Marshes Being Restored at Tijuana Estuary Focus Is on Exotic Plant Removal
June 20, 2006
Contact: Christina S. Johnson, csjohnson@ucsd.edu, 858-822-5334
IMPERIAL BEACH – Important lessons are emerging about how to control exotic plant invasions at the Tijuana Estuary. These lessons are helping wildlife biologists develop what is known as an adaptive management plan for restoring rare coastal habitats.
Adaptive management is a fancy way of saying the reserve is "learning as it goes," gathering scientific information and then incorporating this information into affordable, practical strategies for protecting and improving its wetlands.
Jeff Crooks, research coordinator at the NOAA Tijuana River National Estuarine Research Reserve, looks at an area cleared of invasive salt cedar trees, also known as tamarisk. Credit: C. Johnson, Sea Grant
Because invasive species invasions are a significant roadblock to preserving native habitats, California Sea Grant is supporting scientists to study one particularly problematic exotic plant at the estuary, a flowering invasive Asian tree known as tamarisk or salt cedar.
This wispy tree grows in dense stands along the banks of the Tijuana River and has spread into the tidal reaches of the salt marsh. The trees are changing the way the marsh functions, ecologically and hydrologically, and setting back efforts to create habitats that support rare species, particularly birds.
"The marshes here should be fields of native succulents and grasses about 3 feet high," said Jeff Crooks, research coordinator at the NOAA Tijuana River National Estuarine Research Reserve, explaining the rationale for the emphasis on tamarisk research. "Suddenly you throw in a tree that is 15 feet tall. That changes the system. It changes available light. It changes flow patterns. The trees accrete sediments that fill in the marsh. In the long run, you lose the marsh."
The trees also provide unwanted sentry towers for hawks and other raptors that may hunt rare birds inhabiting the marsh.
Crooks is a former Sea Grant Trainee and collaborator on the ongoing Sea Grant research, which is
In salt marshes, only native plants are re-colonizing areas cleared of tamarisk, a good sign that the restoration process is in motion. Credit: C. Johnson, Sea Grant
being led by Drew Talley, another former Sea Grant Trainee who also has a long-standing interest in marsh ecology and is now the research coordinator at the NOAA San Francisco Bay National Estuarine Research Reserve.
To reduce the impacts of tamarisk, the Tijuana River reserve has been chopping down trees in marsh lands, along the Tijuana River and in upland areas. Removing the trees is the most direct method for preventing them from taking over more land in the short-term, but it is not necessarily an effective long-term restoration strategy, not if the trees have seed banks that allow them to return and not if other problematic exotic plants grow in tamarisk-cleared areas.
A central purpose of the ongoing Sea Grant research is to evaluate what are effective control strategies for different habitats at the reserve.
The results so far show that it is relatively easy to restore native vegetation in tidal salt marshes. This is good news since tidal marshes are among the more rare coastal habitats in California and nationwide. Besides providing habitat for songbirds, water birds and other animals, these wetlands are the kidneys of the coastal environment, filtering pollutants that otherwise flow directly to beaches. Wetlands also buffer inland areas from the direct hit of storms and flooding.
"We are lucky," Crooks said. "Once tamarisk is controlled in the intertidal marsh, there is not re-invasion by other exotic species." The cleared areas are re-colonized by native plants.
"We should take our luck as a wake-up call for prevention," he said. If other invasive plants are introduced to the marsh and become established, the reserve would lose the luxury of having a "one-plant" eradication plan in these areas.
Many invasive plants, including tamarisk, are escapees of landscaped yards. These plants, often attractive ornamental species, are meant to thrive in backyard soils. They are usually intolerant to the saline environment of a tidal salt marsh. Tamarisk is unique in its ability to thrive in such a high-salt habitat; and it is pretty much the only invasive plant in tidal salt marshes at the reserve.
Upland habitats are a different situation. They are quite literally covered in exotic species. Invasive species control is more complicated in these areas, as the Sea Grant research is documenting.
Non-native invasive plants dominate some areas of the reserve, such as the area pictured above. The trees are tamarisk. The flowering succulents are ice plants, an exotic commonly seen along freeways in Southern California. Credit: C. Johnson, Sea Grant
When tamarisk is removed from areas above the tidal line, annual invasives take hold. The tamarisk forest is converted into a field of waist-high weeds, Crooks said.
What this means is that invader control has to be a multi-pronged effort in these areas. "If we remove tamarisk, we have to be prepared to go in and remove the exotic weeds," Crooks said. "We can't just remove tamarisk and think we are done."
The Sea Grant research is documenting the succession of plants that re-colonize tamarisk-cleared areas, including both native plants that return to salt marshes and invasive ones that return to higher-elevation habitats. This information will further help the reserve prioritize its restoration activities, which are restricted by a very tight budget.
One possible activity would be to plant native vegetation in upland areas to "jump start" the restoration process and perhaps stave off exotic weeds. The Sea Grant research will help identify which plants will return on their own and which might be absent without intervention.
"With Sea Grant help, we are continually improving our understanding of what we are doing and how to do it," Crooks said. "The bottom line is tamarisk is leaving. The system is getting better."
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