Tamarisk

Tamarisk and Russian Olive Species

Riparian lands are especially integral and fragile aspects of western ecosystems due to their role in maintaining water quality and quantity, providing ground water recharge, controlling erosion, and dissipating stream energy during flood events (NRST 1997). Unfortunately, many of these water systems and associated riparian lands have been severely degraded over the past 150 years by anthropogenic activities (damming, road building, irrigation, etc.) and invasive plant species, resulting in reduced water quality, altered river regimes and reduced ecological systems and habitats.

Tamarisk (Tamarix spp.) and Russian olive (Elaeagnus angustifolia) are invasive species of particular interest due to their high profile status and negative environmental impacts.

• See Table 1 for an overview of adverse characteristics and potential impacts widely attributed to tamarisk (T) and Russian olive (RO).
• For a more extensive discussion please refer to Tamarisk Ecology and Impacts.

An Introduction to Tamarisk

Tamarisk is a deciduous shrub or small tree that was introduced to the western U.S. in the early nineteenth century for use as an ornamental, in windbreaks, and for erosion control. The exact date of introduction is unknown; however, it is generally understood that tamarisk became a problem in western riparian zones in the mid 1900’s (Robinson 1965, Howe and Knopf 1991).

Originating in central Asia and the Mediterranean, tamarisk is a facultative phreatophyte with an extensive root system well suited to the hot, arid climates and alkaline soils common in the western U.S. These adaptations have allowed it to effectively exploit many of the degraded conditions in southwestern river systems today (e.g., interrupted flow regimes, reduced flooding, increased fire).

By the mid-twentieth century, tamarisk stands dominated many low-elevation (under 6,500 feet) river, lake, and stream banks from Mexico to Canada and into the plains states. 

Tamarisk cover estimates range from 1 to 1.5 million acres of land in the western U.S. and may be as high as 2 million acres (Zimmerman 1997). 

Genetic analysis suggests that tamarisk species invading the U.S. include Tamarix chinensis, T. ramosissima, T. parviflora, T. gallica, and T. aphylla (Gaskin 2002, Gaskin and Schaal 2002).  A hybrid of the first two species appears to be the most successful intruder. There are several ornamental varieties of tamarisk still marketed in the western United States. While these species are non-invasive they do contribute genetic diversity to invasive populations. 

Reproduction

Tamarisk reproduces primarily through wind and water-borne seeds, but a stand may also spread through the vegetative reproduction of broken or buried stems. Seeds are viable for approximately six weeks (Carpenter 1998) and require a wet, open habitat to germinate. 

In the presence of established native vegetation or sprouts, tamarisk seedlings are not strongly competitive

(Sher, Marshall & Gilbert, 2000; Sher, Marshall & Taylor, 2002; Sher & Marshall, 2003). 

Therefore, if native plant communities are intact or conditions favor native plant establishment or growth, tamarisk invasion by seed is not likely to occur. 

However, the when the following conditions coincide with the removal of native canopy due to natural or anthropogenic causes new infestations to occur:

1. Late flooding - Tamarisk seed production generally has a longer season than native vegetation, and therefore is able to take advantage of overbank flooding when native vegetation is not dispersing seed. 
2. Suppression of native vegetation - Herbivory (e.g., cows will eat native saplings), drought, fire, lack of seed, or other disruptive processes can prevent native plants from establishing, and thus allow tamarisk to invade.

Establishment

Once tamarisk seedlings are established (as great as 1,000 indivduals/m2 initially), thick stands are very competitive, excluding natives (Busch and Smith 1995, Taylor et al. 1999). 

Any disruption of the riparian ecosystem appears to make invasion more likely, especially alterations of hydrology (Lonsdale 1993, Décamps Planty-Tabacchi and Tabacchi 1995, Busch & Smith 1995, Springuel et al. 1997, Shafroth et al. 1998). However, there are also numerous documented cases of tamarisk stands where no known disruptions have occurred.

Tamarisk Impacts

Once a tamarisk stand is mature, it will remain the dominant feature of an ecosystem unless removed by human means. Tamarisk has a higher tolerance of fire, drought, and salinity than native species (Horton et al. 1960, Busch et al. 1992, Busch and Smith 1993 & 1995, Shafroth et al. 1995, Cleverly et al. 1997, Smith et al. 1998, Shafroth et al. 1998). 

Tamarisk can increase fire frequency and intensity, drought (Graf 1978), and salinity (Taylor et al. 1999) of a site. Hence, a strong initial infestation will promote a positive feedback mechanism that will lead to more tamarisk invasion. 

In addition to affecting abiotic processes, tamarisk dominance dramatically changes vegetation structure (Busch & Smith 1995) and animal species diversity (Ellis 1995). High invertebrate and bird diversity has been recorded in some tamarisk-dominated areas and tamarisk is valued highly by the bee industry for its abundant flower production. Although some forms of tamarisk (primarily younger, highly branching stands) are favored by cup nesting bird species such as the endangered southwestern willow flycatcher, many endemic species are completely excluded by it, including raptors such as eagles (Ellis 1995).

Because of its potential usefulness to some species, stands of tamarisk mixed with native vegetation were found to have high ecological value in Arizona study sites (Stromberg 1998). In contrast, mature monocultures of tamarisk have a much lower ecosystem value. 

How much Water is Lost?

Limited evidence indicates that water usage per leaf area of tamarisk and the native cottonwood/willow riparian communities is very similar. However, because tamarisk grows in extremely dense thickets, the leaf area per acre may actually be much greater than native stands; thus, water consumption could be greater on a per acre basis (Kolb 2001).

Another aspect of tamarisk water consumption is its deep root system. Tamarisk roots can extend down to 100 feet, much farther than healthy cottonwoods and willows stands which reach a depth of only a few meters (Baum 1978, USDI-BOR 1995). 

This allows tamarisk to grow further back from the river, occupy a larger area, and use more water across the floodplain than native phreatophytes.

This is significant because the upper floodplain terraces adjacent to the riparian corridor typically occupy an area several times larger than the riparian zone itself. In these areas, mesic and xeric plants (such as bunch grasses, sagebrush, rabbit brush, four-wing salt bush, and skunk bush) can be replaced by tamarisk resulting in overall water consumption several times the ecosystem’s natural rate (DeLoach et al. 2002).

Water consumption estimates vary a great deal depending on location, maturity, density of infestation, water quality, and groundwater depth. In 27 research plots, tamarisk had an average annual water usage of 4.2 acre-feet/acre (95% confidence interval = 3.85 to 4.86) (NISC 2006). This agrees strongly with the most sophisticated evapotranspiration studies using eddy-covalence measurements performed for the Bureau of Reclamation (King and Bawazir 2000) of 4.35 feet per year. 

Water use by Russian olive was found to be approximately the same. In many situations this water consumption is equivalent to that of cottonwood/willow vegetation at a similar density.

For dry-land vegetation such as grasses/sage/rabbit brush communities, which are shallow-rooted and get their water primarily from precipitation, the difference in water use is a function of the precipitation received for the area.

 

Summary

 

In general, the following is an assessment of tamarisk and its impacts on riparian systems throughout the West (Carpenter 1998, McDaniel et al. 2004). 

• Tamarisk populations develop in dense thickets, with as many as 3,000 plants per acre that can prevent the establishment of native vegetation (e.g., cottonwoods (Populus spp), willows (Salix spp), sage, grasses, and forbs). 
• As a phreatophyte, tamarisk invades riparian areas, potentially leading to extensive degradation of habitat and loss of biodiversity in the stream corridor. 
• Due to the depths of their extensive root systems tamarisk draw excess salts from the groundwater. These are excreted through leaf glands and deposited on the ground with the leaf litter. This increases surface soil salinity to levels that can prevent the germination of many native plants.
• Tamarisk seeds and leaves lack nutrients and are of little value to most wildlife and livestock. 
• Tamarisk acts as an unnatural ladder fuel (or mid fuel) that serves as a “step” for wildfire to travel up into the canopies of cottonwood trees. Leaf litter from tamarisk also increases the frequency and intensity of wildfires which kill native cottonwood and willows but stimulate tamarisk growth. 
• Dense tamarisk stands on stream banks accumulate sediment in their thick root systems gradually narrowing stream channels and increasing flooding. These changes in stream morphology can impact critical habitat for endangered fish. 
• Dense stands affect agricultural production by invading rangeland, reducing forage, and preventing access to surface water. The non-beneficial use of water also affects irrigation practices as well.  
• Aesthetic values of the stream corridor are degraded, and access to streams for recreation (e.g., boating, fishing, hunting, bird watching) is lost.
• Tamarisk has a reputation for using significantly more water than the native vegetation that it displaces. This non-beneficial user of the West’s limited water resources has been reported to dry up springs, wetlands, and riparian areas by lowering water tables (Carpenter 1998, DeLoach 1997, Weeks et al. 1987).

List of Citations