More info for the terms: adventitious, cactus, cover, density, fire frequency, fire management, frequency, invasive species, leaf area index (LAI), litter, marsh, natural, nonnative species, phenology, prescribed fire, presence, restoration, root crown, severity, shrub, shrubs, tree, wildfire, xeric
Impacts: Tamarisk is one of the most widely distributed and troublesome nonnative, invasive plants along water courses in the southwestern United States [143]. Saltcedar reduces recreational usage of parks, national wildlife refuges, and other riparian areas for camping, hunting and fishing, boating, bird watching and wildlife photography [74,106,122].
There are many environmental changes associated with tamarisk presence and proliferation in southwestern riparian areas, and there are debates over whether tamarisk is a consequence [4,84,144] or a cause [154] of these changes. River impoundment, river diversions, groundwater pumping, agriculture, livestock grazing, and other human activities have altered flow regimes and natural channel dynamics in such a way that the regeneration of many native riparian plants has been reduced along the major drainages in the Southwest [30,70,84,144]. Cottonwood communities along the Colorado River, for example, have decreased from over 5,000 acres (2,000 ha) in the 1600s to less than 500 acres (200 ha) in 1998. Additionally, water in the lower Colorado River has become progressively more saline due to a variety of human-related activities, such as out-of-basin exports, irrigation, and reservoir evaporation [30]. A study by Everitt [84] indicates that spread of tamarisk on the central Rio Grande was opportunistic, driven by a chance coincidence of cultural, economic, and hydrogeomorphic events, following different paths on different reaches. He notes that changes in both the native vegetation and the physical environment were well underway by the time tamarisk became widespread [84].
Saltcedar has replaced many native species partly because it is better adapted to the artificial flow regimes and saline conditions created by river impoundment and diversion [29,32,83,223,228,254]. Howe and Knopf [129] suggest that the combination of paucity of cottonwood regeneration over the last 30 years, rapid colonization during this century by Russian-olive and saltcedar, and current river channel management practices will lead to domination of the Rio Grande riparian woodland by nonnative shrubs over the next 50 to 100 years [129].
Regardless of cause or effect, tamarisk is usually associated with changes in geomorphology, hydrology, soil salinity, FIRE REGIMES, plant community composition, and native wildlife density and diversity [156]. It is also a matter of concern that tamarisk has been found in undisturbed areas such as high elevation streams [74].
Geomorphology: Several literature reviews indicate that tamarisk traps and stabilizes alluvial sediments, reducing the width, depth, and water-holding capacity of river channels and increasing the frequency and severity of overbank flooding [74,107,156,266]. Examples of this occur on the Salt and Gila rivers near Phoenix, Arizona [102], and on the Brazos River in Texas, where this trend has continued over 40 years and has reduced the river's width by up to 71% in some places [35]. It has also been suggested that the dense roots and rhizomes of saltcedar rechannel stream flow and extend riparian zones as stream flow becomes more shallow and spread out [197].
A review by Cooper and others [59] suggests that tamarisk stems change the landscape properties of gravel and cobble islands and bars, as well as those of adjacent channels, by decreasing near-bed flow velocities and increasing the sheer stress required to remobilize the channel bed, while woody roots increase bed cohesiveness (resistance to mobilization). Processes of vertical sediment accretion, bar enlargement and subsequent channel narrowing are evident in both regulated and unregulated study areas along the Green and Yampa rivers [59].
Everitt [84] observes that the tamarisk population explosion in the central Rio Grande in the 1930s came 15 years after large-scale regulation and depletion of flow and 10 years after channel-narrowing was well underway. In the Presidio Valley, between 1935 and 1942, tamarisk occupied a narrow fringe of riverbank, made available by the narrowing channel. These initial pioneers had reached maturity by 1942, when the only overbank flood in a decade spread their seeds across the valley to occupy farmland cleared of native vegetation and point bars and oxbows generated by channel migration. He states there is no evidence that the change in the species of riverbank vegetation had an effect on channel width, flood stage, or the process of channel narrowing [84].
Interactions between riparian vegetation and fluvial form and process in bedrock canyon systems are different from those of large perennial rivers in broad valleys. Hinchman-Birkeland [25] indicates that before tamarisk can become a significant geomorphic agent, it must have a period of low water discharge in which to establish strong root systems that can withstand flood flows. Before this critical mass of plant root density is reached, the vegetation is susceptible to removal by flood events. However, after this density is achieved, plants are capable of stabilizing sediment and affecting channel dimensions. Whether riparian plants ever achieve this density depends on frequency and severity of flood events and riparian plant community dynamics. A study along 2 reaches of the lower Little Colorado River Canyon in Arizona suggests that vegetation there has not yet reached this critical density, although tamarisk does appear to induce small-scale sedimentation on the order of individual plants or stands of plants in the perennial reach. This research offers only a preliminary basis for assessing biogeomorphic relationships on confined rivers, and indicates that vegetation patterns respond to, rather than influence, sandbar form in this canyon riparian system [25].
Hydrology: Several reviews and studies suggest that tamarisk has high transpiration rates and that tamarisk stands use more water than native vegetation, thus drawing down water tables, desiccating floodplains, and lowering flow rates of waterways [35,45,107,121,156,188,211,266]. Dense infestations have dried up springs and pools in California and New Mexico, eliminating habitat for fish and other animals [19,75,145,188]. A study in central Utah, bordering Utah Lake, suggests the longer a community is occupied by saltcedar, the more xeric the habitat becomes [34].
A review by Smith and others [212] suggests that some of the early evapotranspiration estimates for tamarisk may be suspect because they were conducted in open areas, rather than in characteristic dense vegetation, leading to potentially large overestimates due to advective water loss. Tamarisk has leaf-level transpiration rates that are comparable to native species (e.g. [10,97]), whereas sap-flow rates per unit sapwood area are higher than in natives, suggesting that tamarisk maintains higher leaf area than natives, probably due to its greater water stress tolerance. Tamarisk-dominated stands can have extremely high evapotranspiration rates when water tables are high but not necessarily when water tables are low or under droughty conditions [68,211,213]. Results presented by Sala and others [198] indicate that, at least under moderate to high water tables, important variables controlling stand water use by floodplain phreatophytes include stand density and leaf area index (LAI). Because tamarisk stands tend to extend beyond the boundaries of native phreatophytes and to develop higher LAI, water use by tamarisk on a regional scale might be substantially higher than for other riparian species [198]. Any attempt to characterize evapotranspiration of full stands of tamarisk will require a detailed spatial assessment of stand density and an evaluation of water availability relative to atmospheric water demand over time [68].
Saltcedar is able to maintain high leaf gas exchange rates under extremely hot, dry conditions (high vapor pressure deficit, high temperature and low soil water availability) relative to native species [126]. Saltcedar's ability to outcompete native plants in riparian ecosystems of the Southwest that are subject to large interannual fluctuations in water availability can be attributed to the following characteristics: high leaf gas exchange rates, growth when water is abundant, drought tolerance (resistance to cavitation), and maintenance of a viable canopy under dry conditions [67,118,127,170,181]. When riparian species on regulated rivers are exposed to seasonal water stress due to depression of floodplain water tables and elimination of annual floods, there is likely to be a community shift toward more stress-tolerant taxa such as tamarisk [211].
Soil salinity: Tamarisk tends to be more salt tolerant than many of it's associated native species [45,97,204,248]. Increased salinity inhibits growth and germination of many native riparian species [4,107]. This confers a competitive advantage on tamarisk as riparian soils become more saline. Tamarisk species are implicated in the salinization of riparian soils via the deposition of salts excreted by their leaves [4,43,44,107,156,171,204,251]. Again, cause and effect are obscured as increased salinization of waterways, resulting from a number of human-related activities [30], is 1 source of the salts that tamarisk then brings to the surface. Additionally, in the absence of annual flooding to wash away those salts, soil salinity continues to increase [9,97,248,251]. Anderson [9] provides evidence from an area where soil salinity was similar under cottonwood and saltcedar and says that accumulation of salt in upper soil horizons is due almost entirely to the absence of flooding.
FIRE REGIMES: Changes in the nature of disturbance from fire (frequency, intensity, and severity) have been affected both by tamarisk invasion and by other changes in the invaded communities [44]. Fire frequency and fire behavior in tamarisk-invaded communities are thought to be different than in uninvaded communities [42,44,107,231,242]. Additionally, reduced flooding in riparian communities commonly results in excessive accumulations of debris, which in turn increase the frequency, intensity, and severity of fires. This has commonly been blamed on saltcedar, although saltcedar produces no more debris than cottonwood or willow. In the absence of flooding to remove debris, however, accumulation of this material increases to levels that may have a profound effect on the ecology of the system (see Fire Ecology for a discussion of the causes and consequences of fire in these systems) [9].
Plant communities: Many sites along Southwestern river systems are characterized by saltcedar communities with halophytic, fire-tolerant shrubs (e.g. big saltbrush and arrowweed) as codominants, with only senescent individuals of the historically dominant cottonwood and willow remaining. It has been suggested these plants are nearing localized extinction on many riverine systems of the desert Southwest [45,81,212,231]. Causes of these native population declines include invasion by tamarisk, although changes in flow regimes are also implicated.
Several characteristics of tamarisk give it a competitive advantage in the communities it invades, including high seed production and viability, rapid germination and growth, vegetative reproduction, drought and salt tolerance. Under conditions imposed by river impoundment (e.g. increased salinity, reduced flooding, and water table declines), many native plant species cannot compete with tamarisk and may be displaced when it invades [60,107,125]. In some areas, such as desert palm groves in California, the effect of tamarisk dominance on water supplies can be dramatic and may affect the ability of some plants to survive [218]. On many sites (upland ponds, streams, and washes may be exceptions) native plants are lost because the sites have been so altered by human activities that they are, at best, only marginally suitable for natives. Saltcedar replaces some of the dominant native plant species that can no longer survive under these current conditions [9,84]. Additional evidence of this comes from a large number of revegetation projects where native species fail to establish [4,29].
Elimination of the annual flood pulse in the middle Rio Grande Valley has resulted in a dramatic reduction in germination and establishment of native cottonwoods and willows [84,129,169]. The extensive riparian forest along the middle Rio Grande in central New Mexico is largely an ecological legacy of past flooding, and these stands appear to be rapidly senescing. Further, because of flood control, no new stands of cottonwood are being established [169]. This stabilized river flow appears to instead favor establishment of salt cedar and Russian-olive [47,169]. Under some conditions, riparian areas may be converted to an impenetrable saltcedar thicket in less than a decade [107].
Depletion of water, a deeper, more narrow channel stabilized by reservoir releases, and invasion of tamarisk have accompanied rapid loss of cottonwoods along the Arkansas River in eastern Colorado (loss of 31% in 31 years). Losses are less along the South Platte River (9%) where only a few scattered tamarisk are present. Tamarisk invasion along the Arkansas River has been compounded by frequent wildfire within the floodplain in recent decades. These fires are fueled by dense stands of tamarisk and damage or kill many cottonwoods, including whole stands [214].
Stromberg [227] compared the functional role of saltcedar to that of Fremont cottonwood along the San Pedro River in southern Arizona, by comparing 30 soil, geomorphological, and vegetation structural traits. Saltcedar was functionally equivalent to Fremont cottonwood for about half of those traits considered as indicators of riparian ecosystem function. According to Stromberg [227], the functional role of saltcedar is context-specific and variable among rivers. The author urges caution before undertaking regional control measures for saltcedar, especially on the ephemeral or intermittent reaches of the San Pedro River, where saltcedar can serve as an ecologically important functional analog to displaced native species that are no longer able to survive on these sites [227]. Where restoration of native vegetation is impossible or impractical, saltcedar may actually be a key element of the flora, standing in for natives and providing habitat for at least some native plants and animals [9].
Wildlife: The impact of tamarisk invasions upon wildlife species is variable, site specific, and often debated. (Also see Importance to Livestock and Wildlife.) Anecdotal evidence and observations by managers suggest that several species may be affected by tamarisk invasion, although in some cases it is unclear whether impacts are caused by tamarisk itself, or by changes in the ecosystem as a whole.
"Although saltcedar provides habitat and nest sites for some wildlife (e.g. white-winged dove), most authors have concluded that it has little value to most native amphibians, reptiles, birds, and mammals" [154]. Another review suggests that in some areas, tamarisk has reduced or eliminated water supplies for bighorn sheep, pupfish, and salamanders [218]. Saltcedar invasions may have negative impacts on threatened and endangered species such as Amargosa pupfish, warm springs pupfish, and speckled dace in Ash Meadows National Wildlife Refuge, Nevada; desert tortoise, and Nelson bighorn sheep, in Lake Mead National Recreation Area, Nevada [51,194]; Attwater's prairie chicken at Galveston Bay, Texas [194]; Moapa dace in Moapa, Nevada [193]; and about 50 species in New Mexico preserves [193]. A review by Dudley and others [74] indicates that a fire in the Salton Sea National Wildlife Refuge that was fueled partly by saltcedar diminished cattail-bulrush (Typha spp.-Scirpus spp.) habitat for the endangered Yuma clapper rail. In Big Bend National Park, Texas, populations of rodents appear to be affected by tamarisk, some positively, others negatively [26]. 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 [136]. The restriction of water flow in Queen Creek by Whitlow Ranch Dam in Arizona created a 37 acre (15 ha) riparian island upstream, behind the dam, that is dominated by Goodding willow and saltcedar. A survey of herpetofaunas in the area indicated many of the locally expected riparian species were absent, although it is unclear whether biogeographical considerations and flooding patterns are responsible, or if structural and physical conditions of the new habitat are responsible [236].
It is important to note that while there are discrepancies among studies in the extent to which migrant birds use habitats dominated by nonnative species such as tamarisk, several studies have found that tamarisk is used to some degree by several species (e.g. [5,56,87,152,177,264]), and that discrepancies may be related to local differences among study sites or to differences in sampling techniques [264].
A review by Anderson [9] suggests that, while saltcedar thickets support somewhat impoverished animal communities in low-altitude areas, this is less true at higher altitudes. Additionally, saltcedar habitats east of the Colorado River support larger numbers of wildlife species. Along the Pecos River in New Mexico and the Rio Grande and in west Texas, the difference between species richness of typical native riparian birds in saltcedar and native riparian habitats were negligible and differences in densities were minor. Also, both biomass and diversity of insects in saltcedar stands are comparable to those in cottonwood and willows [9].
Studies by Bailey and others [13] indicate that saltcedar effects on leaf litter quality were related to a 2-fold decrease in stream macroinvertebrate richness and a 4-fold decrease in overall macroinvertebrate abundance, as compared with Fremont cottonwood litter, and may also affect higher trophic levels in Wet Beaver Creek, Arizona. Saltcedar leaves had slower colonization by invertebrates during the first 84 days of immersion compared to Fremont cottonwood and sandbar willow leaves, suggesting that leaves from saltcedar contain tannins and/or phenolics that deter invertebrates [182].
Anderson [9] argues that while wildlife diversity and abundance in saltcedar communities along the lower Colorado River are generally lower than in areas dominated by native cottonwood and willow, this is an appropriate standard for comparison only if native tree species can be re-established after the removal of saltcedar. He suggests that after removal of saltcedar in these areas only occasionally has it been possible to return areas to native species and that far more often these areas remain bare or are reinvaded either by saltcedar or by vegetation dominated by arrowweed which has even lower value for wildlife [6].
Control: Tamarisk has been present in North America for almost 200 years, and has been invasive for nearly a century. Efforts to reduce its numbers and control its spread have been ongoing for decades, and it still occupies vast acreages and has substantial impacts in the areas it invades. Once tamarisk is well established it is very difficult and expensive to control, as any stress imposed by control methods (e.g. fire, herbicides, and cutting) increases flowering and seed production; and the entire root system must be killed in order to prevent sprouting. Monitoring, prevention, early detection and local eradication remain the most effective approach to controlling tamarisk [51,85,122,156,192,193,194].
Early detection and control of tamarisk are critical, as it achieves dominance rapidly under favorable conditions [156]. At Afton Canyon in the lower Mojave River in southern California, complete low level, infrared aerial photography of the canyon was completed to assist in determining removal area priorities, based on tamarisk seed sources and wildlife habitat value information [155]. The best time to locate saltcedar is in spring and summer when the flowers are conspicuous, and the best season for removal is late fall or winter when sap is flowing downwards and seed dispersal is not enhanced by removal activities [232]. Care must be taken to avoid soil damage during control efforts, by avoiding high water periods and being careful to avoid compaction on fine-textured soils [107]. When removing aerial plant portions, tamarisk stems should be left on the surface of the ground and never buried in moist soil [93]. When clearing riparian areas of vegetation, the possibilities of channel cutting and wind erosion must be considered [122].
Removing tamarisk, regardless of the method employed, must be followed with the development of an ecologically healthy plant community that is weed resistant and meets other land-use objectives such as wildlife habitat or recreation [9]. The southwestern willow flycatcher recovery plan provides an example of invasive species management with wildlife management as the primary objective [244].
Only a few examples exist where tamarisk was successfully removed and native vegetation re-established. These occur on small invasion sites (2.5-50 acres (1-20 ha)) with more or less discrete borders, and the longest time frame for success is about 10 years after control treatments [18]. In some cases where tamarisk has invaded, it might be worth concentrating efforts to remove tamarisk and restore refugia for rare species in favorable situations, but doing this on a landscape scale may not be possible. On this larger scale, considering alternatives that include saltcedar in surrogate systems may be more appropriate [9]. Barrows [18] urges larger scale removal of tamarisk (e.g. with biocontrol) due to the prolific nature and large-scale seed dispersal of the plant. He also argues that "there is nothing to suggest that these successes are scale-dependent anomalies. In fact, there is every reason to believe that they are indicative of what could be accomplished on a larger scale," although he cites no evidence for this conjecture [18].
Many of the impacts attributed to tamarisk invasion are also causal factors that may have allowed tamarisk to establish and persist [9,18,84]. Hobbs and Humphries [114] advocate an integrated approach to the management of plant invasions that includes "a focus on the invaded system and its management, rather than on the invader" and "identification of the causal factors enhancing ecosystem invasibility" as an effective approach to controlling invasive species. This type of "ecological control" may involve manipulation of factors such as fire disturbance and water and sediment discharges in a way that provides a competitive edge to target native species over invasive species, with an emphasis on removing the ecological stressors that may be underlying the causes of invasion, rather than on direct control of invasive species [114].
In applying this approach to management of tamarisk, Levine and Stromberg [150] examine several descriptive field studies and controlled experiments that contrast response of tamarisk and native riparian trees and shrubs to particular environmental factors. These studies provide the basis for identifying environmental factors that could be manipulated to restore conditions under which the natives are most competitive. There is evidence, for example, that cottonwood trees have increased in abundance at tamarisk-dominated sites in response to appropriately timed flood pulses, high ground water levels and soil moisture, and exclusion of livestock grazing [149,150,203,207,224,226].
Flooding/Altered hydrology/Manipulation of water levels: Several authors have recommended manipulating dam releases and "naturalizing" flow regimes to favor native species over nonnatives [29,127,203,224]. Periodic dam releases to cause "pulse flooding" or "overbank flooding" can have several benefits. Flooding helps to eliminate large accumulations of litter in riparian forests, thus reducing the threat of fires in both native and nonnative dominated forests [81]. Flooding washes accumulated salts from the banks and deposits and moistens bare mineral soil. Timing flood flows or dam releases to coincide with native seed dispersal can also promote natural regeneration of these species [30,120,149,150,198,206,211,248,265].
Timing flood flows later in the season and subjecting saltcedar to the scouring effects of floods and prolonged inundation may increase saltcedar mortality [29,45,224,265], although buried root crowns or aboveground portions of branches and smaller stems will often sprout [107]. Saltcedar seedlings (around 5 weeks old) are more susceptible to summer flooding than are older plants [216]. Prolonged inundation (1 to 3 years) can kill most saltcedar [115,156] and French tamarisk plants [261]. Some studies suggest that saltcedar may be more tolerant of prolonged inundation than cottonwood and willow trees [216,217,254], especially in the fall [95]. Because of the high mortality of cottonwood in response to complete submergence, flooding saltcedar seedlings may not be desirable when submergence of cottonwood seedlings will also occur [217].
In order to design models or prescriptions for flood releases, research is needed to examine inundation, sedimentation, scour effects, drought tolerance, seed production phenology, root growth rates, and establishment requirements for each of the species in a particular system [125,150,203]. Research by Cooper and others [59] suggests that flow prescriptions to restore riparian ecosystems must also consider both the large and small scale geomorphic settings to be affected, as well as the multiple pathways of establishment for both native and undesirable nonnative species. Then the seasonal timing, magnitude, and interannual frequency of flows can be adjusted to match the desired outcomes [59].
For example, cottonwood and willow are favored if germination sites are moistened only during spring, but become dry during summer when tamarisk continue to disperse seeds [225]. Levine and Stromberg [150] recommend, therefore, releasing winter or spring regeneration floods and limiting duration of summer flooding. Later, summer floods of long duration may increase inundation-related mortality of saltcedar seedlings, and those of large magnitude but short duration can scour or bury saltcedar seedlings [150].
Several studies have examined hydrograph components (e.g. timing and magnitude of flood peaks, rate of decline of recession, and magnitude of base flows) and their influence on establishment and survival of saltcedar and native tree species in areas such as the Bill Williams River in Arizona [203], the middle Rio Grande floodplain [238], Big Horn Lake, Wyoming [130], the upper Green River in Utah [60], the Hassayampa River in Arizona [225], and the Yampa and Green rivers [59]. This information is critical in efforts to prescribe reservoir releases designed to promote establishment of native riparian vegetation and to deter the spread of nonnative species.
Curtailment or modification of human activities that lower groundwater beyond the rooting depth of desired native riparian tree species may be worthwhile, because deep groundwater has greater negative physiological impacts on natives than on saltcedar [127]. It is also important to know the range of soil moisture over which native species have a competitive edge in order to manage for soil moisture levels and groundwater depths that favor growth and survivorship of native species during establishment periods. To favor native tree species on cattle-grazed rivers, recruitment zones should be protected year-round from livestock grazing during at least 2 growing seasons to allow seedlings to grow above browse height [224].
Prevention: The most efficient and effective method of managing invasive species is to prevent their invasion and spread [205]. For example, costs for controlling and/or eradicating tamarisk infestations at Ash Meadows National Wildlife Refuge and Lake Mead National Recreation Area exceeded the costs of preventative maintenance by up to a factor of 100 [51].
While the natural flood disturbance regime seems to promote native species and discourage tamarisk (see above), preservation of natural conditions in riparian areas in the Southwest is rarely a factor, as none currently exist, except for mountain reaches in Arizona and New Mexico where some canyons have retained natural flood regimes. The desirability of preserving these areas is great [122].
Other factors managers suggest as discouraging tamarisk invasion include eradication upstream [192], biological control, agency coordination [193], and fencing out cattle [194]. Efforts should be made to prevent site disturbances such as fire, increased soil salinity, ground disturbance [156,193,194], livestock grazing, dams, channelization, sedimentation [193,194], and flooding when tamarisk is dispersing seed [192]. When controlling other invasive species, managers should be aware of the potential for tamarisk invasion. For example, drainage of areas to control cattail in Utah led to establishment of tamarisk [176].
Monitoring is essential to prevent establishment both before and after any control effort, as some saltcedar is capable of resprouting following treatment. In addition, tamarisk seedlings will continue to establish as long as saltcedar infestations persist upwind or upstream of the target area [156] and conditions for germination exist. If tamarisk seed sources are out of the manager's control or if hydrologic regimes are altered, an extended commitment of resources will be needed [194]. Saltcedar can be readily identified on conventional color video imagery in late November when its foliage turns a yellow-orange to orange-brown color prior to leaf drop. The integration of GPS with video imagery permits latitude/longitude coordinates of infestations to be recorded on each image; these coordinates can then be entered into a GIS to map saltcedar populations. This was done along the Colorado River in southwestern Arizona, the Rio Grande River in extreme west Texas, and the Pecos River in west-central Texas [85].
Integrated management: Once established in large stands, tamarisk can rarely be controlled or eradicated with a single method, and many researchers and managers recommend combining physical, biological, chemical, and cultural control methods in some fashion. Removing tamarisk, regardless of the method employed, is useless without also developing an ecologically healthy plant community that is weed resistant and meets other land-use objectives such as wildlife habitat or recreation. Some researchers argue that current conditions of invaded riparian areas in the Southwest are no longer suitable for supporting native species, and removal of tamarisk will lead to dominance by other, less desirable species such as arrowweed [9]. In areas where establishment of a desirable plant community is possible (e.g. small-scale infestations, or areas where historic flow regimes can be mimicked to support native species), removal of tamarisk is best achieved by combining control methods.
At the Bosque del Apache National Wildlife Refuge, both mechanical (bulldozing and rootplowing) and chemical (aerial herbicide application) methods are commonly employed for saltcedar control. Sprenger and others [216] compared these methods in combination with late summer flood treatments, and found mechanical treatment provided better control of saltcedar than chemical treatment and had better cottonwood recruitment. The flood treatment was effective in killing nearly 100% of saltcedar seedlings that were less than 5 weeks old; however, 10-week-old saltcedar were not as susceptible to sustained flooding. Cottonwoods which were totally submerged during the 30 day flood period also suffered high rates of mortality. Mortality of partially submerged cottonwood and saltcedar was similar to unflooded controls [216].
Taylor and McDaniel [237] present integrated management approaches including combinations of herbicide, burning, mechanical control treatment, and revegetation with native species and report success in controlling saltcedar and improving habitat for several species of birds, small mammals, reptiles, and amphibians in the Bosque del Apache National Wildlife Refuge. Saltcedar thickets are first removed using a combination of mechanical, chemical and prescribed fire techniques. The methods used depend on initial plant density. Mechanical control involves removal of aboveground stems followed by removal of underground root crown portions of the plants. Large scale herbicide and/or burning of saltcedar includes aerial application of glyphosate and imazapyr mixed with water and surfactant. Herbicide spraying is done in late flower (September). Prescribed burning is done in September, 2 to 3 years after herbicide application, to remove dead standing saltcedar stems [162]. They found that aerial applications of imazapyr were no more effective than bulldozing as an initial treatment in integrated management approaches [237]. Follow-up control is generally needed for at least a 2-year period to treat root sprouts either mechanically or chemically [162].
Taylor and McDaniel [162] followed saltcedar removal by revegetation with Fremont cottonwood, black willow (Salix nigra), and sandbar willow. Natural regeneration using irrigation water to flood areas using flood schedules to mimic the historic Rio Grande hydrograph, appears to be a more cost effective and preferable method of revegetation than planting and has resulted in a vegetative community dominated by robust native species and occasional saltcedar. By these methods, saltcedar plant densities have been reduced from pretreatment averages of about 7000 plants/ha to about 50 plants/ha 4 to 6 years after treatments. Subsequent flooding for the maintenance of riparian communities is repeated at 5 to 7 year intervals [162].
In a 25 acre (10 ha) wetland in the Coachella Valley Preserve in California that was heavily infested with saltcedar, a control project was initiated in 1986 [17]. Native dominants in the area include California palm, sandbar willow, Fremont cottonwood, common reed, and honey and screwbean mesquite. A 7 acre (3 ha) area with greater than 95% coverage of tamarisk was scraped with a bulldozer, since few natives were present. Where native species were abundant, hand cutting of individual saltcedar trees facilitated native plant retention. Each tamarisk tree was cut as near to the ground as was feasible with a chainsaw or pruning shears and immediately sprayed with herbicide from hand-held or back-pack sprayers. The debris was hauled to centralized piles in inconspicuous places. November through January was the most effective time to achieve first-time kills of tamarisk, probably because the plants are entering dormancy at that time and translocating resources into their roots. The herbicide was most effective when applied immediately after cutting. Waiting more than just a few minutes seemed to increase the likelihood of resprouts (also see [110]). Resprouts were treated with triclopyr. In 5 years, all of the tamarisk was eradicated from the palm oases and wetlands on the preserve. Because the area is surrounded by tamarisk, seeds blow in and perpetual vigilance and maintenance is required to pull seedlings before they become established; requiring about 1 or 2 person-weeks. Natural and artificial seeding during relatively wet springs in 1991 and 1992 resulted in some establishment of native species (except mesquite). Natives established more rapidly on hand-cleared sites than on bulldozed sites [17]. Nine years later, there was almost no sign that tamarisk was once dominant. Native vegetation has returned to acceptable levels and the piles of cut tamarisk, once 10-12 feet (3-4 m) high, are barely noticeable, having degraded to 4 foot (~1 m) piles. At Coachella Valley Preserve, removing tamarisk restored natural habitats and natural processes (such as the water flow) which are vitally important to the survival of many native plants and animals there [159]. The natural flood regime and native species are still intact in this preserve. On sites lacking native species, more extreme efforts are needed [17].
The following table presents other success stories employing cutting and herbicide application on tamarisk:
State Location References AZ Wupatki National Monument [53] Aravaipa [192] Hassayampa [194] Agua Fria River [232] CA Eagle Borax Spring in Death Valley [174,175] Death Valley National Monument [196] Dos Palmas Oasis near the Salton Sea Afton Canyon in the lower Mojave River (used in combination with prescribed burning) [155] CO San Miguel [193] KS Kansas preserves [194] NE Platte reserves [193] NV Ash Meadows preserves UT Utah preserves [194] Zion National Park [110]
Cutting and herbicide application was less successful (21% kill rate) at Petrified Forest National Park, Arizona [28]. Given both past and present anthropogenic degradation on the Agua Fria River, active revegetation consisting of pole planting of Fremont cottonwood and various willow species was pursued. Each pole was hand-irrigated weekly with about 10 gallons (40 liters) of water through the months of June, July, and August, resulting in a 65% survival rate [232].
Imazapyr is popular for the cut stump method, but care must be taken as it can be highly mobile and persistent, and can affect a wide range of plants. Additionally, recent studies report that imazapyr can leak out of the roots of treated plants, and adversely affect surrounding native vegetation [241].
Physical/mechanical: Saltcedar is difficult to kill using only mechanical methods (e.g. cutting, mowing, chaining and bulldozing), as it is able to resprout vigorously from the root crown following removal of aerial plant portions [92]. Even when effective, mechanical control methods can be labor intensive and expensive [193,194]. Early response to small invasions can be successful [192], since seedlings and small plants can be uprooted by hand [156,193], and cutting and pulling are most effective in small, discrete areas [194]. Older plants have brittle stems, and are not easily drawn from the ground [36].
Root plowing and cutting are effective ways of clearing heavy infestations initially, but these methods are successful only when combined with follow-up treatments [156,193]. Root plowing is reported to be one of the more successful mechanical control methods for tamarisk, and is most effective when the soil is relatively dry [122]. It is important to pile and/or burn aboveground vegetation to prevent resprouting of stems and shoots [141]. A root plow modified for deep subsurface placement of herbicides effectively controlled saltcedar on the Cimarron River floodplain in Kansas [117]. Because rootplowing tends to kill a large percentage of any grass cover mixed with the saltcedar, it may lead to serious wind erosion [122]. Additionally, any native plants on the site are likely to be killed.
Along the middle Rio Grande floodplain, saltcedar clearing (using a bulldozer with a front-mounted dirt blade, and stacking debris; followed by root plowing, raking, and stacking) in conjunction with peak river flows in late May or early June encouraged recruitment of native riparian plants [238]. Tamarisk plants were bulldozed and totally removed at a campground along Lake Mead, and numerous sprouts were found coming up in the 1st few months from roots that had not been completely removed. Impact from vehicle traffic and camping activities eventually killed all but about 24 plants which were 8 to 12 feet (2-4 m) tall 7 years after initial removal and used for shade [41]. Chaining with a bulldozer was effective for removing tamarisk on Santa Cruz Island, California [194].
Biweekly cutting of saltcedar at 12 inches (30 cm) above the ground did not kill plants. However, when all foliage was removed from the stump at 2-week intervals, 92% of the plants died the 1st season and the remainder died after retreatment the following year [122]. Mowing in August and September each year followed by inundation from October through April has provided some control at Bosque del Apache [141]. Mowing of small plants followed by grazing can reduce cover [122]. At Organ Pipe Cactus National Monument, managers reduce saltcedar by cutting about 12 inches (30 cm) below the surface [166].
Care must be taken when mechanically clearing when the ground is moist because buried plant parts can develop adventitious roots and form new shrubs [93,122].
Fire: See Fire Management Considerations.
Biological: A review by Dudley and others [74] indicates that 3 Eurasian insects are currently being researched as potential biological control agents for saltcedar. One insect, the tamarisk leaf beetle, was approved for release in 6 states (Texas, Colorado, Utah, Wyoming, Nevada, and California). Concerns regarding use of biological control agents on saltcedar include 1) damage to nontarget plants of environmental or economic concern; 2) the ecological or economic benefits of saltcedar itself; 3) rapid and wholesale control of saltcedar leaving an open niche and inadequate time for native vegetation recovery to support wildlife in the interim; and 4) native vegetation can no longer recover or survive in many systems where saltcedar has invaded. Of particular concern is the role that saltcedar plays as nesting habitat for a substantial number of endangered southwestern willow flycatchers, and release of insects was postponed pending analysis of potential impacts on this species [74].
In early August of 1998 several tamarisk leaf beetles were released at 2 sites in California, and 2 sites in Nevada. Additional releases are planned for another site in California and sites in Texas, Colorado, Wyoming and Utah [46]. It is unclear what effects these releases have had on tamarisk populations.
Dudley and others [74] also indicate that few native insects feed more than occasionally or sporadically on saltcedar and cause it little damage. Except for the Apache cicada in the Grand Canyon, the only insect that appears to have control potential is an introduced leafhopper, and this only in confined spaces. This insect may also provide benefits as a food source for several riparian birds, including the willow flycatcher. Four other Eurasian, saltcedar-specific arthropods have also been accidentally introduced, but have caused little or no damage [74].
Cattle may graze large amounts of saltcedar sprout growth. In one study, cattle removed 40% of saltcedar foliage. All plants outside the fence areas were grazed. Within 1 month, new growth was 4 feet (1.2 m) high in the fenced areas, and about 1 foot (0.3 m) high in grazed plots. After the 1st month, utilization was limited to the terminal ends of saltcedar stems, and 2 years later the stand was so dense that cattle would not enter the area [92].
Chemical: Tamarisk can be very difficult to kill with herbicides alone, and may require repeated treatments to be successful. The root systems are more extensively developed near the ground surface, due to repeated scouring and removal of limbs by floods. These roots can send up shoots where none existed at the time of initial treatment [175], and stress caused by herbicide applications can increase flowering and seed production [107,220]. Herbicides commonly used to control saltcedar include imazapyr, triclopyr, and glyphosate [156].
Stevens and Walker [221] present results from a study using various herbicides to control saltcedar of different age classes with applications in various seasons following stem pruning. Chemicals tested include tebuthiuron, glyphosate, 2,4-D and dicamba, 2,4-D, and picloram. On young tamarisk, glyphosate was the least effective at all application dates, and picloram the most effective. Picloram was not equally effective as other chemicals when applied in the fall. Glyphosate was more effective on older plants than on younger plants, however 2,4-D and 2,4-D combined with dicamba were most effective at controlling older plants [221].
Caution must be used when applying herbicides, especially near water. Temple and others [240] found that phytoplankton primary production, midge density, and midge biomass were negatively correlated with tebuthiuron concentration during peak system productivity. Conversely, no trends were observed at any sample date between an omnivorous fish species and herbicide concentration [240].
The efficacy of herbicides is greatly enhanced when combined with other control methods and/or revegetation [105]. Heavy infestations of tamarisk may require stand thinning via prescribed burning or mechanical removal prior to herbicide application. A commonly used and effective treatment is to cut the shrub off near the ground and immediately apply herbicide to the cut stump. Resprouts are then treated with foliar applications of herbicide. This technique usually results in better than a 90% kill rate (e.g. [155]) (see Integrated management for more details) [156].
Based on a number of research/extension field trials in New Mexico from 1987 to 1998, Duncan and McDaniel [75] concluded that imazapyr applied alone or in combination with glyphosate controlled saltcedar to levels of 90% or greater, especially when applied in August or September. Herbicide activity may be reduced as saltcedar height and stem number increases [75]. Foliar applications of herbicides were also deemed successful at Mad Island Marsh, and Galveston Bay, Texas [194].
Winter is the best time for herbicide application in saltcedar, because plants are dormant and not translocating large quantities of water from the roots. Spring is the least desirable time due to the large upward flow of sap and the lack of translocation of the herbicides to the root system [115].
Cultural: Mature saltcedar is highly susceptible to shading [115,220], and does not dominate in cottonwood stands when the cottonwood is left to develop into mature trees. In mature stands of cottonwood, saltcedar grows only in natural openings and along the outer edge of the stand [47,107].
Replacing saltcedar with native species may lessen or delay recolonization [220]. Unfortunately, conditions on sites where saltcedar dominates or has dominated tend to be unfavorable for establishment of native species. While some suggest that these conditions are caused by saltcedar infestation [220,251], others suggest that they are caused by the alteration of historic flow regimes. Dams, diversions, groundwater pumping and development have resulted in sites with reduced soil moisture, increased depth to the water table, and increased the accumulation of salt in the upper soil horizons. For these reasons much of the former riparian zone along major rivers such as the lower Colorado, Pecos, and Rio Grande is no longer suitable for germination and sapling survival of native species such as cottonwood, willow, honey mesquite and screwbean mesquite [4,9,144]. A more practical approach may be to revegetate with salt-tolerant grasses [251]. Anderson [9] says that he is familiar with a few restoration projects on saltcedar dominated sites that have been successful in the short term but none that can claim long-term success (e.g. 20 years).
At Bosque del Apache NWR, more than 15,000 native trees have been planted in previously saltcedar infested areas with good success, where salinity levels are low enough to permit establishment. Black willow will tolerate salinities up to 2,500 ppm, whereas cottonwood must be confined to areas below 2,000 ppm. Plantings established since 1988 have survival rates exceeding 80% and growth after 18 to 24 months exceeding 25 feet (8 m) on some sites [48]. Establishment of species such as Fremont cottonwood and Goodding willow is facilitated by deep tillage (to the water table) prior to planting [8].
Monitoring efforts along the Virgin and Paria rivers and Kanab Creek in northern Arizona indicate that near the water zone, several native species such as willow, seepwillow and cottonwood can compete with tamarisk, and that arrowweed can increase in the drier floodplain rather than tamarisk and willow. When livestock are restricted to winter use and kept out of riparian areas during the growing seasons on a systematic basis, willows and other palatable woody species can grow and increase to their potential extent [131].
