Dendrochronology, the science that uses tree rings to index time, establishes a physical cross-reference for a time before photography, written records or human beings themselves existed. By reading the size, structure, color and variations of rings on a single tree, scientists can theorize regarding the history and climate of an entire area. The reason that tree rings provide so much information has to do with how the rings form.
As trees grow, their trunks and branches must increase in size to support primary branch and leaf growth. Secondary growth occurs in the cambium, the name for the layers of tissues between the pith, or old growth and epidermis, or bark, of the tree. Each cambium layer contains several smaller layers called xylem and phloem, which swell as they transport carbon dioxide and chlorophyll from leaves downward, nutrients back upward from the ground, and circulate water throughout the plant’s vascular system. Each cambium layer that forms constitutes a tree-ring.
One tree ring forms during each period of growth. Most trees have one period of growth each year, but some trees grow additional “false rings” due to climate or environmental factors that may impose a mid-season period of dormancy or cut growth short. Using other trees for cross-referencing, dendrochronologists can identify false rings. By counting true rings, scientists can determine the exact age of a tree.
As temperatures warm, the sun rises in the sky to warm the ground. Spring showers provide the water to carry chlorophyll produced by new leaves and nutrients collected by new roots. Large, moist cells form over the tightly packed old growth of previous years, and they form the tender, light-colored part of the new secondary growth. As xylem and phloem add more cells outward, an outer layer of the cambium, the cork, adds tissue to the epidermis, which must stretch, or may crack, as the trunk’s girth increases.
As spring passes into summer, heat increases and water availability diminishes; growth slows to conserve resources and smaller, thicker-walled cells create a darker cambium layer. As sunlight intensity decreases and days shorten, the tree prepares for dormancy by making even smaller, thicker-walled cells. As the growing season ends, the tree’s processes slow to allow the tree to survive freezing temperatures and drying winds. The progressively darker cell layer terminates when dormancy sets in, leaving a dense layer for the next spring’s earlywood.
Climatic and environmental factors create variations and irregularities in the ways trees produce their annual cambium layers. Strong winds create looping rings. Fire scars and injuries by falling trees or animals may result in imperfections that grow over year after year, forming dark irregularities or creases. Fallen branches may heal into knots as time passes. Dry-season rings grow smaller and darker than wet-season rings. Smaller rings may also result from shade cast by ancient trees that once surrounded the survivor. Each irregularity provides more information about the tree’s lifetime.