How old is my tree? How to tell a tree’s age in the Northwest

How to tell how old a tree is: the methods compared

There are three ways to age a tree in the field, plus the paper trail. Rings are the closest thing to a birth certificate. Whorls work only on young conifers. Trunk size works on any living tree, but it gives you a range, not a date.

Sources: USDA Forest Service Urban Tree Database (PSW-GTR-253), Understanding i-Tree (GTR-NRS-200-2021) and Silvics of North America; University of Minnesota Extension; PNW 630.

Forest Service researchers ran into the same problem when they built the Urban Tree Database from more than 14,000 city trees. They dated trees from residents, city foresters, street and home construction dates, planting records and aerial and historical photos, and cored a sample in Queens, New York. They still called age “probably the least accurate metric” in the database.

Counting rings and whorls: the precise ways

Rings on a stump or core

Each year the cambium, the growing layer under the bark, adds a new annual ring of wood, as the Forest Service’s urban tree risk guide describes. On a stump, count from the center (the pith) out to the bark on a fresh, smooth cut.

A stump count gives the age at the height of the cut, not since the seed sprouted. Young trees take years to reach breast height: in the Inland Northwest, open-grown western white pine seedlings need about 8 years to reach 4.5 feet, according to the Forest Service’s silvics manual. A cut or core at 4.5 feet misses those years.

For a living tree, foresters use an increment borer, a hand tool that pulls a thin core of wood from the trunk so the rings can be counted; PNW Extension’s forest inventory guide names it as the tool for aging a tree. It has limits. In the Urban Tree Database work, the borer’s length and pockets of decay kept crews from coring some of the biggest trees, and the Forest Service risk guide treats a borer as an invasive tool for checking decay, to use only when there is no other way.

Branch whorls on young conifers

Young pines offer a shortcut. Pines put on a single flush of growth at the tip each spring and then stop, University of Minnesota Extension notes, and many conifers carry their branches in whorls, rings of branches around the trunk. Count the whorls, or the bare trunk sections between them, from the ground to the leader for an approximate age in years. Not sure what you have? Start with pine vs fir vs spruce.

Treat a Douglas fir count as rougher. The silvics manual says Douglas fir can push out a second, late-summer flush, called lammas growth, in the moister parts of its range or in years with unusually heavy summer rain, although most of each year’s growth still comes in the first flush. The method only works while the lowest whorls are still visible, which is why it suits young trees.

How to measure a tree’s diameter at 4.5 feet

Every size-based method, permit rule and forestry table starts from one number: trunk diameter 4.5 feet above the ground, called diameter at breast height (DBH). The Forest Service’s urban tree monitoring guide and PNW Extension’s inventory guide both use 4.5 feet, and Seattle calls the same measurement diameter at standard height (DSH).

1. Find 4.5 feetMeasure up from where the trunk meets the soil. A stick cut to exactly 4.5 feet, which the Forest Service field guide suggests, makes the height repeatable.
2. Wrap the tapePull a cloth tape snug around the trunk, square to the trunk and under any vines. Read circumference in inches.
3. Divide by 3.14Circumference ÷ π = diameter. A trunk 60 inches around is 19.1 inches across. A forester’s diameter tape does the division for you.
4. Note the height you usedIf a branch or bulge forced you higher or lower, write down where you measured so the next reading compares.

Height is a separate measurement with its own tricks; see how to measure tree height.

How old is a tree by circumference? The growth-factor method

The growth-factor method turns diameter into age with one multiplication: diameter in inches × years it takes the tree to add an inch. Everything rides on that factor, so this page builds it from the USDA Forest Service’s i-Tree model, which publishes its growth rates and where they came from.

i-Tree sets average trunk growth for open-grown trees at 0.58 centimeters a year for slow-growing species, 0.84 for moderate and 1.09 for fast, standardized to a 153-day frost-free season. Converted, that is about 4.4, 3.0 and 2.3 years per inch of diameter. The base rates came from street-tree growth studies in central New Jersey, the Twin Cities of Minneapolis and St. Paul, and Chicago, so they describe eastern and midwestern growth.

i-Tree then adjusts for conditions. It scales growth up or down with the local frost-free season, multiplies it by 0.56 for a tree with sun on only two or three sides and by 0.44 for a tree under a closed forest canopy, and cuts it in proportion to crown dieback.

Years per inch of trunk diameter. Source: USDA Forest Service, Understanding i-Tree (GTR-NRS-200-2021), tree growth methods, at the standard 153-day frost-free season. Years per inch = 2.54 ÷ (cm a year × crown light factor), computed by Westcoast-Tree.org.

Worked example: a Douglas fir in an open yard measures 60 inches around. Its diameter is 60 ÷ 3.14 = 19.1 inches. USDA’s PLANTS database rates Douglas fir a moderate grower, so 19.1 × 3.0 gives about 58 years, inside a fast-to-slow span of about 45 to 84 years.

Big trees strain the method in both directions. i-Tree slows growth once a tree passes 80 percent of an assumed maximum diameter, which it sets at 45 inches for species that grow taller than 60 feet, so a constant-rate estimate runs young for a tree that has stopped putting on much wood. Northwest conifers often blow past that limit: the silvics manual calls old-growth Douglas firs 60 to 72 inches across common.

Tree age calculator

Enter the trunk measurement, the species and the setting. The calculator shows the fast-to-slow range from i-Tree’s rates and where your species’ USDA growth class lands in it.

Sources: USDA NRCS PLANTS Database, species characteristics (growth rate, lifespan); scientific names from the USDA Forest Service FIA plant dictionary; years per inch from the matching i-Tree class (Rapid = fast), computed by Westcoast-Tree.org.

Why Northwest trees can beat the formula

Growth is regional. The Urban Tree Database built separate equations for 16 U.S. climate zones because the same species grows differently from one region to another, and its authors stress picking the equation for the right region. The Pacific Northwest reference city was Longview, Washington, where crews measured 994 trees of 22 species in 2001, from bigleaf maple and Oregon ash to Douglas fir and black cottonwood.

Those Northwest equations sit in the Forest Service’s online data archive rather than in the report, so they are not reproduced here. The report’s climate table shows why they matter: Longview logs 2,468 heating degree days a year against 4,436 in Minneapolis, the Midwest reference city (both in the report’s Celsius units), and i-Tree’s base rates come partly from Twin Cities street trees.

Portland’s heritage giant sequoias show how far off a formula can be. The city lists eight with measurements, 22.1 to 26.2 feet around, or 84 to 100 inches across, and notes that many of the city’s sequoias were “planted around 1900.” Run a 25-foot trunk through i-Tree’s rates and you get 223 to 418 years for a tree that, if it went in around 1900, is about 126.

The error can run the other way on hard sites. The silvics manual puts the inland Rocky Mountain variety of Douglas fir at 15 to 40 inches in diameter after 200 to 300 years, 5 to 20 years per inch, and says the coastal variety grows faster and much larger. A tree crowded in a grove or rooted in thin, dry soil can be far older than its trunk suggests.

So use a size-based age as a range. An open-grown yard tree on good soil west of the Cascades may sit at or below the fast end; a shaded, crowded or struggling tree can run past the slow end. Only rings settle it.

Other clues: records, old photos and heritage lists

For a planted tree, paper often beats a tape measure. The Urban Tree Database team used construction dates, noting that outside Queens its study streets were lined with trees of one age, planted when the neighborhood was developed. Their caution applies here too: planting records seldom go back more than 30 to 40 years, and trees are hard to pick out on aerial imagery from before 1990.

• The year your house or subdivision was built, for street trees and foundation plantings.
• Family photos, previous owners and neighbors who remember the planting.
• Historical aerial photos, for trees large enough to show from above.
• City heritage-tree lists, for the oldest and largest trees in town.

Portland keeps a public Heritage Tree list. City Council designates trees for age, size, type, historical association or horticultural value; there are more than 300, searchable in the city’s Heritage Tree database and map. Nominations are due May 1 each year, and designation takes about a year once a nomination is accepted.

Tip: Start your own record. The International Society of Arboriculture’s tree-values brochure suggests photographing your trees every year during the growing season; dated photos also make before-and-after comparisons easier for an insurance claim.

Does a tree’s age matter for permits?

Not directly. Cities in both states regulate by trunk diameter, which is one more reason to measure at 4.5 feet.

Age shows up only through designation. A Portland Heritage Tree can be removed only if it is dead, dying or dangerous, with the consent of the Urban Forestry Commission and City Council, and it needs a permit before any pruning. Other cities’ thresholds are in our guides to Oregon tree removal permits and Washington tree removal permits. This is a summary, not legal advice.

An old tree’s age matters less than its condition. ISA’s mature tree care brochure recommends inspecting mature trees at least once a year and notes that an experienced arborist can read past years’ twig growth for signs of decline; see what an arborist does.