By James L. Cummins
Conservation Corner
Editor’s Note: This is the first of a two-part series about tree rings.
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 trees age can be determined
by counting the rings. Therefore, if the year of cutting is known, counting backwards 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, that 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. In our next installment we will look at how
trees serve as living historical records.
James L. Cummins is executive director of Wildlife Mississippi, a non-profit, conservation
organization founded to conserve, restore, and enhance fish, wildlife, and plant resources
throughout Mississippi. Their website is www.wildlifemiss.org.