monospecific stands (Cleverly et al. 1997).

As described above, the presence of salt cedar in an area tends to increase the salinity of the soil and thus prevent the presence of many native species. Salt cedar readily takes up solutes from the soil and then dumps them above the ground from its salt glands or by dropping its leaves. This is an allelopathic effect in that surrounding plants are unable to grow in these high salt concentrations (Di Tomaso 1996). For example, salt cedar has been shown to tolerate up to 36,000 ppm salt salinity, whereas native floodplain species such as willow and cottonwood can only tolerate up to 1,500 ppm (Weisenborn 1996).

Water consumption is one of the most heavily researched areas in salt cedar's biology primarily because of the strong interest in conserving water throughout the Southwest (Wiesenborn 1996). Water use by salt cedar is generally considered to be high, but evapotranspiration rates often vary throughout the day. Stomates are open during daybreak, when conditions are most cool and humid. The relatively high levels of evapotranspiration have been suggested to be a result of the salt glands rather than the stomates (Di Tomaso 1996; Weisenborn 1996). In general, the longer the community has been invaded by salt cedar the lower the water table in the soil (Brotherson et al. 1984; Cleverly et al. 1997).

Wildfires become more frequent within stands containing high amounts of salt cedar. The deciduous nature and increasing population densities of salt cedar both contribute to a heavy fuel load in infested areas, thus promoting burning (Lovich 1996). Between 1981 and 1992, fires burned 35% of salt cedar infested areas on the lower Colorado River floodplain. During this same time frame, fires burned 2% of the native plant species in a nearby area (Wiesenborn 1996). Salt cedar is a fire-adapted species with more efficient recovery mechanisms than most native species. In this light, salt cedar thrives after a fire occurs and can thus exploit niches once occupied by those native species.

Dense stands of salt cedar and increased flooding have been correlated in previous studies. As these stands increase in density, narrowing the channel that water flows through, the rate of water flow increases and the chances for flooding increases. Also, water is often shunted to areas that usually do not experience water flow because of these dense stands. The result is increased erosion and sedimentation which both increase the chances for flooding to occur in the riparian area (Di Tomaso 1998; Weisenborn 1996). Blackburn et al. (1982), documented salt cedar infestation along the Brazos River in north central Texas beginning in 1941. By 1979 the river's depth had changed from 18.4 ft to 10.2 ft and the width had changed from 515 ft to 220 ft.

Factors Promoting Infestation
There are a number of factors that may contribute to the infestation of salt cedar. Clearing, plowing, and overgrazing, all associated with agriculture, seem to create conditions optimal for salt cedar infestation. Also, off-roading and tree logging both leave disturbances in the natural environment which create barren areas optimal for infestation. One of the most important is the development of water management programs which usually affect natural water flows; the construction of reservoirs, dams, river diversions, flow regulations, and irrigation projects all disrupt the natural flow of rivers. Often these disruptions create conditions unsuitable for the colonization and regeneration of native riparian species, such as willows and cottonwoods because of the usually low water levels and high salinity levels. However, salt cedar persists under these highly saline conditions (Di Tomaso 1998).

Riparian restoration efforts began in the 1960's and were largely supported by federal agencies such as the U. S. Bureau of Reclamation and the U. S. Fish and Wildlife Service (Taylor and McDaniel 1998).  There are four main techniques used in vegetative management: (1) biological, (2) fire, (3) mechanical, and (4) chemical (Brock 1994). One source claimed that saltcedar clearing, a combination of herbicide, burning, and mechanical control techniques, costs from $750 to $1300/ha (Taylor and McDaniel 1998).  There are no current biological controls for salt cedar, however this is an area where much research is currently being conducted. For example, some recent reports show that livestock and certain insects treatments have had effects on the reduction of salt cedar, but neither have been entirely successful. As stated before, salt cedar is a fire adapted species and therefore burning would not be an effective method of management. Mechanical treatment has been conducted, but has been proven to be difficult due to the nature and habitat of the plant. Salt cedar has a relatively large growth form and grows along riparian zones, both of which slow treatment and provide an optimal medium for plant regeneration (Brock 1994). Recent success with a chemical known as Imazapyr, has led to wider herbicide use. One study that began in 1987 documented that Imazapry controlled levels of salt cedar up to 90% (Duncan and McDaniel 1998).

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Authored by Jason Hart
May 2, 1999