Saltcedar: Ecological Interactions and Potential Effects of Biological Control
Dr. Jack DeLoach, USDA-ARS
Grasslands Research, 808 E. Blockland Rd.
Temple, TX 76502
Saltcedar, Tamarix ramosissima, is one of 54 Old World species of Tamarix native across central Asia. No species of the family Tamaricaceae are native in the United States or in the Western Hemisphere. It was introduced into the western United States soon after 1837 as an ornamental and for streambank stabilization. It had become naturalized by the late 1800's and after 1930 it spread rapidly; by the 1950's it was a weed causing major damage in riparian areas from the central Great Plains to the Pacific and from northern Mexico to Montana. Today, it is still becoming more dense and spreading into new areas. It has displaced native plant communities; it has destroyed wildlife habitat causing many animals to decline in population and some to become endangered; it severely reduces bio-diversity; it lowers water tables causing small streams and springs to dry up; it promotes wildfires, increases soil salinity, increases sedimentation and blockage of stream channels, and reduces recreational and agricultural usage of infested areas. North American wildlife species are unable to utilize the tiny fruits and seeds, its foliage is relatively unpalatable, it provides no cavities, and it produces only two insect species in sufficient numbers to be of much value to wildlife. Many frugivores, granivores, insectivores, and cavity dwellers are uncommon or absent in saltcedar thickets.
The unique physiology and ecology of saltcedar enable it to interact to an extraordinary degree with natural factors and anthropogenic ecosystem modifications to increase its competitive advantage over native plants. It is highly tolerant (but cottonwoods, willows and other native plants are very intolerant) of the altered flood cycles below dams, high soil salinity and low water tables, wildfires, livestock browsing, and conventional weed controls. It actively manipulates some of these factors (fire, salinity, watertable depth) to increase their severity. Native plant species are damaged by usually large guilds of insects and plant pathogens but saltcedar was introduced without most of its many natural enemies in Asia (only one of five accidentally introduced arthropod species is damaging). Many insect species have been collected from saltcedar in the United States, but most are only occasional or sporadic; only one native North American insect species slightly damages it.
Under the current paradigm, that mostly abiotic factors (soil salinity, watertable depth, mud-bar nursery sites) determine saltcedar distribution and abundance; it appears to be intrinsically more aggressive and better adapted to present riparian conditions than are the native plants and therefore it is immutably dominant in riparian systems. We propose a modified paradigm in which saltcedar's dominance is determined by both the abiotic and biotic factors, such as direct competition and synergistic interactions that suppress native plants and by lack of effective natural enemies that could suppress saltcedar. Under this scenario, saltcedar is seen to be easily controllable by the introduction of its host-specific insect herbivores from Asia, which in turn would allow the return of the native vegetation in areas where salinity and depth to water table permit.
Along unregulated small streams and springs, numerous control projects have shown that the native vegetation returns rapidly and abundantly after manual control and that springs and intermittent streams flow again, providing water for wildlife. Along highly regulated rivers, some areas are unsuitable for revegetation by cottonwoods and willows because of high salinity and low water tables but suitable areas total several times the amount currently present; most areas are suitable for mesquite, quailbush and other native plants. Proper site selection and planting methods allow 90% survival of cottonwoods, willows and various shrubs. Willows have regrown naturally in extensive areas after floods and direct competition with saltcedar is the only factor restricting willow growth in some areas. In the lower Colorado River valley near Yuma, many areas now have high water tables because of agricultural irrigation. In the Grand Canyon and some other areas, conditions appear acceptable for cottonwoods and willows except for saltcedar competition.
Biological control is expected to gradually reduce saltcedar stands over a period of 3 to 10 years at individual sites, during which native vegetation is expected to establish concurrently. Control is expected to be sporadic, with some large patches of saltcedar remaining for several years as natural vegetation recovery occurs in other areas, with 15 to 25% of the saltcedar remaining perpetually, as has occurred following successful biological control of several other weeds.
Of the 49 endangered or threatened species that live in saltcedar-infested western riparian ecosystems, (2 mammals, 5 birds, 34 fish, 2 reptiles, 2 amphibians, 1 arthropod and 3 plants), most would be benefited by biological control and none would be adversely affected. Only the southwestern subspecies of the willow flycatcher utilizes saltcedar to any important degree (as nesting habitat) and we examine these relations in detail.
The historic range of the southwestern subspecies of the willow flycatcher (SWWFC), Empidonax traillii extimus, was from southern coastal California to central New Mexico and Transpecos Texas north to southern Colorado, Utah and Nevada. Populations of the SWWFC have declined since at least 1948, concurrently with the saltcedar increase, and it was declared endangered in 1995. The other four subspecies are distributed throughout the US and southern Canada and are not endangered. It seems to require a habitat of moderate to broad riparian thickets of dense foliage, and usually with an upper canopy cover, located near water.
In all areas except at low and intermediate elevations in Arizona, it now nests entirely or almost entirely in its historic native vegetation, primarily willows, boxelder maple, or coastal live oak. It no longer breeds in lower Colorado River below Topock Marsh or on the lower Gila River. These areas now appear to be too hot for survival of the SWWFC eggs, caused by the loss of the native habitat and replacement by saltcedar. At several intermediate elevations in Arizona, the SWWFC nests primarily in saltcedar even though willows may be present. These birds appear to select the densest foliage regardless of species; at one site they nest extensively in dense, native buttonbush. The willows here are mostly large with open canopies or else are too small and too disperse. The areas suitable for acceptably dense willows appear to have been preempted by saltcedars that now are large with dense foliage. Saltcedar appears to provide for part, but not all, of the needs of the SWWFC, while the native vegetation, where sufficiently dense, provides for all its needs. The deficiencies of saltcedar appear to be lack of protection from high temperatures, and possibly also from cowbird parasitism and predation, increased risk of fire, and lack of critical food items such as caterpillars and beetle larvae. Nest parasitism by the brown-headed cowbird devastates populations in many areas. Loss of overwintering habitat in Central America may be a major mortality factor but, to date, nothing is known about this.
We conclude that biological control will reduce the saltcedar thickets gradually over time and sporadically over the area, allowing ample time for recovery of native vegetation, leaving areas where little control occurs, and perpetually leaving 15 to 25% of the saltcedar, so that available habitat for the SWWFC will not be reduced.
We project that the return of native vegetation following biological control will lead to increased populations of the SWWFC as well as other wildlife species, and a reestablishment of healthy SWWFC populations in the lower Colorado and lower Gila rivers.