Tree rings, cores and age: what sources say

This page describes what the three named sources say. It does not estimate the age of any particular tree or advise anyone to take a sample from one. The calculator on the tree age estimate page covers a different method, trunk diameter and a growth factor.

What a ring is

NOAA's National Centers for Environmental Information (NCEI), in a news article published March 11, 2016, says a tree adds a new ring around its waistline each year, and that the rings record whether each year had good or bad growing conditions. The University of Arizona Laboratory of Tree-Ring Research (LTRR) describes the ring as made of two parts. In a conifer, it says, the earlywood is light in colour with thin-walled cells of large diameter, and the latewood is dark with thick-walled cells of small diameter. In a broadleaf tree, it says, the earlywood has vessels of large diameter and the latewood has vessels of small diameter. The lab lists three things that vary from ring to ring: total ring width, latewood width and latewood density, and says the variation comes from the environmental conditions when the ring formed.

Why the lab says counting is not enough

The LTRR defines dendrochronology as the dating and study of annual rings in trees. Its page says that ring-counting does not ensure accurate dating of each ring, that studies show it leads to wrong conclusions, and that dendrochronologists insist on assigning a single calendar year to a single ring. It shows two reasons with photographs. A locally absent ring is a ring missing from part of a tree, so one side of a sample can show four rings where another shows three. A false band can look like a ring but is told apart from a true ring by its cell structure. The lab says such samples can be dated, but not by mere counting.

The method it names is crossdating: matching ring-growth patterns across many samples from an area of similar conditions, which lets the exact year of formation be found for each ring. The lab says it most often uses skeleton plotting, marking a tree's ring-width variation on graph paper strips and matching similar patterns between trees. NCEI describes the same step: all cores are crossdated to make certain no rings are misidentified, so every ring is assigned to its correct calendar year.

What the rings are used to read

NCEI says trees that depend heavily on temperature in the growing season have narrow rings in cold periods and wider rings in warm ones, while trees that depend heavily on moisture have wider rings in rainy periods and narrower ones in dry periods. Ring widths are compared with weather-station data, and where the match is strong they can be used to reconstruct climate over the length of the ring record, which it says is often several centuries and occasionally a thousand years or more. It adds that older trees give the longest histories, and that dead trees and remnant logs can be matched to living wood by crossdating to extend the record. The agency says its Paleoclimatology Program distributes the data, and its tree-ring page describes the International Tree-Ring Data Bank as the world's largest public archive of tree ring data, with more than 5,000 sites on six continents.

The National Park Service (page last updated April 27, 2023) says the width, colour and pattern of rings can show whether a tree was thriving or struggling, that an especially wet year may give a broader ring, that a blackened scar can indicate a wildfire, and that other marks could point to an insect infestation. It also says logs in ancient and historic buildings preserve ring records, and that Park Service scientists in the southwestern United States used the dating technique to learn about ancient Puebloan settlements.

What the Park Service found in one age survey

National Capital Region ecologists read rings from 1,518 trees of 36 species with an incremental borer, a tool with a pencil-sized cutting tube. The Park Service compares the effect on the tree to common insect activity and to a medical biopsy. It says the study focused on oak and evergreen species because they give more reliable rings. The oldest tree cored was a white oak at Prince William Forest Park, 239 years old in 2022, and the average age of the trees cored was 90 years. The oldest tree at each park, from its table, was:

Park	Oldest tree cored	Age in 2022
Prince William Forest Park	White oak	239 years
Catoctin Mountain Park	Chestnut oak	197 years
Rock Creek Park	Yellow poplar	192 years
Wolf Trap National Park for the Performing Arts	White oak	180 years
Chesapeake and Ohio Canal National Historical Park	Chestnut oak	178 years
National Capital Parks - East	Chestnut oak	161 years
Harpers Ferry National Historical Park	Chestnut oak	158 years
Manassas National Battlefield	White oak	157 years
George Washington Memorial Parkway	Shagbark hickory	154 years
Monocacy National Battlefield	Chestnut oak	142 years
Antietam National Battlefield	Mockernut hickory	137 years

The Park Service also says the average age of a stand can mislead. Where one tree is found to be older than its neighbours, it may be a tree that stood through past farming, while the younger trees grew in as farmland reverted to forest.

Where this fits

The words for the layers a ring is made of are on the page about how a tree works. How a tree walls off an injury, including the rings added after it, is on the page about compartmentalization.