Tamarisk is one of the most invasive, natural community altering, shrub-trees in the southwestern United States. Estimates of the tamarisk invasion in the southwest include over 600,000 hectares of riparian habitats dominated by this species. Each tamarisk produces 500,000 wind dispersed seeds per year. Once established, tamarisk acts as a facultative halophyte, tolerating salt concentrations up to 15,000 ppm, and secreting salt at 41,000 ppm which is deposited on the soil surface. In addition to increased soil salinity, tamarisk increases fire frequency within the riparian habitats it dominates. The high levels of dead leaves and branches produced by the fast growing tamarisk provide ample fuels for wildfires. After the fires, tamarisk sprouts vigorously, while native riparian trees and shrubs generally do not. The result over time, as a result of both increased soil salinity and fire frequency is a riparian community dominated by tamarisk. Along with the invasive adaptations tamarisk possesses, human alteration of hydrologic regimes (i.e., dams) along streams and rivers has reduced the natural flood processes that willows and cottonwoods thrive under, giving tamarisk one more advantage.
Species affected by the spread of tamarisk include the entire gamut of animals and plants associated with riparian communities. From native amphibians to bighorn sheep, to elf owls to pupfish, to any animal or plant requiring habitat with moist soil or open water for any portion of its life cycle, almost all have been negatively impacted. Nearly the entire bird community along the California shores of the Colorado River has been decimated by habitat loss due in large part to the habitat conversion from willow-cottonwood forests to nearly pure stands of tamarisk. Important desert springs, critical watering holes for desert wildlife, have been sucked dry by tamarisk. Increased soil salinity, as a result of the tamarisk's ability to concentrate salt in its leaves which then form a thick mulch covering the ground, inhibits seeding establishment of native riparian plants and may affect egg viability in amphibians and invertebrates which depend on moist soil for nesting sites. Only doves appear relatively unaffected by tamarisk's invasion.
While most land managers acknowledge the impacts tamarisk has on the riparian flora and fauna, the magnitude of the invasion reduces many to inaction. Some land managers argue that fighting tamarisk is a loosing proposition, so we might as well accept the few values it offers. While biologists have universally found depauperate wildlife populations in tamarisk dominated stands, there are, however, a very few examples of endangered willow flycatchers nesting in tamarisk. Despite its archaic, single species management focus, this finding has caused some to argue for the maintenance of tamarisk within those riparian forests. As a result of these conflicts, there is a ongoing debate as to whether to wage a systematic war on tamarisk.
There have been some dramatic successes in controlling tamarisk. In isolated springs and watersheds, tamarisk has been successfully removed, native vegetation has returned, flowing water has returned to the surface, and native wildlife is thriving. New prospects for biological control offer a possible solution for larger systems; with the new concept of prescribed flooding, those habitats may once again return to a healthy mix of natives. Without a systematic "all out war" on tamarisk, those 500,000 wind dispersed seeds per tree per year will be constantly invading and degrading riparian habitats. The affect of a healthy, seed producing tamarisk plant goes for beyond its canopy and root zone. It extends to any spring, creek, or river its seeds can reach, lowering the habitat quality for nearly all native plants and animals. The debate should not be on whether or not to try to control tamarisk, but focus on how, when and where to direct our control efforts. Anything else will result in a lowering of water availability and wildlife viability throughout the southwestern U. S.
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