by John D. Morris, Ph.D.
Several species of trees live almost indefinitely. The giant sequoia trees of California are known to live over 3,000 years, discerned through tree ring dating. Under normal circumstances, woody trees add one ring per year. A ring typically consists of a light-colored growth portion and a dark-colored portion produced in a stabilization season. However, some trees do not produce annual rings at all, especially those in temperate or tropical regions.
Overlapping and correlating rings have been used to produce “chronologies” of past years. Linear sequences of rings are obtained by cross-matching tree ring patterns from living trees and those from older dead wood. A well-known study involved bristlecone pine trees in California’s White Mountains, but others have employed oak trees in south Germany and pine trees from Northern Ireland. Most chronologies only go back a few centuries, but a few give longer ages than the Bible seems to allow, supposedly up to 10,000 years or so.
Tree rings are more than a record of years. Year-to-year variation in the width of rings records information about the growth conditions in the particular year. Insect infestation clearly manifests itself, as does disease or fire damage. Each of these interrupts the normal growth cycle. Day length, amount of sunshine, water potential, nutrients, age of tree, temperature, rainfall, height above ground, and proximity to a branch all impact tree growth and tree ring production. By assuming the outer ring records the most recent year and that each ring signals one year, a researcher can determine the “date” of a particular ring simply by counting rings.
But how valid is the assumption of one ring per year in a climate where tree-growing conditions are variable? That very assumption is regularly put to the test by research foresters.1 They investigate how a tree grows, how and when it adds a new ring, effect of nutrients, rainfall, etc., over a range of related conditions….