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Species
Tamarix parviflora DC
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Tall shrub, 4 m high, with reddish brown bark. Leaves simple, alternate, sessile, ovate with somewhat auriculate base, 3-5 mm long 1-1.5 mm broad, acuminate. Racemes lateral, vernal, 24 cm long c. 5 mm broad. Flowers pinkish, tetra and pentamerous but more often tetramerous. Bracts longer than pedicel, trullate ovate, acuminate with a diaphanous blunt end, apex subobtuse, membranous at the margin, c. 2 mm long, c. 1 mm broad, pedicel c. 1 mm long. Calyx fused at the base, outer sepals acute, inner obtuse, ovate, denticulate, membranous at the margin, c. 1.5 mm long, 1 mm broad. Petals oblong, obovate, somewhat parabolic, 2-2.25 (-2.5) mm long, 1 mm broad, persistent, rarely subpersistent. Stamens 4 or 5 (depending upon tetra or pentamerous flowers), 3.54 mm long, filaments filiform confluent with the disc lobe, (epilophic disc), anthers 1 mm long, ovate, apiculate, Stigmas 3, somewhat club shaped, ovary conic. Capsule trigonous, c. 4 mm long, c. 2 mm broad.
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More info for the terms: geophyte, phanerophyte
RAUNKIAER [189] LIFE FORM:
Phanerophyte
Geophyte
Tamarisk honey is "typically described as being amber in color with a strong disagreeable taste. It sometimes impairs the color and flavor of honey from other sources, if not extracted in a timely fashion. Still, to beekeepers in the arid Southwest whose bees are near tamarisk, the plant is of major importance, both for its nectar and for the pollen it supplies" [65]. Saltcedar stands can be a refuge for honey bees, especially during the season that insecticides are applied to croplands. Management of saltcedar for honey production needs more research [122].
Wood Products: Production of fuelwood from saltcedar areas is probably not very important [122].
Habitat and Ecology
Systems
- Terrestrial
- Freshwater
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More info for the terms: association, cover, density, invasive species, presence, shrub, tree
Livestock: Cattle and sheep tend to browse heavily on young tamarisk seedlings and mature plants if the stand is open [107]. More commonly, livestock tend to browse native plants (e.g. cottonwood and willow), giving tamarisk the competitive advantage in areas grazed by livestock [70,224]. In Arizona, tamarisk is seldom browsed by livestock, but is used by cattle for cover in river bottoms [139].
Wildlife: The species richness and diversity of wildlife in tamarisk differs from one location to another. Biogeographical considerations, specifically elevation and climatic gradients, may at least partially explain this phenomenon [133].
Extensive stands of tamarisk may lack the plant diversity and food sources associated with some native riparian communities. In some cases, native communities may be more likely to support greater species diversity and numbers than tamarisk monocultures [6,107,237], although this may not be true in all cases [133]. Tamarisk does affect habitat dynamics for a number of birds, mammals, insects, and aquatic species where it invades.
Riparian floodplains in the southwestern U.S. support some of the highest concentrations of breeding bird species in temperate North America in both abundance and diversity [87,234]. Several authors suggest that replacement of native woody vegetation with nonnative, invasive species such as tamarisk may result in a reduction in avian diversity and species richness (e.g. [87,108]). The effect of saltcedar on avian species is dependent on the type of bird, density and growth characteristics of saltcedar (e.g. [122]), elevation and temperature [133], and overall structure of the community. Additionally, breeding bird densities, diversity and species richness differ from location to location, season to season, and year to year within the same vegetative association [234].
Several studies suggest that saltcedar communities do not support as high a density and/or diversity of native bird species as do native plant communities on the lower Colorado River [5,6,7,56,164]. Along the Rio Grande in west Texas, saltcedar has a high total bird population in the breeding season consisting mostly of white-winged doves, but during the winter bird densities are very low [82]. Other studies have found a high degree of avian use of tamarisk along the middle Pecos River in Texas and New Mexico (e.g. [133,152]) and along the Rio Grande River (e.g. [79,148]) for at least some bird species. Brown [37] found that 5 of 6 species of obligate riparian birds preferred saltcedar as a nest site over native riparian vegetation along the upper Colorado River in Grand Canyon National Park, and Brown and Trosset [38] conclude that "the tamarisk community created by the operation of Glen Canyon Dam represents the ecological equivalent of native habitat for some riparian birds, and its presence has enhanced breeding habitat for these 11 species of birds."
Saltcedar provides habitat for a number of bird species including (but not limited to) white-winged and mourning dove [122], Mississippi kite [98], black-throated sparrow [253], summer tanager [5], yellow-billed cuckoo, yellow-breasted chat, rufous-sided towhee [152], and the endangered southwestern willow flycatcher [37,177]. According to Cohan and others [56], a large number of species that use saltcedar belong to genera of the Old World where saltcedar evolved. Among them, ground feeders, granivores, or species such as doves often showed a preference for or did not avoid saltcedar; frugivores do not use saltcedar; and very few insectivores used saltcedar, although palatable insects can be present in large numbers [56]. Other studies have found that tamarisk stands do provide habitat for several insectivorous birds (e.g. [5,37,98,152,177,253]). In the Bosque del Apache National Wildlife Refuge in the Middle Rio Grande Valley, timber gleaners such as the white-breasted nuthatch, were never detected in saltcedar and timber drillers and cavity nesters were rare in saltcedar [79]. Ellis [79] suggests that many species could successfully transition to saltcedar communities, while others (i.e. cavity nesters and timber gleaners) are more strongly tied to native vegetation. Similarly, at least 6 of the bird species studied on the lower Colorado River by Meents and others [164] may be threatened by the continuous decline or removal of native vegetation.
The number and type of bird species supported by tamarisk communities depends, in part, on density of saltcedar and composition and structure of the community. For example, mixed communities containing saltcedar may support more bird species and have higher densities than saltcedar monocultures [56]. Tall, dense stands of saltcedar along the lower Colorado River are valuable for nesting doves and less common bird species, such as the summer tanager, that are normally restricted to cottonwood-willow communities [5]. Floodplain grassland areas on the middle Pecos River are low in bird abundance and species richness when compared to tamarisk habitat. These areas are, however, important to grassland birds, and removing tamarisk from the Pecos River would provide these species with additional habitat, while reducing habitat for other species, including yellow-billed cuckoo, yellow-breasted chat, and rufous-sided towhee [152]. Farley and others [87] suggest that most bird species in the central and southern Rio Grande Valley benefit from a mosaic of riparian woodlands containing mixtures of native tree and shrub species of different size classes. Breeding pairs of the southwestern willow flycatcher spend most of their time beneath the overstory canopy in willows or saltcedar in the Rio Grande Valley [177]. The southwestern willow flycatcher nests in native vegetation where available, but also nests in thickets dominated by tamarisk and Russian-olive [244,264]. In one study, the willow flycatcher was captured more frequently in saltcedar than in other habitats in the fall. Several other migrant species were captured more frequently in saltcedar than in other habitats, in spring, fall, or both seasons [264]. Some tamarisk stands mimic, to some degree, the riparian woodland structure once provided by willows, and tamarisk is used as a nesting substrate by the flycatcher. Flycatcher productivity in tamarisk-dominated sites has been variously found to be equal to or lower than in sites dominated by native willow species. From the standpoint of flycatcher productivity and survivorship, the suitability of nonnnative-dominated sites is not known [244]. See the southwestern willow flycatcher recovery plan for more information.
Vertebrate herbivores on tamarisk include beaver, while other rodents and/or lagomorphs may rarely consume young foliage [220]. Bank beavers will eat young shoots of saltcedar, but they prefer cottonwood and willow over saltcedar by about 9 to 1, thereby continually transforming a floodplain habitat to saltcedar dominance [115]. At Big Bend National Park, beaver eat willow saplings but not saltcedar, exacerbating the invasion. Beaver populations appear to be suffering from saltcedar invasion, and populations of other rodents also appear to be affected, some positively and others negatively [26]. Monotypic stands of saltcedar are little used by beaver or mule deer [108]. In Owens Valley, California, pocket gophers were found to cause damage and mortality to saltcedar plants in several stands by chewing through the roots [157]. Along the shoreline of Lake Powell, black-tailed jackrabbits use saltcedar as a major food source [253]. Both cottonwood and saltcedar communities provide important cover and forage for calving elk in the Cimarron National Grassland in Kansas [24].
A survey of herpetofauna on a riparian island upstream from the Whitlow Ranch Dam in Arizona, dominated by Goodding willow and saltcedar, indicated many of the locally expected riparian species were absent. It is unclear whether biogeographical considerations and flooding patterns are responsible, or if structural and physical conditions of the new habitat are responsible [236]. Reptile abundance and diversity were greater on an unaltered, mature, gallery-type stand of cottonwood and willow than on an altered site dominated by honey mesquite (Prosopis glandulosa) and tamarisk in central Arizona [136].
Watts and others [255] provide a list of insects associated with saltcedar in New Mexico. Stevens [220] reports over 200 species of invertebrate herbivores associated with tamarisk in the U.S., only 6 of which are sufficiently common in northern Arizona to qualify as pests of this plant. Insect herbivory does not appear to affect tamarisk growth. Herbivory must exceed 75% of the foliage before reduction of apical growth rate occurs [220].
Vegetated shorelines of the Colorado River, consisting mainly of nonnative saltcedar, had nearly twice the densities of subadult humpback chub compared to talus and debris fans. This may have important implications for humpback chub recovery and management of the Colorado River through Grand Canyon, where shorelines were not historically vegetated, and saltcedar invasion has created a unique habitat [57].
Palatability/nutritional value: The scale-like leaves of tamarisk tend to be unpalatable to grazers [218]. The nutritional value of tamarisk is not known, although it is reported to have very low crude protein content [115].
Cover value: Saltcedar provides some cover for livestock and wildlife (e.g.[24,139]). Saltcedar provides nesting sites for many bird species (see above discussion for pertinent references).
Habitat and Ecology
Systems
- Terrestrial
- Freshwater
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More info for the term: phenology
Tamarisk seedlings establish shortly after seed dispersal if conditions for germination and establishment are appropriate (see Seedling establishment/growth). Some authors report that tamarisk seedlings can flower and produce seed in their 1st year [254], but most begin to reproduce in their 3rd year or later [107]. Flowering can be in spring or summer or both. Some tamarisk plants can produce seed from May to October, and peak seed production and seedling establishment is typically during June, depending on location.
During 15 years of observation, Stevens [220] reports that most saltcedar plants in northern Arizona produced 2 flushes of foliage, 1 in April and May, and the other in late July. At low elevations in northern Arizona, flowering commenced in late April about 1 month after bud-break and peaked from mid-May to early June. Saltcedar plants that were not water-stressed continued to bloom at low to moderate levels throughout the growing season, while water-stressed individuals ceased reproduction following the spring blooming period [220]. On the San Pedro River in southern Arizona, saltcedar flowered between late April and early October [254]. In Kansas, flowering in tamarisk started during late May and continued until late October with maximum anthesis in mid-August. Plants often supported all flowering stages, from buds to mature seeds, at one time [165].
On the Salt River in Arizona, seasonal development in saltcedar is as follows: buds begin swelling in mid-February, racemes begin blooming on last season's wood in mid-March, achieve full bloom in early April and disperse seeds by mid-April. Terminal panicles on new shoots begin budding by mid-April, and are in full flower in early May, as other panicles begin to bud. This cycle continues until the last bloom in September [119]. The morphology of the flowering structure is different between spring and summer on the same plant. Those plants that flower in both seasons bear spring flowers composed of lateral racemes arising from old wood, and summer panicles (or sometimes racemes) form on young shoots of the current season [123].
The following table provides some flowering dates for tamarisk species as reported by location:
Location | Species | Flowering dates | Reference |
AZ | saltcedar | March to August | [139] |
AZ | small-flowered tamarisk | spring | [119] |
AR | saltcedar | May through September | [132] |
Carolinas | French tamarisk | April-July | [187] |
Great Plains | saltcedar | May-October | [103] |
northern Great Plains wetlands | saltcedar | June thru September | [146] |
NM | small-flowered tamarisk | March-April | |
NM | saltcedar | April to November | [3] |
north-central Texas | saltcedar | June through July, October | [71] |
Texas to southern North Carolina | saltcedar | March to October | [76] |
western Utah | May 11- May 23 | [27] |
Seed dispersal in tamarisk begins shortly after flowering. Horton and others [120] found most tamarisk seed was produced between April and July, and was produced intermittently until late September or October along the Salt River in southern Arizona. Along the Gila River in Arizona, the 1st seeds are released during the week of May 4 to May 11 and throughout the following 5 1/2 months ending in mid-October. The peak of seed production is in mid-June, with a 2nd, minor, peak in mid-August [254]. On sand flats on the lower levels of the floodplain of the south Canadian River in central Oklahoma tamarisk begins fruiting in late May, and continues to produce flowers and disperse seeds throughout the summer [252].
The phenology of tamarisk relative to co-occurring natives varies from site to site. On the San Pedro River in southern Arizona, Fremont cottonwood produced seed before saltcedar and willow began flowering [254]. On the sand flats of the lower levels of the floodplain of the south Canadian River in central Oklahoma, tamarisk reaches full verdure at the same time as cottonwood and willow (late March), but initiates flowering about 2 weeks later [252]. On the Upper Green River in Utah, cottonwood seed rain is nearly complete in early August, when saltcedar seed dispersal is at its peak. Saltcedar seed dispersal at this site continued until mid-September [60].
Phenological observations along the Rio Grande valley indicate that development of tamarisk floral and vegetative parts is approximately 20 days later at Albuquerque than at El Paso [47]. Similarly, a study along the Bill Williams River in Arizona reported that seed dispersal began in late March at a downstream site and by early April at the upstream site. Seed dispersal continued into October [203].
On sand flats of the south Canadian River in central Oklahoma, tamarisk seedling establishment began June 15 [252]. In western Utah initial growth of established plants began in early April [27].
Tamarix tetrandra is a species of flowering plant in the family Tamaricaceae, native to south eastern Europe, Turkey, Bulgaria and Crimea. Growing to 3 m (10 ft) tall and broad, it is a small deciduous tree with almost black arching branches, and tiny scale-like leaves arranged along the branches. Racemes of pale pink flowers are produced in late spring.[1]
The binomial Tamarix tetrandra means "four-stamen tamarisk".[2]
This plant is particularly associated with temperate coastal areas, but can also be grown inland in a sunny position with protection from winter winds. It has gained the Royal Horticultural Society's Award of Garden Merit.[2]
References[edit]
- ^ RHS A-Z encyclopedia of garden plants. United Kingdom: Dorling Kindersley. 2008. p. 1136. ISBN 1405332964.
- ^ a b "RHS Plant Selector - Tamarix tetrandra". Retrieved 5 June 2013.
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More info for the terms: codominant, density, frequency, selection, severity
Tamarisk tolerates a wide range of environmental conditions. It has been suggested that this tolerance is a result of multiple species of tamarisk, with previously disjunct distributions across the Eurasian continent, hybridizing into a single species complex in North America [35,212].
As a facultative phreatophyte, tamarisk is mostly found on lakeshores and in riparian floodplain habitats, on seasonally submerged sites, and in fine fluvial substrates [33,71,220,259]. In the southwestern United States, it first established along major drainages and then invaded outlying ephemeral water courses, canyon bottoms, isolated marshes, wet pastures, springs, desert oases, and rangelands [212,249,256,257]. Saltcedar is widely distributed in these habitats in New Mexico while small-flowered tamarisk and French tamarisk are only rarely found there [3]. In northern Great Plains wetlands, saltcedar is found on stream banks, floodplains, ditches, alkaline or saline flats [146], and is found in river flood plains, salt marshes, and roadsides in Texas, Oklahoma, Kansas, western Nebraska, western South Dakota and western North Dakota [103]. In California, tamarisk is abundant where surface or subsurface water is available for most of the year, including stream banks, lake and pond margins, springs, canals, ditches, and some washes. Disturbed sites, including burned areas, are particularly favorable for tamarisk establishment. It survives and even thrives on saline soils where most native, woody riparian plants cannot [153,156]. Saltcedar is a seaside plant from Texas to southern North Carolina and is spreading in the coastal prairie along the Gulf of Mexico [76,101].
The extent to which tamarisk assumes dominance in these various habitats is a function of climate, and current and historical disturbance regimes. The influence of riparian flow regimes on water Table depth, seasonal water availability, flooding, sedimentation and native plant community structure and composition is also an important determinant of relative tamarisk dominance [83,102,212].
Elevation: Tamarisk can grow from below sea level to more than 6,600 feet (2,000 m) elevation [115,235,239]. In Death Valley National Monument saltcedar is a potential invader of all streams, ponds, marshes and wet ground below 5,200 feet (1,585 m) [153]. In New Mexico saltcedar is found along water courses 3,000-6,500 feet (915-1,980 m) [160]. In Arizona, tamarisk is abundant along streams in most of the state below 5,000 feet (1,525 m) [139], and, while it grows in the Southwest at elevation up to 11,000 feet (3350 m), it does not spread rapidly above 4,000 feet (1220 m) [226].
In California, small-flowered tamarisk is common in washes, slopes, sand dunes and roadsides <2,600 feet (<800 m) [111], and in Utah, small-flowered tamarisk is found along seeps and streams at 2,800 to 5,600 feet (850-1,710 m) [259].
Climate: Saltcedar generally grows best in the warmer climate of the semiarid southwestern U.S., reaching its greatest extent along major lowland watercourses such as the Colorado and Rio Grande rivers and many of their tributaries such as the Pecos and Salt Rivers [107]. Tamarisk can also be found in palm oases in the Colorado Desert of California, where the average maximum July temperature is 107 degrees Fahrenheit (42 °C) and the average minimum January temperature is 39 degrees Fahrenheit (4 °C). At the other extreme, near the northern edge of its North American range, tamarisk is found on sites along the Bighorn, Powder, and Yellowstone rivers in southeastern Montana [149,249]. In these areas, tamarisk routinely dies back to the ground and the oldest live stems are generally much younger than the entire plant. It may also be susceptible to sporadic, harsh, climate-driven events such as freezing, flooding, or ice scour at these extremes [149]. Hudson [130] found slower seedling growth, lower initial densities, later flowering, and shorter growing season for saltcedar in the cold, semiarid desert environment of Bighorn Lake in northern Wyoming, as compared to seedling studies in the Southwest. These results indicate that recruitment rates of saltcedar may be somewhat slower, and invasive potential less certain in colder environments [130]. Similarly, Stromberg [226] found tamarisk growth at 3,700 to 4,300 feet (1,130-1,300 m) in southern Arizona was substantially slower than at 330 feet (100 m). Sexton and others [201] also found that tamarisk seedlings from northern sites were smaller and invested more energy in roots when grown at low temperatures. Throughout its current distribution in both Canada and the U.S., saltcedar has not become a major problem along colder, high elevation streams, or along low elevation streams in northern Canada [107]. However, it has been suggested that the invasive potential of northern populations could increase if populations persist long enough to experience multiple episodes of selection [201].
Disturbance: Tamarisk communities are frequently associated with past disturbances and/or changes in historic disturbance regimes. Engineering features on most western rivers for management of water and electric power have resulted in increased evaporation and associated salinity, changes in erosion and sedimentation rates, and other physicochemical changes [135]. In the central Rio Grande Valley, changes in both physical environment and native vegetation were well underway by the time tamarisk became widespread. Tamarisk occupied land made available by agricultural and urban development and by upstream water development. There is no evidence that it actively displaced native species nor that it played an active role in changing the hydraulic or morphologic properties of the river [84].
A review by Dudley and others [74] presents several examples of saltcedar invasion in areas not severely altered by human activities:
State | Location |
AZ | middle Gila river |
tributary streams at Lake Mead National Recreation Area | |
CA | Mojave River at Afton Canyon |
Coyote Creek in Anza-Borrego State Park | |
CO | San Miguel River |
TX | Brazos River |
UT | Colorado River in Canyonlands National Park |
NV | Virgin River |
Along the relatively undisturbed Escalante River in southeastern Utah, Irvine and West [134] found saltcedar only in sections of the canyon where large boulders provided protection from the full force of floods, or on medium and high terraces susceptible to only occasional flooding. Saltcedar will likely increase in density on these sites if flood frequency and severity and average river flow are diminished by impoundments [134].
Saltcedar often dominates sites downstream from large storage dams, presumably because it competes better in areas with placid waters and not so well under conditions of frequent or severe flooding [52,83,239]. Saltcedar occurrence greatly increased as flood frequency and severity, and average river flow diminished on dammed waterways [120]. It may be unable to germinate or establish on terraces more than a few meters from the water edge (e.g. [2]), but can persist on these sites once established. Along the San Pedro River in Arizona, young saltcedar communities occurred in the lowest bottomlands while older communities were above the lowest bottomlands, often in striated patterns which represented former streamflow patterns [178,235]. Habitat along newly-formed bars and cut banks of terraces in canyon-bound river reaches apparently remain too prone to scour for establishment of many species, including saltcedar. Absence of saltcedar in canyon-bound segments of southwestern streams may be due to the erosive nature of such habitats [120,167].
Water availability: Saltcedar is less sensitive to changes in ground water availability than native riparian trees with which it is commonly associated. Greater tolerance of water stress can lead to saltcedar dominance on relatively dry riparian sites [127,211,226]. Saltcedar showed no change in water availability, leaf gas exchange, or canopy dieback with increasing groundwater depth at either the dam-regulated or the free-flowing rivers studied in Arizona. Both Fremont cottonwood and Goodding willow experienced decreased water availability and leaf gas exchange, and increased canopy dieback with increasing groundwater depth at these sites. This suggests a competitive advantage for saltcedar with increasing depth to ground water. Leaf gas exchange was lower and dieback was greater for saltcedar at the free-flowing river, but there was no saltcedar mortality at either river [127]. Similarly, in the middle basin of the San Pedro River, saltcedar dominates only drier sites where surface and ground water conditions no longer support cottonwood-willow forests [226]. Following a groundwater decline of 3.7 feet (1.1 m), 92 to 100% of Fremont cottonwood and Goodding willow saplings died, while 0 to 13% of saltcedar saplings died. At sites with perennial or near-perennial streamflow, saltcedar is codominant with Fremont cottonwood. However, saltcedar has been declining in importance at these sites, perhaps due to recent occurrence of conditions that favor cottonwood establishment (e.g. frequent winter flooding, high rates of stream flow during spring, exclusion of livestock). In the upper basin of the San Pedro River, saltcedar has increased in relative abundance at sites that show evidence of groundwater decline. Saltcedar is relatively sparse in the upper basin, perhaps due to the combination of cooler temperatures and occurrence of perennial or near-perennial stream flows in most areas [226].
Establishment of tamarisk is highly dependent on surface moisture conditions, and the height and fluctuations of the water table. Tamarisk requires moist soils for germination and seedling establishment and usually establishes on sites where surface soils are moist in the spring and early summer, and subsoils remain saturated throughout the growing season [107]. Saltcedar does not develop rapidly when the water table is less than 3 feet (~1 m). In New Mexico, areas flooded during the growing season have a higher density of tamarisk than areas which have an early spring flood and a limited supply of underground water throughout the growing season [47]. Once established, tamarisk can survive periods of both inundation and drought [220]. Inundation frequently occurs for long periods of time (70-90 days) in areas where saltcedar occurs [35]. Mature saltcedar plants survived complete submergence for as long as 70 days, and under partial submergence plants survived up to 98 days [254]. Saltcedar can survive almost indefinitely in the absence of surface saturation of the soil [35,83,249]. During complete drawdowns in marshes of the Great Salt Lake, salinity increases dramatically and inhibits germination of most species, with the exception of saltcedar, saltgrass, and saltmarsh bulrush (Scirpus robustus). Drawdowns of longer than 1 season appear to promote establishment of saltcedar [210].
Soils: Saltcedar is able to tolerate wide variations in soil and mineral types [36]. Saltcedar can survive in salinities exceeding 50,000 ppm [220]. Tamarisk is well adapted to the saline and alkaline soils of the Great Basin [171,249]; the saline meadows around Utah Lake [36]; and the saline soils and open salt flats in the Great Salt Plains in Oklahoma and Kansas [245].
Increased soil salinity and decreased soil moisture may favor saltcedar over native and other nonnative riparian species [23,30,151,248]. In a comparison of sites infested and not infested with saltcedar and Russian-olive at Utah Lake, Carman and Brotherson [49] found that saltcedar usually occurs on soils with higher soluble salt concentrations than Russian-olive (700-15,000 ppm compared to 100-3500 ppm for Russian-olive). Two greenhouse experiments compared effects of salt stress and water stress on saltcedar and 4 native species collected from the lower Colorado River, Mexico. At the control salinity level, Fremont cottonwood, Goodding willow, and mule's fat were able to extract water from the soil equal to that of saltcedar and arrowweed. Yet, at elevated salinity levels saltcedar and arrowweed exhibited a superior water-use ability in dry soils. Under flooded conditions all native riparian species outperformed saltcedar [248].
Increased soil salinity and decreased soil moisture are now typical characteristics of many western river systems with altered hydrology caused by human activities [23,30]. A review by Briggs and Cornelius [30] reports US EPA estimates of increased salinity concentrations in the Colorado River between 1944 and 1988 of 3%, 37%, and 12% caused by out-of-basin exports, irrigation, and reservoir evaporation, respectively. Without occasional overbank flooding to wash salts from streambanks, native species are unable to germinate and establish, and saltcedar maintains dominance [29,45,265]. Saltcedar has replaced many native species partly because it is better adapted to conditions that are a result of artificial flow regimes created by impoundment [29,150,254].
Changes in nutrient concentrations in surface waters are also brought about by the introduction of treated municipal effluent. Marler and others [158] investigated the growth response of Fremont cottonwood, Goodding willow, and saltcedar in laboratory experiments and found that all 3 species responded positively to increased levels of ammonium, nitrate and phosphate. The authors note this positive response is characteristic of species adapted to episodic disturbances, such as floods, where the ability to rapidly acquire available resources may ensure seedling survival to the next growing season. Saltcedar showed an increase in stem number, shoot biomass and total biomass at these higher nutrient concentrations while natives did not, suggesting that at these concentrations, saltcedar establishment may be favored. Concentrations below these levels may provide the greatest potential for re-establishment of cottonwood and willow [158].