Archive of Spotlight Feature Articles

Invasion and Impacts of Tamarisk in Tijuana Estuary Salt Marshes, and Ecosystem Recovery After Its Removal

By Christina Johnson

A non-native tree called tamarisk has invaded salt marshes at the NOAA Tijuana River National Estuarine Research Reserve, a 2,500-acre preserve adjacent to the U.S.-Mexico border in southern San Diego County.

The reserve is one of 26 NOAA-supported research reserves and is managed by the U.S. Fish & Wildlife Service and the California Park Service. In early 2005, the reserve was designated a “Wetland of International Importance” by the United Nations Ramsar Convention.

(Left) Tamarisk in bloom. Photo: Jil M. Swearingen, USDI National Park Service (Right) Reserve Research Coordinator Jeff Crooks points to tamarisk growing along the Tijuana River banks. Photos: Christina Johnson, California Sea Grant

Research Project

In 2004, California Sea Grant funded a research project to examine the ecological effects of tamarisk, an Asian tree introduced for erosion control by the Spanish. Though little is known about its effects on salt marsh, biologists fear it may diminish light to ground plants, increase sedimentation and change soil properties, thus altering the hydrodynamics of marshlands. It may also change food webs in nearby habitats.

Led by Drew Talley, a biologist at UC Davis and former Sea Grant trainee, a team of scientists is documenting the extent of tamarisk at the reserve and developing techniques for monitoring its range. They will also examine the degree to which areas cleaned of tamarisk recover to their pre-invaded state. Ultimately what is learned will help scientists predict areas most at risk of invasion and develop cost-effective strategies for eradicating tamarisk. It will also provide managers with needed information on the processes by which native communities recover from a non-native invasion.

Talley and collaborators will test two hypotheses:

1. Tamarisk significantly alters ecosystem properties and water flows.
2. Removing tamarisk returns these properties and water flows to pre-invasion conditions.

The scientists will document the history of tamarisk in the reserve by analyzing old aerial photos and mapping areas that are purple – blooming tamarisk has purple flowers. The scientists will also develop a remote-sensing technique to identify tamarisk when it is not in bloom based on how light is scattered through leaves and branches. This new technique will have broad applications in monitoring non-native plant invasions.

The scientists will also examine the effects of tamarisk on birds. More than 370 bird species have been documented at the reserve, many of which are rare or endangered. Any effective tamarisk eradication plan must be safe for birds.

Another issue: tamarisk can reach heights of 10 meters while native vegetation typically never grows above one meter. Some biologists fear that tamarisk provides raptors with sentry towers from which they may scout prey, including federally protected ground-nesting birds.

Partners

The California Coastal Conservancy, the U.S. Fish & Wildlife Service and the California Department of Parks and Recreation are all supporting the tamarisk eradication and control plan at the reserve. The project, which began in 2001, consists of trying different control strategies at various demonstration sites. The Sea Grant research will answer fundamental questions about tamarisk’s biology and ecological impacts. A model being developed during this project will help managers predict sites vulnerable to invasion.

Outreach

The results of this project will be included in an interpretive display at the reserve’s visitor center. Scientists also plan to lead a workshop about their work for resource managers on both sides of the U.S.-Mexican border.

[6/13/05]

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