National Geographic : 1974 Jul
CROSS-SECTIONOF PINUS PONDEROSA This tree kept a lifelong diary for all of us to read Each year a tree grows a new layer just inside the bark. Early in the season, fast-growing, light-colored cells form. Then growth slows, leaving a thin line of darker cells to mark the passing year. This cracked and drying stump of a ponderosa pine reveals in its annual growth rings much about its 53-year life. Its center rings, evenly-spaced, show 14 years of steady, normal growth. Then came four years of drought, indicated by crowded, narrow rings. Fire crackled through the forest, searing one side of the maturing pine, and leaving an open wound that took seven years to heal. As neighboring trees competed for sunlight, the tree's rings narrowed, until in its 46th year a series of very narrow rings tells of perhaps more drought, an insect blight, or a period of heavy cone production. By studying nearby trees, experts can pinpoint the cause. Scientists study tree rings to find ways of improving timber growth, to gain greater under standing of the weather, and even to date archeological ruins. Around 1900 astronomer Andrew Ellicott Douglass turned from his study of sun spots to examine tree stumps. Douglass thought that if he could link ring width to weather, he would find a chronology of climatic changes written in the wood of fallen trees. His studies gave us a valuable history of weather going back many hundreds of years. It also gave archeologists a unique tool for dating ruins. Experts had long puzzled over the age of the pre-Columbian ruin of Pueblo Bonito in northwest New Mexico. Douglass, working under a National Geographic Society grant, drilled cores from the house timbers at this long deserted site, matched sequences of thick and thin tree rings with his "yardstick" of ring patterns, and was able to date positively the pueblo's earliest construction at around A.D. 900. In establishing a relationship between tree ring variation and climate, Dr. Douglass pioneered a new science now known as dendrochronology. He also founded the Laboratory of Tree-Ring Research at the University of Arizona, where today's dendrochronologists, by cross-dating living and dead specimens of California's bristlecone pines, have pushed back a continuous chronology of over 7,000 years. They have precisely dated hundreds of archeological sites, and have vastly improved our picture of the paleoclimate of western North America. Not every reader shares the dendrochronologist's enthusiasm for reading tree rings. But many do share the scientist's enduring curiosity about the wonderful world around us. And to help satisfy that curiosity, they turn each month to the pages of NATIONAL GEOGRAPHIC.