To really grok the research, it's helpful to understand how trees transport water. Trees draw ground water up through specialized tubes called xylem, relying on intermolecular forces between water molecules and themselves, and water molecules and the sides of the tubes, to create a single column of unbroken water in each xylem tube. But as groundwater dries up, the trees must pull harder on the remaining water; if the pressure is greater than the strength of the intermolecular forces, the column of water breaks and an air bubble forms. This process is called cavitation. Too many air bubbles can mean death for the tree.