The day is hot and humid, as Robert Jackson descends into the mouth of Powell Cave and down a ladder that drops into the limestone bedrock of the Edwards Plateau about 240 kilometers west of Austin. He follows an ancient streambed through several large caverns and a few tight crawl spaces, until he arrives at a point about 18 meters below the surface, where a crystal-clear stream bubbles out of the rock. Several thick tree roots burst out from the limestone, reach down, and suck moisture from the water. Some of them have been wired with electronic probes. Applying pulses of heat, Jackson and his lab can gauge water flow by how fast that heat dissipates. Jackson, a biologist at Duke University in Durham, North Carolina, is investigating how roots transfer water from this depth up to the surface. "A single taproot can provide a third or more of the tree's water during a drought," says Jackson.
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