Upon closer examination of trees, one can discover growth records, water and climate records and even the vegetative history of an area. For this reason, trees are known as nature’s historians.
When you look at the top of a tree stump you find a series of concentric rings, known as tree rings. These rings become larger and larger in diameter outward from the center of the stump.
Tree rings contain individualized characteristics that reveal fascinating facts about water and climate conditions in the life of the tree. For example, several unusually narrow rings may indicate extreme drought years.
Because each tree ring represents a year of life for the tree, the tree’s age can be determined by counting the rings. Therefore, if the year of cutting is known, counting backward from the outer ring can determine the age of the tree. This process is called “dating,” and the study of tree rings for the purpose of dating is called dendrochronology.
So, what are tree rings and how are they formed? To grow, trees must increase the size of their xylem — the woody portion exclusive of bark — by adding new tissue between the existing xylem and bark.
Growth depends on the amount of soil moisture within the season of growth — an annual cycle beginning with the opening of leaf buds in spring and ending with the dropping of leaves in autumn. The growing tree forms two types of cells; large, thin walled cells when soil moisture is abundant and small, thick walled cells when soil moisture is less available.
The production of these cells determines the tree ring boundaries. The wood between these two boundaries is formed during one growth season and is popularly referred to as a growth ring, or tree ring.
At the end of its first summer, once germinated from seed, a seedling has one increment of growth layer. Somewhat cone-shaped, this increment is a fraction of an inch in diameter and only a few inches in height. This increment will not grow or change dimensions after the year in which it is formed. It will remain precisely the same size and in the same position for the life of the tree.
The second year, another growth ring is formed around and above that of the first year. By the end of the third year, yet another increment of growth forms around that of the preceding year and similar rings are formed in each succeeding year. Thus, the growth increments of each year increase the diameter and height of the tree. Remember, once a ring has been formed, it does not change in size or position during the life of the tree.
Because the rings, once formed, cannot be changed, they can serve to tell a wide variety of things about their environment through the years.
Tree Rings as Historical Records: By aging the trees of an area, a time reference can be established to determine climate/environmental conditions for that area.
For example, trees can indicate: 1) a timeline for when farmland was abandoned or no longer cultivated; 2) a minimum length of time since new land surfaces were created (i.e., flooding, glaciers, landfills, road cuts, etc.); and 3) a reference of time since a land surface first became suitable for germination and growth of tree seedlings.
If the tree rings can be dated, then the exact year during which a tree was damaged can be determined. This would make it possible to date the event responsible for the damage. Also, if a patch of bark is damaged, the date of damage can be determined by counting the number of rings in an adjacent undamaged area. This method has been used in identifying catastrophic events such as ice storms, forest fires and even hurricanes.
Identifying such events is possible even in partially tipped trees. Terminal shoots of trees generally grow upward. When a tree is tipped over, new shoots emerge and grow vertically at an angle to the axis of the bent over trunk. Therefore, the event that caused the damage can be dated by counting the number of rings in the new shoots that are now growing vertically from the trunk.
Use of Tree Ring Studies: Tree ring dating has been perfected to the point of being used to date events and conditions. Calibrated records have been used to classify past conditions into two categories: cool and wet or hot and dry.
The Water Resources Division of the U.S. Geological Survey maintains a Tree Ring Laboratory near Washington, D.C. There, research is conducted relating tree growth with hydrology.
Currently, studies are being conducted to help hydrologists determine conditions in areas where no records exist. They have also been successful in expanding records into periods of time before information was collected.
It has been discovered that tree ring records in humid areas can be successfully cross-dated, enabling researchers to calibrate tree ring records with hydrologic records. This will allow for estimation of yearly variation in stream flow of small streams and water level variation in wetland areas.
Tree ring studies have proven that information about past environmental conditions may be reconstructed and have revealed the many ways in which environmental factors affect the growth of a tree. Therefore, trees serve as natural recorders of their environment.
Take time to look up how tree rings are used in various studies to fully appreciate the importance of tree rings.
James L. Cummins is executive director of Wildlife Mississippi, a nonprofit, conservation organization founded to conserve, restore and enhance fish, wildlife and plant resources throughout Mississippi. The website is www.wildlifemiss.org.
