Tamarix (saltcedar) is one of four genera of Tamaricaceae and is represent by 90 species worldwide. The genus was named after the Tamaris River in Spain and consists of halophytic shrubs and small trees native to Western Europe, the Mediterranean, North Africa, and northeast China and India (Baum 1967). Eight species of Tamarix have been introduced into the United States, primarily as ornamentals or for wind breaks and shade. Of these species, five are present in the southwest. Most species are weedy, particularly T. parviflora, previously known as T. tetrandra, and T. ramosissima, previously known as T. pentandra. One species which is less weedy is the large evergreen tree, athel (T. aphylla). Table 1 compares the morphology and abundance of the saltcedar species found in southwestern United States.
Saltcedar species are phreatophytes (deep-rooted to reach water table) that depend on groundwater for their water supply. However, under some conditions saltcedar can grow where no groundwater is accessible. Thus, it is classified as a facultative rather than obligate phreatophyte (Kerpez and Smith 1987).
Shoot Growth. Weedy saltcedars can grow to heights of 3 to 4 m in a single growing season under favorable conditions (Sisneros 1991). Furthermore, mature saltcedar is remarkably tolerant to a variety of stress conditions, including heat, cold, drought, flood, and high concentrations of dissolved solids. By dropping its leaves and halting growth, saltcedar can withstand lengthy periods of drought. In contrast, mature plants can also survive complete submergence for as long as 70 days (Kerpez and Smith 1987).
Root Growth. The root system of saltcedar is extensive, and is largely responsible for its competitiveness and survival under stress. Initially, the primary root grows steadily downward with little branching until it reaches the water table, which can be at depths of 3 m or deeper (Brotherson and Winkel 1986). Once the water table is reached, secondary root branching becomes profuse.
Adventitious roots easily develop from submerged or buried saltcedar stems. Thus, expansion in saltcedar infested areas can also be through vegetative growth (Kerpez and Smith 1987).
Reproduction. Seedlings mature rapidly and produce small, white or pinkish flowers often by the end of the first year of growth (Neill 1985). Flowers have four or five sepals and petals, three to five styles, and stamens borne on a fleshy, lobed, hypogynous disk. The fruit is a 3 to 5-valved capsule (Kerpez and Smith 1987). Seeds have a tuft of hair on the end to aid in wind dispersal or can also be carried and deposited along sandbars and riverbanks by water (Brotherson and Field 1987). A single large saltcedar plant can produce a half million seeds per year, primarily from late May to October.
Germination. Seeds which develop from mature plants are quite small and light (0.1 mg) (Sisneros 1991), and will germinate on saturated soils or while afloat. Once wetted, fresh seeds usually germinate within 24 h (Kerpez and Smith 1987). Due to their short-lived viability, saltcedar seeds must come in contact with suitable moisture within a few weeks of dispersal. Seedling mortality is high when soils are scoured or dry up too quickly, or when seedlings are submergence for four to six weeks following germination (Shrader 1977).
After summer rains, saltcedar seedlings can rapidly colonize new moist areas because flowering and fruiting cycles provide a continual supply of available seeds (Engel-Wilson and Ohmart 1978). This strategy is of considerable advantage over native riparian species since saltcedar can exploit suitable germinating conditions over a longer time interval (Howe and Knopf 1991).
Seedling Establishment. Establishment of saltcedar seedlings occurs in high seasonally saturated soils (Brotherson and Winkel 1986, Brotherson and Field 1987). This requirement is most often met along river or reservoir banks where slowly receding water levels create optimum seed beds. In the initial stages of establishment, roots grow slowly the first two to four weeks, and will not survive more than one day if the soil dries. Although the seedlings can survive submerged for a few weeks, they are easily uprooted by even a weak current and do not tolerate flooding within a period of several months subsequent to germination (Kerpez and Smith 1987).
On some occasions saltcedar can become established in typically dry locations if these areas experience an unusually wet spring and early summer, or if rivers or lakes temporarily flood their boundaries (Carman and Brotherson 1982). Once established, saltcedar can survive almost indefinitely in the absence of surface saturation of the soil (Brotherson and Field 1987).
Saltcedar grows to about 1,650 m (5,400 ft) in elevation (Brotherson and Winkel 1986) and prefers very saline soils. Typically, saltcedar occupies sites with intermediate moisture, high water tables, and little erosion. However, mature plants can withstand long periods of water inundation (70-90 days). They can resprout vegetatively after fire, severe flood, or treatment with herbicides and are able to accommodate wide variations in soil and mineral gradients (Brotherson and Field 1987).
Soil. Successful stands of saltcedar are generally found in non-rocky soils composed of silt loams and silt clay loams high in organic matter (Brotherson and Field 1987).
Salinity. Saltcedar is not an obligate halophyte but can survive in areas where groundwater concentration of dissolved solids approaches 15,000 ppm (Carman and Brotherson 1982), but typically occur in areas averaging about 6,000 ppm salt (Brotherson and Winkel 1986).
Following fire, higher alluvium salinity and elevated concentrations of phytotoxic boron can delay the reestablishment of native trees and shrubs, particularly Populus and Salix. These areas are very susceptible to invasion by salt tolerance species of Tamarix (Busch and Smith 1993).
Acidity. Saltcedar has a slight preference for alkaline conditions (pH = 7.5) compared to other native shrubs (Brotherson and Winkel 1986).
Allelopathy Saltcedar exudes excess salt crystals from openings in its scale-like leaves (Neill 1985). It has been reported to contain 41,000 ppm dissolved solids in the guttation sap (Duncan et al. 1993). Not only can these glands concentrate salt, but they also secrete various other ions, including boron (Busch and Smith 1993). These salts are eventually deposited on the soil surface under the plant, sometimes forming a hard crust (Kerpez and Smith 1987). Such deposits of salt-encrusted needles can inhibit the germination of other species (Egan et al. 1993). A combination of this allelopathic effect and the extensive lateral root system contribute to the ability of saltcedar to outcompete other vegetation for space and water (Brotherson and Field 1987). In some communities, saltcedar is the dominant overstory species, whereas salt tolerant grasses, such as saltgrass (Distichlis spicata), dominate the understory (Brotherson and Winkel 1986).
Water Acquisition. The longer the community has been invaded by saltcedar the greater will be the capacity to lower the water table in the soil (Brotherson et al. 1984). With this overall drying out of the habitat, more xeric plant species will occupy the understory in established saltcedar stands.
A dense stand of saltcedar will grow where the water table is between 1.5 and 6 m from the surface (Table 1). Water use of saltcedar is generally considered high, but evapotranspiration rates can vary with water table depth and soil salinity. Under dry or extremely hot conditions, saltcedar does not always transpire at potential rates (Davenport et al. 1982). Water conservation under these situations is of ecological significance as it enables Tamarix species which grow in hot desert environments to open their stomata just at daybreak during the coolest and most humid hours of the day. This allows the plants to acquire adequate CO2 without losing much water. The stomata close during the hotter afternoon hours, further reducing water loss (Hagemeyer and Waisel 1990). Summer evapotranspiration rates can also vary considerably with stand density and other stress conditions (Davenport et al. 1982).
As a facultative phreatophyte, Tamarix species are capable of extracting soil moisture from less saturated soils in areas with deeper water tables. This appears to be an adaptation that obligate native phreatophytes such as Populus and Salix do not possess (Busch et al. 1992), and may partially explain the competitive exclusion of these native shrubs by saltcedar in southwestern riparian areas.
Table 1. Comparison of the naturalized five Tamarix species.
|T. aphylla||tree <12 m||not overlapping, strongly clasping||5-parted, nectar disk lobes wider than long, stamens alternate disk lobes||escaped populations uncommon, often cultivated|
|T. chinensis||tree <10 m||overlapping, oblong to narrowly lanceolate||5-parted, nectar disk lobes wider than long, stamens alternate disk lobes||uncommon|
|T. gallica||shrub or tree <8 m||overlapping, linear to narrowly lanceolate||5-parted, nectar disk lobes longer than wide, stamens together with disk lobes||uncommon|
|T. parviflora||shrub or tree 1.5-5 m||overlapping, linear||4-parted, nectar disk lobes longer than wide, stamens together with disk lobes||common, serious weed problem|
|T. ramosissima||shrub or tree <8 m||overlapping, ovate||5-parted, nectar disk lobes wider than long, stamens alternate disk lobes||common, serious weed problem|
Table 2. Characteristics of floodplain zones at varying groundwater depths (from Shrader 1977).
|Zone||Depth to groundwater (m)||Saltcedar growth||Other vegetation||Water salvage prospects||Other uses|
|1||1||dwarfed & multi-stemmed||vigorous saltgrass & bermudagrass||little||good grazing, flood passage, minimal wildlife use|
|2||1.5-2.5||major stands||excellent saltgrass||large savings||wildlife utilization (doves), some grazing, bees|
|3||2.5-6||major stands||xeric types||great water savings||wildlife utilization (doves), bees|
|4||>6||scattered individuals||xeric types||none expected||limited use|
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