Saltcedar
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Introduction and Spread
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Biology and Ecology
Common Names and Synonyms
Athel tamarisk (Tamarix aphylla (L.) Karsten); French tamarisk (Tamarix gallica L.); Chinese tamarisk (Tamarix chinensis Loureiro); saltcedar (Tamarix ramosissima Ledebour, Tamarix pentandra Pallas, Tamarix gallica L.); smallflower tamarisk (Tamarix parviflora De Candolle, Tamarix tetrandra Pallas) (Baum 1978).
Taxonomy and identification
Tamarix is one of four genera in the Tamaricaceae and is represented by 54 species worldwide (Baum 1967). Tamarix taxonomy is somewhat disputed and authors generally include nomenclature different from each other (Allred 2002; Baum 1967, 1978; Crins 1989; DiTomaso 1998; Gaskin 2003; Martin and Hutchinson 1981; McClintock 1951; Welsh et. al. 1987). In addition to scientific synonyms, Gaskin (2003) listed several more putative invasive taxa in the United States including T. africana Poir, T. aralensis Bunge, T. canariensis Willd., T. juniperina Bunge, and T. tetragyna Ehrenb. Most species with the exception of T. aphylla are regarded as weedy (DiTomaso 1998), and T. ramosissima is generally regarded as the most serious invasive species. The accepted common names of tamarisk and saltcedar are usually applied to many of the 90 species of the plant (Graf 1978), but in the southwestern United States, the common names usually apply to T. ramosissima (Baum 1967) or T. chinensis (Allred 2002). Many taxonomists consider these to be indistinguishable and the same species, in which case T. chinensis is the proper nomenclature (Gaskin and Schaal 2003).
General botanical characteristics
Tamarix or saltcedar, commonly have numerous basal branches or trucks that are brown or blackish and younger branches that are reddish-brown or purplish. Leaves are scale-like, alternate, sessile or sheathing, with salt-secreting glands. Flowers are small, perfect, and subtended by a small bract (Allred 2002). Petals are white, pinkish, or reddish - different colors often are found on the same plant or even the same flowering branch. Of the four principle invasive species, two (T. gallica and T. parviflora) flower mainly in spring, though they may also flower in summer, and two flower in both spring and summer (T. ramosissima and T. chinensis) (Allred 2002; DiTomaso and Healy 2003). Other than T. aphylla that has sheathing leaves, the other three species are not distinguishable by vegetative features.
Growth form
Saltcedar has principally two growth forms as a mature plant, either highly multi-stemmed or a main stem tree that seldom grows over 30 ft tall. The natural tendency is to grow into a tree, however, if the top growth is disturbed in any way then saltcedar becomes more shrub-like with multiple stems. Plant age can not be easily discerned by the growth form. Stems support slender, contorted branches which are covered when young by small scale-like leaves. Small leaves, sunken stomata and water-storing tissue adapt it to life in wet or dry climates. The ability of saltcedar to thrive under a variety of conditions is undoubtedly due to the remarkable adaptability of its widespread and deep root system.
Root biology
In order to determine variations in root depth, lateral development, and degree of branching, Tomanek and Ziegler (1960) studied 35 excavated plants that varied in age from a few days to four years. In nearly all cases, roots extended to the capillary fringe above a constant source of water. Roots were never found extending into the saturated zone except where the water table had recently risen. They found that the degree of development of the root systems varied in several respects. Smaller roots branch and re-branch near the extremities, providing a finely subdivided and effective absorptive tap root which went down to the water table. However, on most large plants the major part of the system was made up of lateral growth of primary, secondary, and adventitious roots. The determining factor in branch root formation and development may possibly be the location of the water table.
According to Tomanek and Ziegler (1960) where environmental conditions permit, saltcedar develops a widespread and deep root system. They found lateral roots to spread 30 ft from a single plant and that most roots were from 12 to 16 inches beneath the surface. On one excavated plant the tap root extended below 16 ft and was still 3/16 inch in diameter. While they did not determine total root depth, they hypothesized it is 25 ft or more.
Reproduction
Flowers are borne in dense panicles developing from base to apex in individual racemes. Typically blooms are found from early spring (mid-April) to frost (early-November), with a major peak in late spring and early summer, although cycles of heavy and light production occur throughout the season. It is believed that time of flowering depends upon the age of the individual branch and that branches on the same plant flower differently (Tomanek and Ziegler 1960). Plants as young as 1 year old often can flower and produce viable seeds. Individual flowers contain 20 or more ovules, each of which is capable of developing into a seed. A flowering branch three inches long often supports an average of 50 to 60 flowers, and a single mature plant can produce >500,000 seeds in single season.
Seeds are small with a tuft of hair on the end to aid in wind dispersal (Merkel and Hopkins 1957). These seeds can also float on water where they are deposited along sandbars and riverbanks (Brotherson and Field 1987). Tamarix seed is somewhat cylindrical and very small (0.17 mm in diameter and 0.45 mm long, weight 0.0001 gm) (Tomanek and Ziegler 1960). Seed is typically viable for only a few weeks after maturity (Reynolds and Alexander 1974; Young et al. 2004). Close synchrony between seed availability and damp soil moisture conditions are a must for germination and seedling survival (Warren and Turner 1975). During periods of heavy production, seed in nearby waterways is often high as well as seed deposited on the soil.
A study of seed dispersal by Tomanek and Ziegler 1960 estimated distance traveled by saltcedar seeds. Boards, one foot square were painted black and layered with grease to capture and hold seed. These boards were mounted on posts at varying distances from known seed sources. As distance from the seed source increased, chances for a seed to hit a board one square foot in size became more remote, with the furthest seed recovered up to 790 ft away. They stressed that their data not be interpreted to mean that seed will travel only 790 ft, but rather to emphasize that the light and fluffy seed can move great distances, probably several miles from any known source. In a similar study by Young et al. (2004), they used sticky traps to capture seed dispersed at varying distances from a saltcedar infestation on the Walker River Delta in Nevada. They reported the seed rain at the source as 4,600 seeds per m2, but about 0.1 km down wind 2,400 seeds per m2 were recorded, and 51 seeds per m2 were captured 8 km down wind from the infestation.
Saltcedar seed can germinate over a wide range of constant or alternating temperatures, but has no great advantage over seeds of native woody species in a common riparian environment (Young et al 2004). A competitive difference, however, is that native species have very specific and relatively short periods of seed dispersal in late spring, whereas saltcedar seed production continues throughout the entire summer. This seed has no dormancy requirement but remains viable for only about 5 weeks under normal conditions (Everitt 1980). Because of their short-lived viability, saltcedar seeds must come in contact with moist seedbed conditions within a few weeks of dispersal (DiTomaso 1998). Receding flood waters, receding lake margins or fluctuating stream banks with exposed sandbars often provide the ideal environment for dense uniform seedling establishment over broad areas. Once wetted, seed usually germinate within 24 hours of receiving moisture (Tomanek and Ziegler 1960).
Seedlings are probably more susceptible to competition or adverse environmental conditions than other stages of development. For seedlings to establish successfully, they require a combination of saturated soil for the first 2 to 4 weeks of life, open sunny ground, and the absence of competition (DiTomaso 1998, Brotherson and Field 1987, Brotherson and winkel 1986). In the initial stages of establishment, roots grow slowly within the first 4 weeks and will not survive more than a day if the soil dries (Kerpez and Smith 1987). Seedling establishment also requires at least 4 to 6 weeks without subsequent inundation (DiTomaso 1998, Shrader 1977). Although seedlings can survive submerged for a few weeks, they are easily uprooted by currents or disturbance. Data indicate the ideal water level for development and growth of saltcedar seedlings to be near the soil surface to 1 inch deep.
Saltcedar seedlings grown in the greenhouse in pots containing loamy soil grew primary roots at an average rate of about 1 mm per day or 16 mm after 3 weeks (Tomanek and Ziegler 1960). In this study, during early stages of development, roots in the field resembled those grown from greenhouse containers. Secondary root development was generally initiated by the third week after germination, but was sometimes delayed until the fifth or sixth week. At the end of the first month, the primary roots initiated lateral growth if plants are kept in nearly saturated soil moisture. By the sixth week of the study, primary roots reached about 35 mm and averaged 6 to 7 secondary roots on most plants. Within 10 to 11 weeks roots reached the bottom of pots (30 inch phytometers) and in most cases were only slightly branched in the upper 16 to 24 inches. Below 24 inches roots were extensively branched.
Seedling survival will vary considerably under different environmental conditions. Within a season the population of cohorts may fluctuate widely as seedlings may die during short dry unfavorable periods only to be replaced by new seedlings during wet favorable periods. Tomanek and Ziegler (1960) estimated that during a full season the survival of any one young population might be < 10% and, by the time a community becomes established, would surely be < 1%.
Vegetative reproduction by secondary growth or layering from roots and stems is common with saltcedar. Basal shoots which become nearly prostrate either naturally or accidentally are capable of producing adventitious roots from any portion of the stem in contact with moist soil. Anchored shoots send branches upwards giving a dense multi-stemmed appearance and produce new plants that superficially appear independent. Only by examination of underground parts is it apparent that they are shared plants. Whether or not adventitious roots play a primary part in survival of the plant is not known. However, without layering saltcedar would not be as successful in maintaining its place in stream channel and riparian environments. Layering is also a primary mechanism that allows saltcedar to be resilient to control strategies that harm top growth.
