The Transformation of Dryland Rivers: The Future of Introduced Tamarisk in the U.S.

A tamarisk plant. Photograph by Pamela Nagler, U.S. Geological Survey.

Streamside (riparian) habitat covers less than 10 percent of the Southwestern landscape, yet it is the most critical ecosystem in drylands: it shelters more than 90 percent of wildlife species and does other ecological work. Across the Southwest, the nonnative shrub-tree Tamarix — tamarisk, or saltcedar — has taken hold in these habitats, bringing large changes to how rivers behave biologically and physically.

Why tamarisk succeeded

• Its seeds are spread by wind and water all through spring and summer.
• Its small, needle-like leaves exude salt, letting it tolerate high salinity, drought and heat.
• It may be favoured along rivers whose floods have been altered; that are saltier because of dams and diversions; that carry less water under drought and higher temperatures; or where groundwater is falling from over-pumping and little recharge.

Tamarisk spread in the early 20th century, just as many riparian habitats were losing their ecological function: rivers were dammed and water diverted for irrigation at the same time as — but separately from — tamarisk's introduction.

The tamarisk leaf beetle

Concern over its spread led to the release of a biological control agent, Diorhabda carinulata (the northern tamarisk leaf beetle), from 2001, by the U.S. Department of Agriculture's Animal and Plant Health Inspection Service, in California, Colorado, Nevada, Texas, Utah and Wyoming. More tamarisk leaf beetle species have been introduced since, and they have spread into neighbouring states and northern Mexico.

USGS scientists and partners track where the beetles are, how fast they spread and what they do to tamarisk, using remote sensing, time-lapse photography and ground measurements. The beetles usually strip nearly 100 percent of the leaves, but one defoliation rarely kills a plant. There can be several in a year; heavy branch die-back — part of the canopy dying — or death usually comes only after repeated defoliation over several years. Why plants respond differently may depend on genetics, soil microbes, or conditions such as air temperature, streamflow, depth to groundwater, and soil and river salinity.

The Virgin River floodplain from tower-mounted cameras on May 19, 2011 (top) and, with the tamarisk defoliated, on June 19, 2011 (bottom). Photographs by Pamela Nagler, U.S. Geological Survey.

Tamarisk as wildlife habitat

Riparian habitats are migration routes for nesting and breeding birds, whose movements evolved around native trees. Tamarisk shelters many birds, reptiles and small mammals, though its value varies by species and by how much tamarisk there is. It grows in single-species stands or in mixed stands with native cottonwoods, willows and shrubs, and the two work differently:

Tamarisk canopies can be warmer than native ones — for some birds and animals, the difference between life and death — and beetle defoliation greatly raises canopy temperature and sunlight and lowers humidity, which can cause birds to abandon or lose nests. Still, many birds nest successfully in tamarisk, and small mammals and reptiles use it about as much as native trees. Where a single-species stand is the only habitat left, defoliation may degrade it further.

Single-species stands of tamarisk on the lower Colorado River at Cibola National Wildlife Refuge, California. Photograph by Pamela Nagler, U.S. Geological Survey.

The southwestern willow flycatcher

The endangered southwestern willow flycatcher (Empidonax traillii extimus) shows how tangled this is. Many of its breeding sites are dominated by tamarisk, and it will nest in tamarisk, mixed or native stands wherever there is standing water or moist soil. Defoliation can raise nest abandonment, adding to the threat.

The beetles were not expected to reach the bird's range: they were thought to spread only a few miles a year and to be limited by day length, since they need a certain number of hours of darkness to complete their life cycle. In fact they can move about 25 miles (40 km) a year, and have rapidly evolved to complete their life cycle much farther south than predicted — so they now defoliate tamarisk where the flycatcher lives.

Water use

Earlier studies said tamarisk used far more water than native plants, prompting calls to eradicate it to save water. More recent research consistently shows tamarisk uses about the same as, or less than, many native woody plants. Removing it and planting natives is therefore unlikely to free water for farms, cities or other uses.

Riparian vegetation "water footprint": mean yearly water use from the scientific literature, one droplet per 100 millimetres. By Pamela L. Nagler; illustration by Kimber Petersen, U.S. Geological Survey.

The values are means; real use varies with conditions and leaf cover.

Tamarisk is more drought-tolerant than native cottonwoods and willows: its roots reach deeper groundwater and it survives dry spells longer, so it can grow farther from the main channel than natives. Where it expands, water use could rise slightly and groundwater recharge fall in places. But plant water use depends on the mix of species, stand density, age, rooting depth and more — so decisions about reducing it involve much more than picking a target species.

The beetles were expected to raise stream levels. Defoliation does cut transpiration — water lost through leaves — and saves water at plant, reach and landscape scale, depending on the size, age and extent of the stands. But the savings are short-lived: new leaves usually grow within a month and water use returns to normal. Repeated defoliation may give small savings if fewer leaves grow back or branches or plants die — and even then other vegetation usually fills in within a few years.

Fire

Riparian areas once burned rarely, and most native trees are not adapted to fire. Tamarisk is highly flammable compared with cottonwoods and willows, because of chemicals in its tissues and drier leaves and stems, and it recovers from fire better than they do. Defoliation makes things worse: drier stands and a thicker layer of flammable leaf litter mean more frequent, more intense, faster-spreading fires.

Tamarisk resprouting after a fire in 2006. Photograph by Pamela Nagler, U.S. Geological Survey.

Managing tamarisk

The same strategy can give different results in different places, and managers disagree about why tamarisk succeeded:

Restoration is sometimes not feasible, sometimes needs many rounds of adaptive management, and sometimes happens passively once natural flows return. Native trees depend on floods to spread and germinate their seeds, so they need occasional flooding beyond levees and banks. Pulse flows — dam releases that mimic natural floods — can moisten soil, cool water and add oxygen to it, flush wastes, open space for native trees to germinate and grow, and deliver nutrients to the aquatic food web.

Tamarisk and native tree height, 2001. Photograph by Craig Westenberg, U.S. Geological Survey.

Tamarisk and native tree cohorts from overbank floods, 2002. U.S. Geological Survey aerial photograph.

Looking back

Tamarisk's effects vary widely — with whether it forms dense single stands or part of a diverse community, with how dominant and how old the stands are, and above all with local conditions. Even so, the evidence is that tamarisk:

• provides suitable habitat for some wildlife;
• uses about as much water as most native woody plants;
• increases the intensity and frequency of riparian wildfire.

With the amount and quality of Southwestern riparian habitat much reduced, what remains matters more — so losing tamarisk could harm some wildlife, especially species that use it heavily and live where it dominates.

Looking forward

Tamarisk and the leaf beetle will remain major players in the Southwest; the beetle's long-term effect on riparian ecosystems is unclear. Federal partners, managers and nongovernmental groups use USGS science to shape adaptive management as the region grows warmer and drier. Research priorities include tracking the beetles and the defoliation, long-term changes in water use and fire regimes, how plant communities and river flow change once tamarisk dies, best practices for restoring invaded rivers, and monitoring the southwestern willow flycatcher and other wildlife that depend on tamarisk.

Sources

• Nagler, P.L., Hull, J.B., van Riper, C., II, Shafroth, P.B., and Yackulic, C.B., 2020, The transformation of dryland rivers: The future of introduced tamarisk in the U.S.: U.S. Geological Survey Fact Sheet 2020-3061. https://pubs.usgs.gov/publication/fs20203061
• The captions, the water-footprint values and the figures come from the fact sheet's PDF, which the web version lacks. The diagram thanks "the late Arjen Hoestra" for his water footprint concept; the name is Arjen Hoekstra. Two photographs with non-federal credits are left out.
• Rewritten in hubnx's own words.