Tree Rings: How They Form and What They Tell Us
It’s well-known that tree rings show a specimen’s age, but they also contain other kinds of historical data about a tree’s lifetime and growing environment. In this article, we’ll go over how tree rings form, what they can tell us, and how that information can help us best manage Nashville’s individual trees and collective canopy.
How Are Tree Rings Formed?
As trees grow tall by extending their branches, they also grow wide to support their height. Trunk thickness is achieved by growing a new layer of wood under the bark each year. This fresh layer of wood is the tree’s xylem, which is the tissue that transports nutrients and water from the roots throughout the tree.
In the spring, when there is abundant light and moisture, the tree’s xylem grows larger and wider cells to help move nutrients and water more efficiently, growing more quickly and utilizing the season’s available resources. In the fall, with less light and cooler temperatures, the xylem cells are smaller and more compact since the tree is slowing down for the dark winter months.
The difference in the density of the xylem over the year creates the pattern that can be analyzed for age and other information. Since these rings can be made of very small layers, people have to use microscopes and chemical treatments to make rings more visible for analysis.
The resulting two-toned ring can show a single year’s growth, with the thickness of each line varying based on rainfall, nutrient availability, and other environmental factors that affected the growth of a tree during a season.
How Are Tree Rings Examined?
When a tree gets cut down, the interior of the trunk is exposed, and the rings are visible and able to be counted very easily. However, trees are most valuable and beneficial while they are standing, so if a tree’s exact age needs to be known, an arborist can use a tool called an “increment borer” to extract a small core sample and examine it under a microscope. A tree’s age might be able to be approximated by its size alone, but the closer the look, the more information can be found.
What Do Tree Rings Tell Us?
Tree rings have proven to be a useful scientific tool. “Dendrochronology” is the study of a tree’s ring record, which, in addition to showing its age, can contain data about environmental factors such as temperature and precipitation in a given year, as well as disturbances a tree or an area may have been exposed to.
For example, wide rings show favorable growing conditions such as warm and wet weather, while thin rings can demonstrate cool temperatures or drought or other adverse growing conditions. Broken rings, misshapen patterns, or even missing rings may be evidence of damage from insects, disease, climate extremes, or another type of harm or stress.
"Dendrochronological drill hg" by Hannes Grobe, Alfred Wegener Institute, licensed under CC BY 2.5.
Tree rings can also contain particles from pollution or wildfires that a specimen was exposed to, which can help scientists better understand local climate and geological history (including the water cycle, solar events, landslides, and river flows).
In addition to providing a historical record of a tree’s life and environment, wood rings can help us date historical structures and objects made of wood, like buildings and boats, and even better understand historical events whose written record may align or be supported by tree-based evidence (such as human settlement patterns or significantly impactful weather events like devastating drought or a natural disaster). Timelines can be built by lining up tree rings of trees of various ages from the same area, which all would have been exposed to similar conditions in a given year.
How Is Tree Ring Information Useful for Landscaping and Tree Care?
Knowledge gained from ring analysis can help tree professionals and property owners make decisions about trees in the landscape.
Determining Tree Health: If a tree shows signs of decline, knowing how long the decline has been going on or how severe can help guide decision-making.
Landscaping Decisions: If a homeowner is planning major landscaping changes, understanding the tree’s age can help determine how much care or special treatment it might need.
Legal or Insurance Reasons: If the tree is involved in a dispute (such as property damage or legal matters), knowing its age with certainty might be important for evidence or insurance claims.
Tree Preservation: Some mature or historic trees may be protected by law, and knowing the tree's exact age can help ensure it is being properly preserved or managed.
Planting Strategies: Tree ring studies help identify species that thrive in urban conditions by analyzing growth patterns under different stressors (including pollution exposure and drought). They also guide site selection by showing which areas support healthy, long-lived trees.
Long-Term Tree Care
Knowing the age and resilience of a tree species or specimen helps with understanding its ecological role and care needs, both of which support us in managing trees for health, productivity, and longevity.
Healthy, mature trees produce the most benefits, and their ability to grow up strong starts at planting! Browse the Nashville Tree Conservation Corps’ tree sale for a range of hardy species to plant and care for in your yard. You can also donate a tree to be planted, sign up to volunteer and help us plant, or make a financial donation to NTCC to support our mission of canopy care!
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