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Tree Rings

Bayou-Diversity (18 October 2020) One day a forester walked into an unfamiliar patch of woods. She chose a medium-sized white oak tree as a subject, collected a sample, and returned to the lab. In a couple of days she drafted the following history of the tree. “The white oak sprouted from an acorn in the year 1885. For the first twelve years of its life the tree grew slowly as a result of being shaded from sunlight by large trees nearby. In 1897 the closest large tree, perhaps a parent of the sapling, blew over in a spring storm and allowed sunlight to reach the young tree. It grew rapidly for the next seventeen years. Then, beginning in 1914, a severe drought slowed growth for three years until favorable precipitation returned to the area. For the following forty-two years growth was normal. In the fall of 1959 a fire raged through the forest severely injuring the white oak. It survived because of its thick bark but barely grew at all for eight years as a pathogenic fungus attacked through the fire scar. In 1963, a neighboring tree to the east fell hard against the white oak causing a permanent lean to the west at a thirty-degree angle. At the time of this sampling in 2010 the tree is 125 years old.”

Although this story is hypothetical, it is typical of many life histories of trees that are determined through the science of dendrochronology. In temperate climates trees form growth rings in response to changing seasonal conditions. Each ring has two parts: a wide, light part called the early wood, and a narrow, dark part known as late wood. The early wood is formed during the wet, spring growing season. The late wood forms during the drier transition period from summer to fall and winter when growth slows. By studying the rings, information about the climate over a period of time and evidence of environmental disturbances such as fires and floods can be learned. The shape and width of the annual rings differ from year to year because of varying growth conditions. A ring formed in a wet favorable growing season may be very wide, while rings formed when the tree is stressed will be much narrower. Fire scars and insect damage are often visible in the rings.

To study a tree’s growth rings without harming the tree, scientists use a technique called coring. A hollow tubular instrument known as an increment borer is used to drill into the center of a tree trunk and extract a narrow cylinder of wood. Growth rings on this core sample appear as lines that can be counted, measured, and studied for abnormalities. Similar observations can also be made by examining the stump of a freshly cut tree or the end of a sawn log. By studying tree rings in very long-lived species such as baldcypress and overlapping data from living trees with that obtained from old fallen logs, climate, fire, and flood histories can be developed for an area that go back more than 2,000 years. This information helps us understand how forests change over time, and in some cases can be used to predict the frequency of future fires and floods.

So as you enjoy the shade tree in your back yard, remember that history and lessons for the future are being recorded season by season just under the bark.


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