The Transformation of dryland rivers: The future of introduced tamarisk in the U.S.

Abstract

Tamarix spp. (tamarisk or saltcedar), a shrub-like tree, was intentionally introduced to the U.S. from Asia in the mid-1800s. Tamarisk thrives in today’s human-altered streamside (riparian) habitats and can be found along wetlands, rivers, lakes, and streams across the western U.S. In 2001, a biological control agent, Diorhabda spp. (tamarisk leaf beetle), was released in six states, and has since spread throughout the southwestern U.S. and northern Mexico. Beetle defoliation of tamarisk has altered tamarisk’s water use and effectiveness as erosion control, as well as dynamics of native and nonnative plant and wildlife species. The full effects of the tamarisk leaf beetle on ecosystem function remain unknown. The U.S. Geological Survey collaborates with Tribal, State, Federal agencies, and other institutions to provide current, fact-based information on the effects of tamarisk and the tamarisk leaf beetle on managed resources, and provides sound science for conservation and restoration of riparian habitats in the southwestern U.S. Tamarisk and Riparian Systems Streamside (riparian) habitat occupies less than 10 percent of the Southwestern landscape. Yet, it is the most critical ecosystem in drylands, providing habitat for more than 90 percent of wildlife species and provides other ecological functions. Thus, riparian habitats receive considerable attention, resources, and management action. The success of nonnative Tamarix spp. (tamarisk or saltcedar) in riparian habitats across the Southwest has led to large changes in biological and geomorphological processes. Several factors have contributed to the success of tamarisk in the western U.S. Tamarisk produce seeds that are dispersed by wind and water throughout the spring and summer. Tamarisk has small, needle-like, salt-exuding leaves that allow them to tolerate high-levels of salinity, drought, and heat. Tamarisk may be favored in areas along river courses (1) that have altered flood regimes, (2) that are saltier due to the effects of dams and water diversions, (3) with less available water because streams are undergoing pressures from drought and increased temperatures, and (4) with declining groundwater levels owing to over-extraction and limited recharge. The spread of tamarisk in the west coincided with a decline in the ecological function of many riparian habitats in the early 20th century. Rivers and streams were dammed and (or) water was diverted for irrigation purposes separate from but concurrent with the introduction of tamarisk to the U.S. Increasing concern over the spread of tamarisk led to the release of a biological control agent, Diorhabda carinulata (northern tamarisk leaf beetle), starting in 2001. The beetles were released in six states— California, Colorado, Nevada, Texas, Utah, and Wyoming—by the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. Since 2001, additional tamarisk leaf beetle species have been introduced and have spread to adjoining states and northern Mexico. Photograph of a tamarisk plant. Photograph by Pamela Nagler, U.S. Geological Survey. U.S. Department of the Interior U.S. Geological Survey Fact Sheet 2020–3061 April 2021 Monitoring Beetle Defoliation and Tamarisk Response Several ecosystem functions, such as wildlife habitat and water use, are affected by tamarisk biocontrol. USGS scientists and collaborators use remote sensing, time-lapse digital photography, and ground measurement techniques to monitor where the beetles are, how fast they spread, and what effects they are having on tamarisk. Although beetles typically cause nearly 100 percent defoliation, tamarisk is rarely killed by a single event. Several defoliation events can occur in a single year, but substantial branch die-back (a portion, but not all, of the canopy dies) or tamarisk mortality typically results after multiple years of repeated defoliation events. Difference in die-back and mortality response could be linked to plant genetics, soil microbe, or environmental conditions, such as, air temperature, streamflow patterns, depth to groundwater, soil salinity, and in-stream water salinity. Tamarisk as Wildlife Habitat Riparian habitats support many wildlife species and act as migratory routes for nesting and breeding birds. The movement of birds along rivers and streams evolved to coincide with available food and habitat from native trees. Tamarisk provides habitat for a wide variety of birds, reptiles, and small mammals, although the quality of that habitat may differ for different wildlife species and on how much tamarisk is present. Tamarisk can form large, single-species groups of trees called stands, or it can grow in mixed stands alongside native riparian trees and shrubs, such as cottonwoods and willows. Single-species tamarisk and mixed stands have different ecosystem functionality. Single-species tamarisk stands host fewer arthropods, such as insects and spiders, which are important prey for many birds, reptiles, and small mammals. Temperatures in tamarisk canopies can be warmer than those of native trees—a difference between life and death for some birds and other animals that live there. Defoliation by tamarisk Photograph showing tamarisk biocontrol, the Diorhabda spp. (tamarisk leaf beetle). Photograph by Dan Bean. Photograph of a southwestern willow flycatcher. Photograph by Osvel Hinojosa-Huerta, Cornell Lab of Ornithology. leaf beetles greatly increases temperature, reduces humidity, and increases solar radiation in tree canopies that can lead to nest abandonment and failure. Yet many bird species successfully nest in tamarisk, and small mammals and reptiles use tamarisk as habitat at roughly equal rates as native trees. Mixed stands support more biodiversity than native stands. Mixed stands have a higher number of arthropod species than single-species tamarisk stands or native stands, resulting in more food for reptiles, birds, and small mammals. Defoliation of single species stands by tamarisk leaf beetles may result in the further degradation of the only remaining habitat. Endangered Southwestern Willow Flycatchers and Tamarisk Leaf Beetles The case of the endangered Empidonax traillii extimus (southwestern willow flycatcher) exemplifies the complexities associated with wildlife habitat use of riparian habitats in the context of tamarisk biocontrol. Many breeding sites of this bird species are dominated by tamarisk. The birds will nest in singlespecies tamarisk, mixed, or native stands if standing water or moist soils are present. Defoliation by tamarisk leaf beetles can lead to higher rates of nest abandonment, further threatening this endangered species. Tamarisk leaf beetles were not expected to move south into the bird’s range because (1) the beetles were expected to spread only a few miles per year, and (2) their range was thought to be limited by daylength requirements; the beetles need a certain number of nighttime hours to complete their lifecycle. However, tamarisk leaf beetles can move about 25 miles (40 kilometers) per year and have rapidly evolved to complete their lifecycles much farther south than originally predicted. This has resulted in beetles defoliating tamarisk in the southwestern willow flycatcher range. Water Use by Tamarisk Previous studies reported that tamarisk uses water at much higher rates than native plants, resulting in calls for tamarisk’s eradication to conserve water resources. However, more recent research has consistently shown that tamarisk uses about the same or less water than many native woody plants. Therefore, the removal and replacement of tamarisk with native species will likely not increase water availability for agriculture, municipalities, and other uses. Beetle defoliation of tamarisk has altered tamarisk’s water use, in addition to the effectiveness of tamarisk in erosion control, and riparian health related to native plant and wildlife species. Tamarisk is more drought tolerant than native cottonwood and willow species; its roots can access deeper groundwater sources, and it can withstand drier conditions for longer periods. The combination of deeper roots and higher drought tolerance allows tamarisk to occupy areas that are farther away from primary rivers or stream channels than native species. Consequently, the expansion of tamarisk in some riparian systems could result in slight increases in water usage and reduced groundwater recharge in some areas. If management goals are only concerned with reducing water use from vegetation, then replacing tamarisk with native cottonwood and willow is not likely to reduce water loss because native species, in some cases, use more water than tamarisk. However, water use of plants depends on many factors, including the assemblage of plant species, stand density, age, and rooting depth among other factors. Management decision making about reducing water use by vegetation includes many factors beyond identifying a target species. Tamarisk leaf beetles were expected to help increase water levels in streams by reducing tamarisk cover through defoliation. Green-leaf die-back, or even plant death, results when foliage is removed and transpiration (water loss via plant leaves) is reduced. This in turn, results in water savings at the plant-level, river reach-level, and (or) landscape-level following beetle defoliation. Water savings depend on how large the defoliated tamarisk stands were, their age, and how much area they covered. Furthermore, water savings are often short-lived. Once a tamarisk has grown new foliage, typically within one month, water use returns to pre-defoliation levels. Small water savings could result after multiple defoliation events if (1) fewer leaves are produced, (2) there is branch die-back and (or) (3) mortality occurs. However, even when tamarisk mortality does occur, water savings may be relatively short-lived, because other vegetation typically grows in place of tamarisk within a few years. Tama