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.

Investigating the Structure and Function of Wood


The Centennial Edition of the Wood Handbook was published in 2010.

Wood has evolved over millions of years to serve three main functions for itself:

  • Conduction of water from the roots to the leaves
  • Mechanical support of the plant body
  • Storage of biochemicals

As a complex biological structure, a composite material of many chemical and cell types acting together to serve the needs of a living plant, wood is a versatile material suited to many uses.

As Alex Wiedenhoeft writes in Chapter Three of the Wood Handbook, there is no property of wood – physical, mechanical, chemical, biological, or technological-that is not fundamentally derived from the fact that wood is formed to meet the needs of the living tree. Wiedenhoeft is a research botanist at FPL, working within the Engineering Properties of Wood, Wood Based Materials and Structures group and affiliated with the Center for Wood Anatomy Research.

The three primary functions of trees have influenced the evolution of approximately 20,000 different species of woody plants, each with unique properties, uses, and capabilities, in both plant and human contexts. Chapter Three categorizes the biological structure of wood at decreasing scales, from the whole tree and tree types, to axial and radial systems, vascular cambium, growth rings, and cellular differences. Wiedenhoeft also examines the appearance of wood as sawn lumber in terms of color and luster, grain and textures, slope of grain, knots, and decorative features. Chapter Three wraps up with a section on the importance of wood identification from a scientific perspective.

The Wood Handbook’s Enduring International Appeal

The Wood Handbook – Wood as an Engineering Material is the Forest Products Laboratory’s most popular publication. From Anchorage to Key West, thousands of Americans download all or part of the Wood Handbook every month. Just as impressive is the international scope of interest in this instructive tome.


The Wood Handbook – A global wood resource.

The graphic here demonstrates the Wood Handbook’s enduring international appeal. For a full size version visit the FPL Flickr page. The island nation of Malyasia provides a strong audience as do the wood enthusiasts of western Europe, northern Asia, and the Middle East. Canada and Scandinavian countries, well known for vibrant timber industries, and South America also show strong interest in wood as an engineering material.

Citizens from about 180 nations worldwide have downloaded all or part of the Wood Handbook over the past 4.5 years for a total of 713,006 downloads. A Centennial Edition was published in 2010 with a popular new introductory chapter: Wood as a Sustainable Building Material.

Characteristics and Availability of Commercially Important Woods

“Throughout history, the unique characteristics and abundance of wood have made it a natural material for homes and other structures, furniture, tools, vehicles, and decorative objects. Today, for the same reasons, wood is prized for a multitude of uses.”

So begins Wood Handbook’s Chapter Two: Characteristics and Availability of Commercially Important Woods by Michael C. Wiemann, a research botanist at FPL.


FPL botanist Michael Wiemann

Variations in the characteristics and proportions of the building blocks of wood – primarily cellulose, lignin, and hemicelluloses – make woods “heavy or light, stiff or flexible, and hard or soft” writes Wiemann. To use wood most effectively in engineering applications, “specific characteristics or physical properties must be considered.”

Chapter Two covers timber resources and uses, describing the roughly 100 species available in the U.S. and 30 species commonly imported in the form of logs, cants, lumber, and veneer for industrial uses, building trade, and crafts.

A comprehensive Scientific Name Index – including latin and common names – is also available in Chapter Two, starting on page 2-41. The full index lists domestic and imported hardwood and softwood species names.

Wood as a Sustainable Building Material

The Wood Handbook is an unparalleled resource and has many practical and technical uses but, literally, it all starts here: Chapter One – Wood as a Sustainable Building Material.

WoodHandbook100As FPL research general engineer Bob Falk writes in the introduction to the chapter’s first section, Wood as a Green Building Material, “over the past decade, the concept of green building has become more mainstream and the public is becoming aware of the potential environmental benefits of this alternative to conventional construction.” From the benefits of improving a building’s energy efficiency to reducing negative impacts on public health, wood building products can help achieve many of these goals.

Green building is defined as the practice of increasing the efficiency with which buildings use resources while reducing building impacts on human health and the environment – through better siting, design, material selection, construction, operation, maintenance, and removal – over the complete building life cycle.

Chapter One covers topcs such as embodied energy, carbon impacts, and sustainability as well as forest certification programs like the Forest Stewardship Council, the Sustainable Forestry Initiative, and the American Tree Farm System, among others.