History as Tree Rings Tell It
This is a true story of an astronomer who learned more about the sun by looking around him than by looking up into the sky. As a result, he established a new science.
The name of the science is dendrochronology. It deals with growth-rings of trees.
The rings offer clear records of the weather of the past and give us new information on human history.
A tree grows well in favorable years and slowly in years of drought or other hardship. The change from good years to bad years leaves a pattern of rings in a cross section of the tree trunk. For instance, three good years followed by three years of drought form three widely separated rings which are close together.
The date of a serious drought can be determined by counting from the present year’s ring of a growing tree. Suppose a drought was 50 years ago. The we find its mark by counting the rings in from the bark. There will be 50 rings before we reach the closely packed rings.
An old timber we find somewhere may have the same pattern of closely packed rings. But this time the pattern appears near the outer edge instead of 50 rings in from the bark. Its earlier growth rings carry the weather farther back and reveal other patterns that are like the outer rings of timbers even older. Here is the perfect record of the climate in the past.
From this record we learn that our climate is not changing. We could establish this information in other way. Detailed weather reports in the United States go back fewer than 100 years. But oak trees in the Middle West give us a record back to the year 1536. Pine trees in the Northwest provide information back to 1268.
Studying such evidence, we learn that there has been no change in the amount of precipitation for about 650 years. The trees record droughts of centuries ago. The droughts, though longer and drier than any we have known in recent years, were always followed by plentiful rainfall.
Dendrochronologist also have discovered that a drought affecting all parts of the country at one time has probably never occurred. There was a drought in the Middle West in 1675, but it was very wet in the Northwest that year and the southwest had its usual rainfall.
Just before the American Revolution, from 1772 to 1774, the Middle West had another long dry period. The Northwest was rather dry, too. But two of those three years were splendid growing years in the Southwest.
Trees and Weather
The father of this science of dendrochronology was Andrew Ellicott Douglass, an astronomer, born in Vermont and later a professor at the university of Arizona. He was a student of sunspots and needed weather records of earlier centuries.
One day he had an idea: “Why not find out from the trees?” these were the steps in his reasoning: sunspots are a sign of storms on the surface of the sun. the sunspots are known to affect the earth’s weather. Weather affects plant growth. Thus trees might furnish a record of weather and therefore a record of sunspots centuries ago.
So Douglass began to study tree rings, and in 1904 he discovered the principle which was to make a science of what began as a simple idea. He measured rings of trees freshly cut near Flagstaff, Arizona. Twenty-one rings in from the bark, indicating 1883, he saw a group of thing rings. On an old stump, he recognized the same rings: but they were only 11 rings in from the bark. This, he reasoned, indicated that the stump was left from a tree cut in 1894.
The owner of the stump was asked when the trees in that area were cut down. “In 1894,” he answered .
Then Douglass found that by comparing ring patterns he could trace history back from living trees to trees long dead. Where the story stopped in one tree he could find it continuing in a stump, in the timbers of an old cabin or in old logs preserved in swamps or lakes.
But even trees still standing carry the story back quite far living cedar trees in eastern Tennessee started growing a full century before Columbus. California has giant evergreen trees still alive after 30 centuries. There is a special tool which cuts out a smell, narrow section from the bark to the center of the tree. The rings of an old tree can thus be studied without destroying the tree.
Universities are doing research in dendrochronology. Entomologists have used the science to learn the growth of forests. From trees climatologists have learned a large number of facts not known before.
Trees and History
Archaeologists use the science of dendrochronology to determine the age of ruins and to study ancient peoples. Here, for instance, is one bit of history that trees helped uncover. Near the little town of Thoreau, New Mexico, you find the Chaco Canyon, treeless and sandy. You can drive for miles and never meet a human being. Yet vast ruined pueblos suggest that at least 100,000 people once lived there.
How long ago? Timbers preserved in the ruins tell us that Chaco Canyon was well peopled in 1066. It probably remained so until the middle of the 12th century. Then, the record shows, the population left.
Why? After a study of the situation, Douglass concluded that lack of trees caused the people to leave. Forests originally grew to the edge of the canyon. Poles used by the Indians for building purposes were from pine trees. Pine forests apparently grew near the place where the Indians built their pueblos, for they had no beasts of burden. Today the nearest pine forests are 60 miles away.
Such a large population needed great quantities of wood, and hence the forests were gradually destroyed. As the trees were cut down and used, the ground no longer held moisture. Water from the rains rushed through the canyon, making it deeper and wider. A man-made desert finally remained where fields had been, and the people departed. That’s the story of the Chaco Canyon as told by tree records.
Today there are many students of dendrochronology. Following the example of the founder of the science, they are working harder than ever before to discover any new knowledge that tree rings may reveal.
Astronomer, a person who studies astronomy. The science dealing with the sun, moon, stars and other heavenly bodies.
Dendrochronology, the science dealing with the growth-rings of tress
Drought, a long period of dry weather when plant growth is affected by the lack of water
Cross section, a cut made across something.
Tree trunk, the main body of a tree. Branches grow out from the trunk of a tree.
Timber, a log; any large piece of wood prepared for use in building
Climate, the weather conditions at a place over a period of time
Precipitation, rain, snow and other forms of water falling from the sky
Dendrochronologists, scientists in the field of dendrochronology
Sunspots, signs of storms on the surface of the sun
Stump, the part of a tree which remains above the ground after the tree has been cut down.
Swamps, wet, soft land
Before Columbus, before 1492, the year in which Christopher Columbus discovered America
Research, careful search for new facts in a field of knowledge
Entomologists, scientists in the field of entomology, the study of insects
Climatologists, scientists in the field of climatology, which deals with climate and the conditions that affect climate
Archaeologists, scientist in the field of archaeology, the study of ancient people and their way of life
Canyon, a deep, narrow valley, often with a stream flowing through it
Pueblos, villages of Indians houses built for large numbers of people
Beasts of burden, animals used to carry things
Moisture, slight wetness