The late Dr. Alex Shigo, author of accessible books on trees and former Chief Scientist of the U.S. Forest Service, described trees thusly:

“Trees are superior survival organisms. They live longer, grow taller, and become more massive than any organism ever to inhabit earth.” 

Dr. Alex Shigo

In short, trees are tough. Street life for a tree involves air pollution, yard sale signs, climbing kids, tire swings, knife carvings and car accidents. In the woods, trees endure drought, floods, fires, deer stands and enumerable pathogens. Regardless their environment, trees document their travails in their annual growth rings. These concentric increments, made of light and dark cells, indicate the amount of wood added in one season.  

The mechanics of trees, so orderly and resilient, fascinate. Let’s take a moment to peel back the bark and appreciate what comprises a tree and its growth rings:

• Bark: this outer layer protects the tree from extreme temperatures, certain bugs and diseases, and even fire.
• Phloem: also called the “inner bark,” the phloem transports sap produced in the leaves down and throughout the tree. Compare this role to that of the sapwood (xylem).
• Cambium: amazing layer of thin cells that produces inner bark (phloem) on one side and sapwood (xylem) on the other.
• Xylem: sapwood conducts water and necessary nutrients up the tree, versus the phloem which transports ready sap down the tree. When looking at the cross-section of a tree, the relatively youthful sapwood is lighter, partly because it has more water than the…
• Heartwood in the middle of the stem, which is comprised of older, inactive cells that provide structural strength. 
• Pith: central core of the tree and in twigs.

In most of North America, the growing season for trees starts in the spring when the cambium produces larger cells with thin walls that form lighter-colored “springwood” or earlywood. Growth slows later in the summer when trees produce the smaller and thicker walled cells that for the “summerwood” or latewood that is the darker part of each annual growth ring.