Historic photographs taken by Ray Turner of the U.S. Geological Survey showed foothill palo verde trees on the laboratory grounds in locations which now have only buffelgrass. Such trees can live more than 100 years, so Eilts suspected that buffelgrass had something to do with the palo verdes' disappearance.

To study how buffelgrass affects the trees, Eilts used an instrument called a pressure chamber to find out how much water stress the trees were experiencing. First he tested foothill palo verde trees surrounded by buffelgrass. The next year, he removed the grass from under some of those trees and repeated the experiments. Removing the buffelgrass significantly reduced the trees' water stress.

The grass is reducing the water available to the trees, he said. "This means that in any given year the water deficit experienced by the trees with grass at their bases is greater compared with those trees free of the influence of buffelgrass."

Eilts also measured how much of their branches the trees shed.

Palo verdes self-prune little during an non-drought year, said Huxman. But when times are tough, buffelgrass makes it tougher. Eilts found that in a drought year, trees surrounded by buffelgrass lost about 24 percent of their total branches, whereas trees free of buffelgrass shed only about 16 percent.

Huxman said, "Buffelgrass takes the drought and accentuates it."

The ecologists' findings contradict current hypotheses about the relationship between trees, grasses and water in arid climates. Eilts said, "One would predict that the presence of buffelgrass would have the least affect on the deep-rooted woody plants such as the palo verde trees."

Instead, the buffelgrass reduces water available to the trees' deep roots. "Effectively, what is happening to the trees in the grass is that they are experien
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Contact: Mari N. Jensen, UA News Services
mnjensen@email.arizona.edu
520-626-9635
University of Arizona
1-Aug-2004