What Exactly Are Growth Rings?
I received an email with an interesting picture of a small piece of cedar that was approximately 1.25 inches thick. It was purchased many years ago for a toy that they built for their son and when they recently stumbled upon a piece of that same wood, they cleaned it up and polished it to see just how many growth rings were present. The sender said they counted 176 growth rings per inch in that small piece of wood. What exactly is a growth ring and what can they tell us about the tree it came from?
Growth rings are formed on an annual basis by trees growing in temperate regions. During the cold winter months, activity in the cambium layer of trees is inactive. The cambium is the region of a tree where cell division takes place that adds new wood and increases the diameter of a tree. In the spring when temperatures begin to warm up, activity in the cambium will be reactivated. Increased temperature is one of the most important factors related to the beginning of cell division in the spring. It is also believed that hormonal signals from the tips of the stems play a role in the onset of cell division in the spring.
The exact mechanism that triggers growth to stop in the fall of the year is a bit of a mystery. The decreasing length of days has been shown to be a factor as well as freezing or near-freezing temperatures. However, some trees will stop growth before temperatures are near freezing and days that are short enough to induce dormancy. Studies, where trees were artificially exposed to light to simulate long days, showed that growth can be extended but it will eventually cease. This suggests that growth inhibitors created by the tree may play a role in inducing dormancy in trees, but more research is needed to be conclusive.
When cell division is initiated in the spring it proceeds rapidly and then begins to slow down in late summer and finally completely stopping in the fall. This pattern of growth forms one growth ring per year. This also creates two fairly distinct zones within each growth ring which are called earlywood (or springwood) and latewood (or summerwood). Earlywood tends to be lighter in color than the latewood.
The reason for the difference in color can be seen at the microscopic level. If a growth ring is examined under a microscope the earlywood portion of the growth ring will be seen to have thin cell walls. At this time of the growing season, the bulk of the sugars being used for growth are coming from stored reserves that are found in the tree such as the roots. As the leaves of the tree become completely formed and photosynthesis is in full swing and creating lots of sugars, the cell walls begin to get thicker due to the abundance of available sugars. This creates the darker color found in the latewood.
Some hardwood trees such as oak and ash do not have a distinct color variation between earlywood and latewood. Instead, the growth ring is characterized by large, open pores in the earlywood followed by much smaller pores in the latewood. There will be more fibers in the latewood of hardwood trees than in the earlywood and the latewood has a higher density than the earlywood due to the thicker cell walls found there.
What kind of information can we gather from looking at growth rings? The most understood piece of information that can be gathered from growth rings is the age of the tree. We know that in the temperate regions one growth ring is created annually so by simply counting each growth ring an accurate age can be determined for the tree. Remember to count the early and the latewood portion of each growth ring as one year, not two.
Occasionally a tree may create a false ring. A false ring can occur when an event such as a severe drought or insect defoliation occurs. If the event is severe enough to cause the formation of latewood cells earlier than normal, and then the conditions change within the same growing season that cause growth to resume to normal, it is possible for the tree to develop a false ring. When the growth returns to normal it could start with the rapid growth of thin-walled earlywood cells first and then progress normally into the latewood cells. In a false ring, you would have two earlywood and two latewood portions in one growth ring. False rings are often very small and hard to depict without a microscope.
Another piece of information we can gather from growth rings is how fast the tree grew. The diameter of a tree is not a good indicator of the age of the tree. For example, it is not unusual to have a tree that is 16 inches in diameter next to a tree of the same species that is perhaps 8 inches in diameter that are the same age. If the growth rings of both trees were examined, the smaller tree will have growth rings that are much smaller than the larger tree. This is related to the amount of sugar the tree is able to produce. The larger tree outcompeted the smaller tree and decreased the amount of sunlight it could get due to its crown creating shade over the smaller tree. This in turn caused slower growth. Meanwhile the larger tree has its crown above the other tree and is getting full sunlight and producing lots of sugars which allows it to make larger cells and grow faster. Forest management activities can help increase the growth of favorable trees. Even something as simple as a tree dying in the woods can increase the growth rate of other trees nearby.
Growth rings can affect how a board will look and perform when made into products. When a board is sawn in a way that the growth rings are parallel to the width of the board it is said to be flat-sawn or plain-sawn. This can give the face of the board a cathedral pattern where the outline of the growth rings create high arches in the board.
Rarely are the growth rings perfectly parallel to the width of the board but will have some amount of curvature to them. A flat-sawn board will always cup toward the direction of the bark. Woodworkers will often alternate the direction of the growth rings when gluing up a panel so that the forces will counterbalance each other and hopefully keep the panel flat.
A board sawn in such a way as to have the growth rings parallel to the thickness of the board is said to be quarter-sawn. Boards that are sawn in this manner have a distinct look. The edges of the growth rings appear on the face of this type of board and creates a very straight-lined appearance. Depending on other characteristics of the species that the board was sawn from, features such as ray flecks may appear on the surface which can be a highly desirable look. White oak is known to have a very distinct ray fleck that shows up when it is quarter-sawn. Quarter-sawn lumber has a much lower tendency to cup and is often used in applications where the cupping must be kept to a minimum.
Growth rings seem like a simple component of a tree that people use to determine the age of the tree. In reality, the processes that go into creating a growth ring are very complicated and not completely understood. They affect the density of the wood that comes from the tree and can even affect how a board looks and how it will warp. The next time you examine the growth rings on a board or a stump, take a little extra time to analyze what they are really showing, just like the person who emailed me with questions about a cedar board with 176 growth rings per inch did.
