The Texas state government is interested in understanding the effects of such trees on water uptake because the Edwards aquifer supplies a large area of central Texas with drinking water, including the city of San Antonio. The water supply from this aquifer has been affected by the change in the landscape above from a savanna to forest, due to grazing and human prevention of natural wild fires over the last 150 years.
In their experiments, Jackson and his colleagues descended into the caves and sampled roots of four tree species -- a juniper, an evergreen oak, a deciduous oak and the deciduous gum bumelia, which is also known as the chittamwood.
"The evergreen oak we studied is the dominant oak on the Edwards Plateau," said Jackson. "Because it needs a lot of water to maintain its year-round foliage, it has the deepest roots, going some 60 to 75 feet into the bedrock to find water."
In their studies, the researchers did genetic fingerprinting on both the roots and above ground trees to match the roots to specific trees. The fingerprinting is the same technique used in criminal forensics.
They then measured the diameters and wall thicknesses of the water-conducting root xylems. And in laboratory experiments, they determined how vulnerable specific sections of root were to "cavitation," in which mechanical stress or drought induces bubbles that block water flow.
The researchers found that trees adjust their root anatomy very effectively to work in different environments. Shallow roots and especially stems have thicker walls and smaller interior diameters because they must support the tree more, and because they experience more drying and temperature extremes that cause cavitation. Deeper roots, more protected from such demands, can have larger diameters and thinner walls to maximize the conduit size and enhance water flow.
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Contact: Dennis Meredith
dennis.meredith@duke.edu
919- 681-8054
Duke University
19-Aug-2004