Time and Cycles
"Logs of Straw: Dendrochronology"
Trees are some of nature's most accurate timekeepers. Their growth layers, appearing as rings in the cross section of the tree trunk, record evidence of floods, droughts, insect attacks, lightning strikes, and even earthquakes.
Tree growth depends on local conditions, which include the availability of water. Because the water cycle, or hydrologic cycle, is uneven, that is, the amount of water in the environment varies from year to year, scientists use tree-ring patterns to reconstruct regional patterns of drought and climatic change. This field of study, known as dendrochronology, was begun in the early 1900's by an American astronomer named Andrew Ellicott Douglass.
While working at an observatory in his native Arizona, Douglass began to collect pine trunk cross sections to study their annual growth rings. He thought there might be a connection between sunspot activity and drought. Such a connection could be established, he believed, through natural records of vegetation growth.
Douglass was not the first to notice that some growth rings in trees are thicker than others. In the climate where Douglass was working, the varying widths clearly resulted from varying amounts of rainfall. In drier growing seasons narrow rings were formed, and in growing seasons in which water was more plentiful, wide rings occurred.
In addition to correlating the narrow rings to periods of drought and, in turn, to sunspot records, Douglass had to establish the actual year each tree ring represented. Because absolute ages can be determined through dendrochronology, the science has since proven useful far beyond the narrow study to which Douglass applied it.
Computer analysis and other methods developed since Douglass' time have allowed scientists to better understand certain large-scale climatic changes that have occurred in past centuries. Likewise, highly localized analyses are possible. Archaeologists use tree rings to date timber from log cabins and Native American pueblos by matching the rings from the cut timbers of homes to rings in very old trees nearby. Matching these patterns can show the year when a tree was cut and, thus, reveal the age of a dwelling.
To determine whether changes now occurring in climate are part of the Earth's normal pattern or are induced by human activity, scientists rely on the history of climatic changes both locally and globally as revealed by tree rings, ice cores, pollen samples, and the fossil record. Computers are used to detect possible patterns and cycles from these many sources. In dendrochronology, large data bases allow scientists to compare the ring records of many trees and to construct maps of former regional climates. The evidence collected so far suggests that climatic change is simply a part of life on Earth. The extent to which human activity affects the way the global climate is changing now is not yet fully understood.
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