Paul R. Sheppard, University of Arizona (IMAGE) University of Arizona Caption Paul Sheppard of the University of Arizona's Laboratory of Tree-Ring Research seated at the microscope he uses to count the annual rings in tree cores. He is holding one of the pencil-thin cores mounted on a block of wood for ease of handling. Credit Laboratory of Tree-Ring Research, the University of Arizona. Usage Restrictions Photo must be used only in connection with a story about the research described in this release. Photo credit must be given as: Laboratory of Tree-Ring Research, The University of Arizona. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.