The biological characteristics of saltcedar (Tamarix ramosissima) that make it an aggressive weed also can cause undesirable physical changes in the plant's environment. The four most-common physical changes are briefly described here. They are: (1) increased soil salinity inhibiting native plant germination and growth, (2) increased water consumption and loss, (3) increased wildfire frequency, and (4) increased frequency and intensity of flooding.
Suggested by its name, saltcedar has the ability to excrete salts from glands on its leaves allowing the plant to tolerate saline soils and groundwater. Rather than excluding salts from the roots as do most plants, saltcedar freely takes up salts and then voids them aboveground. The ions excreted (eg. Na, K, Ca, Cl) are the same as those found in the soil surrounding the plant's roots (Berry 1970). The plant therefore acts as a conduit transporting salts from the groundwater to the leaf surface. The process also concentrates the salts; salt-gland exudate containing 41,000 ppm total solids has been measured on plants rooted in groundwater containing 2,000 ppm total solids (Gatewood et al. 1950). Because saltcedar is deciduous, all of the salts exuded eventually reach the soil surface, and salinity beneath the plant can increase as dropped leaves accumulate year after year until rainfall carries the salts through the soil and back to the groundwater.
Lacking a similar adaptation to saline soils, many native riparian plants can be affected by the salts transported by saltcedar to the soil surface. Growth by cottonwood (Populus fremontii) and Goodding's willow (Salix gooddingii) is inhibited by salinity greater than 1,500 ppm, whereas saltcedar can tolerate soil salinity up to 36,000 ppm (Jackson et al. 1990). We examined the soil salinity and the groundwater depth and fluctuation, two additional factors critical to riparian plant growth, in areas dominated by saltcedar on the lower Colorado River floodplain to estimate site suitability for restoring native plants. Of the 18,762 acres evaluated, 10 percent was found suitable for cottonwood or willow (Salix gooddingii or S. exigua), 45 percent was found suitable for honey mesquite (Prosopis glandulosa) or screwbean mesquite (P. pubescens), and 45 percent was found suitable for quailbush (Atriplex lentiformis) (Bureau of Reclamation 1995).
Water consumption has been saltcedar's most-studied physical property, primarily resulting from interest in removing or replacing the plant to conserve water. The term evapotranspiration is used to describe water loss per land area and includes evaporation from soil and transpiration, the biological process whereby plants lose water from their stomates while taking up carbon dioxide. Saltcedar is atypical, because it loses water by transpiration and evaporation due to the water evaporated from the salt glands.
Evapotranspiration is easy to understand but difficult to measure. Most studies have used one of four techniques: (1) planting plants in tanks and measuring water loss within the tank, (2) measuring the flow of xylem from the roots to the leaves, (3) measuring the decrease in flow within a stream and crediting part of the decrease to evapotranspiration, and (4) measuring microclimate to estimate the movement of water vapor upwards from the plant canopy. Each method has its advantages and disadvantages. For example, one can accurately measure xylem flow within several branches, but extrapolating the measurement over an entire acre may be difficult.
Differences between methods of measuring evapotranspiration and study sites has produced a wide range of water uptake estimates for saltcedar, varying from 1.4 feet per year to 10.5 feet per year (Bureau of Reclamation 1992). A recent study by Desert Research Institute (Ball et al. 1994) using the Bowen Ratio/Energy Balance method estimated an evapotranspiration rate of 2.3-2.5 feet per year from monotypic stands of saltcedar adjacent to the lower Colorado River near Blythe, California. The Bowen Ratio/Energy Balance method estimates evapotranspiration by measuring the amount of energy (mostly sunlight) absorbed by the plant community and used to evaporate water. As a comparison, the same study estimated an evapotranspiration rate of 1.6 feet per year for honey mesquite and 2.3 feet per year for quailbush. Maximum evapotranspiration by saltcedar was observed in the early morning when salt gland exudate was most visible, suggesting that the glands may contribute more to saltcedar's water loss than do the stomates.
The most observable impact of saltcedar on available water has been instances where surface water has visibly increased following plant removal. Two notable examples include saltcedar management projects at Eagle Borax Works Springs in Death Valley National Park (Rowlands 1990) and at Spring Lake near Artesia, New Mexico (Keith Duncan, pers. comm.). At Eagle Borax Works Springs, historical records described a natural spring and its associated ponds progressively drying-up concurrent with the spread of saltcedar beginning in 1950. In 1971, the park staff conducted a controlled burn of 10 acres to restore the site, and 8 weeks later the water elevation had risen 1.2 feet and a 1-acre pond had reappeared. At Spring Lake, saltcedar had invaded and covered a 13-acre spring-fed lake, eliminating its surface water by 1968. Saltcedar was effectively controlled with herbicides in 1989, and by 1992 the water table had resurfaced.
Wildfires are an increasingly common occurance in saltcedar along the lower Colorado River, partly the result of increasing population densities along the river's shorelines. The prevalence of fires in saltcedar has been attributed to the accumulation of leaf litter (Kerpez and Smith 1987) and dead and scenescent woody material (Busch 1995). Between 1981 and 1992, fires burned 9,281 acres, or 35 percent, of saltcedar-dominated vegetation on the lower Colorado River floodplain (Busch 1995). By comparison, fires in communities of honey or screwbean mesquite during the same period destroyed 629 acres, or 2 percent of the existing mesquite acreage. Assuming that the fires during the 12 years examined are representative and did not significantly overlap, a given stand of saltcedar would be expected to burn every 34 years.
Not only does saltcedar readily burn, its adaptation to saline conditions allows it to thrive in the elevated soil salinities that fires often produce (Busch and Smith 1993). In addition, the plant can quickly resprout from below ground after its above-ground parts have been completely burned away. Saltcedar's greater propensity to burn, and its tolerance for post-fire conditions, suggests that fires may be a significant factor promoting the plant's spread along the lower Colorado River.
Saltcedar also has been implicated to increase the frequency and intensity of flooding resulting in increased soil erosion. Dense stands of saltcedar covering a river's floodplain can impede high flows and cause the water to spread out and inundate areas not normally flooded (Robinson 1965). Saltcedar's restriction of the river channel also increases sedimentation, further intensifying flooding, and causing flows to meander outside the original channel and erode soil. The progression of saltcedar encroachment and its effect on sedimentation and flooding on the Brazos River in northcentral Texas has been described by Blackburn et al. (1982). As saltcedar deposited onto the floodplain prior to 1941 began to spread, the width of the river channel began to narrow. In 1941, the river channel's mean width along a 75-mile reach was 515 feet, but by 1979 the mean width had reduced to 220 feet. Saltcedar's encroachment also increased sediment deposition onto the floodplain, and flood events with similar flow rates produced flood stages of 10.2 feet in 1941 compared to 18.4 feet in 1971. Higher flood stage in 1971 caused a greater area to be inundated.
Clearing saltcedar has been used as a means of restoring the river channel's original flow capacity. A comparison of cleared and uncleared reaches of the Gila River near Safford, Arizona, found that clearing saltcedar increased flow velocity by 30 percent and decreased water depth by 13 percent (Great Western Research 1989). As an example of a full-scale project removing saltcedar to accomodate high flows, the Flood Control District of Maricopa County mechanically cleared a 1,000-foot wide corridor within approximately 34 miles of the Gila River near Phoenix, Arizona (Dick Perreault, pers. comm.). The corridor succeeded in reducing flooding, however an extreme flood in 1993 exceeded the corridor's flow capacity and caused erosion and property damage at the floodplain's margins. The cleared corridor has not been rehabilitated since the 1993 floods due to increased regulatory requirements.
Ball, J.T., Picone, J.B. and P.D. Ross. 1994. Evapotranspiration by Riparian Vegetation along the Lower Colorado River. Biological Sciences Center, Desert Research Institute, Reno, Nevada. Final Report for Bureau of Reclamation, Lower Colorado Region, Boulder City, Nevada, 188 pp.
Berry, W.L. 1970. Characteristics of salts secreted by Tamarix aphylla. American Journal of Botany 57:1226-1230.
Blackburn, W.H., Knight, R.W and J.L. Schuster. 1982. Saltcedar influence on sedimentation in the Brazos River. Journal of Soil and Water Conservation 37(5):298-301.
Bureau of Reclamation. 1992. Vegetation Management Study, Lower Colorado River; Phase I Report. Lower Colorado Region, Boulder City, Nevada. 103 pp.+ maps.
Bureau of Reclamation. 1995. Vegetation Management Study, Lower Colorado River; Phase II Report. Lower Colorado Region, Boulder City, Nevada. 72 pp.
Busch, D.E. 1995. Effects of fire on southwestern riparian plant community structure. The Southwestern Naturalist 40(3):259-267.
Busch, D.E., and S.D. Smith. 1993. Effects of fire on water and salinity relations of riparian woody taxa. Oecologia 94:186-194.
Gatewood, J.S, Robinson, T~W., Colby, B.R., Hem, J.D. and L.C. Halpenny. 1950. Use of water by bottom-land vegetation in lower Safford Valley, Arizona. U.S. Dept. of Interior, Geological Survey Water-Supply Paper 1103. 210 pp.
Great Western Research, Inc. 1989. Economic Analysis of Harmful and Beneficial Aspects of Saltcedar. Mesa, Arizona. Final Report for Bureau of Reclamation, Lower Colorado Region, Boulder City, Nevada, 259 pp.
Jackson, J., Ball, J.T. and M.R. Rose. 1990. Assessment of the Salinity Tolerance of Eight Sonoran Desert Riparian Trees and Shrubs. Final Report for Bureau of Reclamation, Yuma Projects Office, Yuma, Arizona, 102 pp.
Kerpez, T.A and N.S. Smith. 1987. Saltcedar control for wildlife habitat improvement in the southwestern United States. U.S. Dept. of Interior, Fish and Wildlife Service Resource Publication 169. 16 pp.
Robinson, T.W. 1965. Introduction, spread and areal extent of saltcedar (Tamarix) in the western states. U.S. Dept. of Interior, Geological Survey Professional Paper 491-A. 11 pp. + map.
Rowlands, P.G. 1990. History and treatment of the saltcedar problem in Death Valley National Monument. Pages 46-56 in M.R. Kunzmann, R.R. Johnson and P.S. Bennett [eds..], Tamarisk Control in Southwestern United States. U.S. Dept. of Interior, National Park Service, Cooperative National Park Resources Studies Unit, University of Arizona, Tucson. 144 pp.
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