Majestic Trees of America
The rings of a tree are nature’s way of marking the passage of time. Each year new growth adds wood to the trunk of the tree, as the tree grows outward and upward.
If a slice from the tree trunk could be viewed, it would look very much like the “tree cookie” at the left. Each ring in this tree cookie represents another year of growth. Thinner rings show years when the tree struggled to get enough water and sunshine.
Many trees in America today are more than a century old, and the giant sequoias of California may be as much as 3,000 years old.
Enter the timeline and discover the many historic moments when trees helped shape our country’s history.
More About Tree Rings
Most trees in North America add new wood to their trunk each year in a typical way. The new tree tissue is added just inside the bark by a paper-thin layer of specialized cells called the cambium. When spring arrives, the cambium cells begin to divide. Those cells pushed to the outside add to the tree’s phloem, a narrow band of inner bark that is a living highway for the transport of nutrients created in the leaves during photosynthesis down to the rest of the tree. New cells created on the inside become the xylem (sapwood), tiny straw-like cells that carry the tree’s water supply up from the roots. It is this section of the tree that creates the rings that tell us much about the past.
In the spring when the annual cycle begins and rainfall is usually more abundant, layer after layer of large cells (springwood or earlywood) are added, slowly swelling the tree to greater diameter. But as the season wears on and soil moisture becomes less available to the roots, the cambium adds new cells more slowly. These cells (summerwood or latewood) are thicker-walled, smaller, and darker. Finally, due either to cold in the north and at high altitudes or lack of precipitation elsewhere, growth ceases altogether. Each spring the cycle repeats itself. The new lighter-colored spring wood contrasts with the darker-colored summerwood from the previous year creating a sharp line that marks the division of the years.
Tree rings tell their tale because the annual miracle of growth is sensitive to a wide range of environmental conditions. Best known is the relationship of rainfall to ring width (the wetter the year, the wider the ring). But tree rings can also reveal history of fire, disease, or forest crowding.
To study tree rings, stumps are often useful, but core samples have proven even better. Using a hollow instrument called an increment borer, scientists can drill into the center of the tree trunk. A long, narrow cylinder of wood about the diameter of a pencil is removed through the tube of the borer. The borer is removed, leaving just a small hole in the trunk. The tree is able to quickly seal over this small wound and heal itself. The annual rings appear as lines on the wood core cylinder providing the same information as a “slice” of the tree. The core sample can be smoothed and studied under a microscope.