Southwestern Riparian Zones, Tamarisk Plants, and the Tamarisk Beetle

Science Center Objects

Introductions of bio-control beetles (genus Diorhabda) are causing defoliation and dieback of exotic Tamarix spp. in riparian zones across the western U.S., yet the factors that determine the plant communities that follow Tamarix decline are poorly understood. In particular, Tamarix-dominated soils are often higher in nutrients, organic matter, and salts than nearby soils, and these soil attributes may influence the trajectory of community change following Tamarix defoliation and mortality. This project aims to assess the physical and chemical drivers of plant colonization after beetle-induced Tamarix decline using long-term observational studies, field and greenhouse manipulation experiments, and a focus on providing resource managers with more information regarding management decisions that could help improve southwestern riparian zone community composition and function.

A tape measure disappearing into a stand of tamarisk and a whiteboard containing information about the site.
What the beginning of a Tamarix transect looks like. Each transect (using a tape measure in this case) travels perpendicular to the river and ranges from tens of feet to hundreds of feet. Along the transect we collect information about plant community composition, riparian zone architecture, light, leaf litter depth, and soil chemistry (fall 2010). (Credit: Sasha Reed, USGS. Public domain.)

Background & Importance

In arid and semiarid regions of the western U.S., much of the riparian vegetation is dominated by non-native shrubs and trees in the genus Tamarix, which have replaced native vegetation in many areas over the past several decades.  In recent years, specialist herbivores (beetles) in the genus Diorhabda have been introduced for the biological control of Tamarix.  These beetles can cause substantial defoliation and mortality of Tamarix, paving the way for subsequent changes to plant community composition and structure, and consequent effects on wildlife populations and ecosystem processes (for example, wildfire, hydrological dynamics, sediment dynamics, nutrient cycling). The nature of these potential vegetation changes and the site conditions (for example, soils) that are more or less likely to promote undesirable successional trajectories, such as secondary weed invasion, or successful restoration treatments are poorly understood.

In addition, there is significant concern about how changes in vegetation composition and physiognomy might affect water use in the riparian zone, and how utilization of riparian habitat by wildlife might change, especially neo-tropical migratory birds (e.g., southwestern willow flycatcher), but also small mammals, lizards, and invertebrates. Research and monitoring of ecosystem changes associated with biological control of Tamarix could greatly benefit efforts to predict when and where management actions may be needed, particularly to facilitate maintenance or restoration of native plant communities and their associated wildlife species. This research addresses important unknowns in how beetle-induced Tamarix defoliation and mortality will affect future riparian plant communities and function, using a mix of observational, experimental, and laboratory approaches.

General Methods

The USGS conducts a variety of Tamarix research, and this project complements those efforts by concentrating on the ecosystem consequences for Tamarix defoliation and mortality. In particular, we are focusing on ‘what comes next’, in the context of Tamarix decline and mortality. For example, do soil conditions created by Tamarix – both when it is alive and as it dies due to beetle defoliation – benefit native plants, or instead other exotic plant? Are there management techniques that could help set the stage for a native plant community that is serving key ecosystem functions, such as habitat for important wildlife communities? What effect does Tamarix defoliation and mortality have on water quantity and quality, soil fertility, and plant community composition?

To achieve these research goals, we are using multiple complementary methods. First, we are using long-term observation of multiple reaches of the Colorado and Virgin Rivers to assess how beetle defoliation is affecting native and exotic plant communities through time. Second, we are monitoring how defoliation and mortality affect key environmental conditions – such as soil fertility, salinity, and litterfall chemistry. Third, we are using experimental manipulations in the greenhouse and in Colorado River riparian zones to determine the exact environmental characteristics help determine the riparian plant community that follows Tamarix defoliation and mortality. Our ultimate goal is to provide information that will help resource managers find successful and economical approaches to a healthy, multi-functional riparian community.

USGS researcher struggling to set up a line transect (tape measure) in a dense thicket of tamarisk.
USGS scientist collecting data on one of our Colorado River Tamarix transects. These are difficult data to collect due to dense thickets Tamarisk can create (spring 2014). (Credit: Sasha Reed, USGS. Public domain.)
Native and nonnative riparian species growing in a greenhouse
A greenhouse experiment where we planted common native and nonnative southwestern riparian plant species to determine how a number of treatments affected their germination and growth. Treatments included adding different heights of Tamarix litter, different heights of wood chips, nutrient additions, salt additions, and leaf litter leachate (spring 2014).(Credit: Pat Shafroth, USGS. Public domain.)