Beneath the Bark : Tree Rings Tell Many Tales

We’re all familiar with the obvious changes northern-latitude trees go through as winter approaches, but did you know that there’s more to a tree’s seasonal changes than autumn’s brightly-colored foliage?

Researchers at the Forest Products Laboratory (FPL) study the both the external and internal structure of trees, and FPL’s Center for Wood Anatomy Research notes in the Wood Handbook that changing temperatures affect far more than the crimson and orange hues of fall.

When a tree grows, the wood is produced one layer of cell divisions at a time — but we do know from experience that in many woods, large groups of cells are produced at the same time, and these groups act together to serve the tree.

Transverse sections of woods showing types of growth rings. Ring development in softwoods ranges from no transition (A) to an abrupt transition between earlywood and late wood (C). Hardwoods (D-F) exhibit a similar range. The arrows delimit growth periods when present.

These collections of cells produced over the same time interval are known as growth increments. Because of the tree’s internal biological processes, these increments are arranged into layers. More commonly, these layers are referred to as growth rings.

In temperate portions of the world (and anywhere else with distinct, regular seasonality) trees form their wood in annual growth increments. All of the wood produced in one growing season is organized together into the recognizable, functional entity of the growth ring. In many tropical woods however, growth rings are not evident, as their climate zones lack seasonality.

Woods that form distinct growth rings, and this includes most woods that are likely to be used as engineering materials in North America, show three fundamental patterns within each growth ring: no change in cell pattern across the ring; a gradual reduction of the inner diameter of conducting elements from the earlywood to the latewood; and a sudden and distinct change in the inner diameter of the conducting elements across the ring.

The orientation of these rings can effect the tensile strength and elasticity of a wood product, and industry professionals must take this into consideration when deciding how a tree should be used.

In addition, most know that by counting the annual rings, researchers can determine the age of the tree, but analyzing growth rings can also tell us about the environmental conditions present when they were forming, including moisture levels in the soil and air, temperature, and sunlight.

In larger trees, annual rings can represent decades, if not centuries, of growth.

Abnormal rings can also be linked to traumatic events in the tree’s past, like forest fires, disease, or climate events, and the rings become not only a record of the life of the individual tree, but of the forest and environment as a whole. Many other disciplines, like archaeology, can use this information (known as dendrochronology) to support their own research, making wood one of the best record keepers on the planet.

For more information, please see Chapter 3 of The Wood Handbook, Wood as an Engineering Material.