Trees are living museums and their rings are able to shed light on climate change, colony collapses, archeology, tsunamis, fire histories and radio carbon dating.

Each year tree species in the temperate and boreal forests grow a combination of early-wood and late-wood forming one annual ring, which normally circles the tree just beneath its bark.

Tree growth, like all plant growth, depends upon both environmental and physical factors such as temperature, sunshine, wind, soil properties, how steep the slope is, snow and rainfall. During the growing season when snowmelt and rainfall are plentiful, growth rings are wide. Conversely, when drought occurs tree ring growth is significantly diminished.

Tree growth in stressful environments such as in rocky soil, along cliff faces or at the tops of mountains are very sensitive to environmental factors and display excellent variation in ring to ring growth — known as variation sensitivity. It’s these tree ring patterns that scientists study.

Interestingly, Leonardo da Vinci examined tree rings when he studied growth patterns in the 1400s. But it wasn’t until 1894 that University of Arizona astronomy professor Andrew Douglass began to unravel how tree rings could be used to date beams from Native American ruins in southwest United States and determine ages of the colossal Sierra Nevada Sequoias.

Douglass first discovered that narrow tree rings corresponded to dry years and that tree rings were an accurate indicator of sunspot activity; and that “Little Ice Age” of the late 1600s displayed very little solar activity.

Incidentally, from his workshop in Cremona, Italy, Antonio Stradivarius (1644-1737) created the most exquisite violins, violas, cellos, and guitars made from the dense, slow-grown wood during the coldest period from 1645-1715 — including his most famous violin “Messiah” of 1716, all producing unparalleled acoustics.

Douglass founded dendrochronology or the science of tree ring patterns. Scientists use a non-destructive method of sampling whereby they carefully extract pencil width cores from living trees. And by counting each annual ring, which contains a lighter band of early-wood and a darker band of late-wood, researchers are accurately able to determine the tree’s age.

Tree core samples are compared to form a chronology or an arrangement of events in time. By matching overlapping patterns of tree ring growth dendrochronologists use a technique called cross-dating. It’s this method that has enabled almost 10,000 continuous years of tree growth from living, standing dead and pieces of wood of Great Basin bristlecone pines on the White Mountains of eastern California to precisely be recorded.

Dendrochronology has some fascinating applications. For instance, tree rings from 700-year-old-bald cypress trees in the Carolinas revealed why the early settlers at Roanoke in 1587 A.D. perished. They arrived at the beginning of a prolonged drought. Bald cypress tree rings from two decades later also showed that the Jamestown colonists arrived during a seven-year drought lasting from 1606 to 1613 A.D.

Tree ring patterns from western red cedar growing along coastal Washington clearly showed that an earthquake occurred in January 1770. It measured between 9 and 9.3 on the Richter scale. When compared to Japanese tsunami records it was discerned to have taken place on January 26, 1770; and its exact time was pinpointed 10 hours before the unrelenting wave train pummeled the Japanese coastline.

In 1929, Dougalss determined the exact ages of prehistoric ruins of the southwest Peoples of the Four Corners by taking tree rings from pieces of wood in the dwellings and bits of charcoal, and cross-dating them. 

Professor Thomas Swetnam of the University of Arizona has used dendrochronology to reconstruct reliable regional fire histories across western North America dating back to about 500 A.D.

All living things absorb carbon-14 — a radioactive form of carbon — from the atmosphere. It can be measured using a Geiger counter. When an organism dies, carbon-12 remains stable while the unstable carbon-14 isotope begins to decay at a known rate.

In the 1960s carbon-14 was measured in Great Basin bristlecone pines and it showed scientists that there was a change in radioactive carbon-14 production over hundreds of years due to the bombardment of cosmic radiation. The radioactive dates did not match the tree rings, and in fact the tree ring dates were older.

Bristlecone pine chronologies were used to re-calibrate the radio carbon dating process. 

This in turn exposed that the European peoples discovered the principles of copper metallurgy and gold smelting 3,000 years before the Romans, Greeks or Celts. Brittany’s great stone tombs are the oldest known buildings. The megalith graves of Europe are older than the pyramids.

Great Basin bristlecone pines were dubbed “the trees that rewrote history.”


Dr. Reese Halter is a public speaker and the founder of the international conservation institute Global Forest Science. His latest book is “The Incomparable Honeybee and the Economics of Pollination.” Contact him through

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