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The not-so-thirsty tamarisk

March 11, 2009 | 12:01 pm

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In Western circles, tamarisk, a small invasive tree from Eurasia, has a nasty reputation as a water hog that sucks stream beds dry and lowers water tables at an alarming rate.

But new research suggests the shrubby tree's bad rep may be undeserved.

Conventional wisdom says a single tamarisk, also known as saltcedar, can gulp 200 gallons of water a day from riverbanks. The real figure may be a tenth of that, says Philip Dennison, a University of Utah assistant professor of geography who is monitoring the effects of a recently introduced bug that attacks tamarisk.

The saltcedar leaf beetle munches away the tree's wispy foliage. As part of a study to be published online this month in the journal Remote Sensing of Environment, Dennison and fellow researchers are mapping the leaf loss with satellite imagery.

Since leaf area is linked to a plant's water use, the images are also an index of saltcedar's water consumption, which Dennison says has been confirmed with on-the-ground sap flow measurements. "There's not any good scientific evidence" for the 200-gallon figure, Dennison said. It's more like 20.

The leaf beetle has been introduced by government agencies as a biological control for the pesky saltcedar, which is common along southwestern streambanks. "People who are expecting the beetles to produce a big water savings through defoliation--we're not going to see that," Dennison said.

Moreover, if tamarisk groves die from the leaf loss, it's likely they will be replaced by other invasive plants that could use more water. "The impact of this defoliation is largely unknown," warned study co-author Kevin Hultine, a research assistant professor of biology at the University of Utah. "The net impact of controlling tamarisk could be positive or negative."

-- Bettina Boxall

Photo: Pre- (left) and post-beetle satellite images of tamarisk growth along the Dolores and Colorado rivers in Utah.  The defoliation is evident from the change in color from red to darker colors. The red fields are alfalfa crops. Credit: Philip Dennison /University of Utah, from NASA data 

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