Tamarisk and invasive plants

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Figure 1. (A) Tamarisk-dominated habitat in which the Southwestern Willow Flycatcher bred at St. George, Utah, prior to tamarisk defoliation. (B) The same habitat during defoliation in 2008. An active flycatcher nest eventually failed in this habitat. (Photos by P. Wheeler, Utah Division of Wildlife Resources. Modified from figure compilation by Paxton et al., 2011.)

Invasive species are those introduced to new geographic areas by human activity (i.e., non-native) and have deleterious impacts on ecosystems and society. Invasive plants typically outcompete native plants, potentially creating monocultures, decreasing biodiversity, and changing disturbance regimes such as fire. Because of their harm to native species and ecosystem services, land managers often undertake extensive control efforts. Control strategies can include chemical (herbicides), physical (mowing/removal), and biological (e.g., defoliating beetles) approaches. If invasive plants can be removed, native habitat and its original functions may be restored.

Like elsewhere in the West, many invasive plant species have been introduced in the Colorado River Basin, both intentionally and accidentally, with impacts on protected species, ecosystem services, environmental health, and water availability. Like other parts of the West, the spread of invasive plants in the Basin has a been facilitated by human alterations of natural disturbance regimes (e.g., flooding, wildfires) as well as by human-caused disturbances of plants and soils (e.g., grazing, road construction).

Perhaps the most infamous invasive plant present in the Colorado River Basin is the tamarisk (Tamarix spp.), also known as saltcedar. The tamarisk is a shrub-like tree intentionally introduced to the U.S. in the 19th century for erosion control on riverbanks and railroads. Following its introduction, tamarisk spread rapidly along river corridors of the Southwest, growing in dense, monotypic stands (Figure 1). Tamarisk has replaced stands of native species such as willow along thousands of miles of stream and river corridor in the Upper and Lower Basins, altering soil chemistry, displacing native plant communities, impairing critical wildlife habitat, and–possibly–consuming more water than the native species it has replaced, though this widespread notion has been challenged.

Due to the difficulties in traditional eradication methods such as herbicides, controlled burns, and physical removal, the saltcedar leaf beetle (Diorhabda elongata) was introduced to southwestern waterways as a biological control agent. The introduction of the beetle has been largely successful in reducing the abundance of tamarisk; however, the use of the beetle, itself non-native, remains contentious. While tamarisk has been treated as a nuisance in much of the Colorado River basin, it has also provided habitat for the endangered Southwestern Willow Flycatcher (Figure 1).

Russian olive (Elaeagnus angustifolia) is a similar invasive shrubby tree that crowds out native plant and tree species and degrades the habitat for many bird and mammal species. Another widespread riparian invasive, limited to the Lower Basin, is giant reed (Arundo donax; aka giant cane), a bamboo-like grass up to 20 feet tall.

In upland areas of the Colorado River basin, especially in grasslands and shrublands that have historically had or presently have cattle or sheep grazing, a plethora of invasive plants now dominate or otherwise impact millions of acres of land, including cheatgrass (Bromus tectorum), buffelgrass (Pennisetum ciliare), Canada thistle (Cirsium arvense), musk thistle (Carduus natans), and Russian knapweed (Rhaponticum repens). Cheatgrass and buffelgrass are both notorious for facilitating fire ignition, spread, and intensity in habitats that previously had much lower levels of fire, threatening many native plant and animal species.

Russian thistle (Salsola tragus), also known as tumbleweed, is a common invasive and early colonizer of disturbed landscapes. It grows in riparian zones in the years after a disturbance and in dry upland locations associated with disturbance. Awned barnyard grass (Echinochloa crus-galli) is a 1.5 m tall invasive riparian grass from southeast Asia that can have considerable impacts to crops and can remove large amounts of nitrogen from soils.

Data and tools

USGS INHABIT (Invasive Species Habitat Tool)

This web-mapping tool allows users to select an invasive plant species, including those mentioned above and over 200 other species, and map the modeled potential for that species to occur in new locations based on its known occurrences and the environmental characteristics of those locations. The known occurrences can also be displayed (“training data”) along with the current range (“range polygon”).

USDA PLANTS (Plant List of Attributes, Names, Taxonomy, and Symbols)

This database contains information on thousands of plant species found in the U.S., including invasive species. Use the ‘Basic Search’ in the upper left to find the profile page for the species of interest. You can then click on the ‘Plant Guide’ links towards the upper right, see the range map on that page (zoom in to see the county level), and click the tabs at the top to see other information about that species.

Additional resources

USDA Field Guides

Field Guides for Invasive and Weed Species Management for the southwestern region has ~10 page guides on 44 different species that occur in Arizona and/or New Mexico (and may occur in other parts of the Colorado River Basin). The Guides cover species description, ecology, and management (including control methods). Specific pages of interest include: