Tree Clocks

Leonardo da Vinci seems first to have noticed that widths of tree rings vary in wet and dry years, but there is a big gap between his discovery and a history of tree rings that chronicles Midwest weather as far back as 1300 A.D.--a study released this week by Anthropologist Florence May Hawley of the University of Chicago.

Systematic study of rings, so that old wood can be dated by its growth patterns, was begun in 1904 by Astronomer Andrew Ellicott Douglass of the University of Arizona, who was interested in solar activity. Douglass became able to look at a pine ladder or rafter from a prehistoric Indian pueblo, date it exactly as far back as 11 A.D. Sequoia wood from the High Sierra can now be dated back to 1305 B.C. Since the weather, and therefore tree growth, varies from place to place, master tree-ring charts must be worked out for different districts of the U.S. Florence Hawley's district is the biggest yet mapped: the Mississippi Valley south of Wisconsin.

Working in the studio of the late Sculptor Lorado Taft at Chicago, she analyzed materials from living trees, old log cabins and bridges, refuse from sawmills and barrel factories, charcoal from old campfires, relics from Indian caves and graves, a ton of tree sections supplied by a Wisconsin paper manufacturer who had just taken a paper contract for LIFE. She patiently placed pieces hundreds of years old in sequence by their overlapping ring patterns, and reconstructed growth calendars for several types of trees.

TVA sponsored part of Miss Hawley's work to learn about past cycles of drought and deluge revealed in the old trees in order to learn about future problems in harnessing a great watershed. Federal entomologists were interested because weather cycles affect the number of insect pests which they may have to fight. Meteorologists welcome the tree-ring studies because weather forecasts derive in part from elaborate past data. And archeologists can read history in tree rings: Douglass, for example, showed that the dreadful drought which started the decline of the high culture of the Pueblo Indians began in 1276, ended in 1299 A.D.

Florence Hawley is now studying a group of timbers from an old ship found on Manitoulin Island in Lake Huron. If she finds their rings correspond to those of oaks growing near Niagara Falls in 1667, the ship is probably Explorer Robert LaSalle's Griffin.

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