All You Need to Know to Age a Tree (It's More Than You Think)

I am often asked to estimate the age of a tree.

There it is, standing proud. It has a certain girth and a certain height relative to those around it. Its growth proceeds at a certain rate, right? It should be basic math. Spoiler alert: Trees are travelers in a country largely undiscovered by us. Take two Douglas-firs I recently aged with certainty, thanks to a friend with an increment borer.

The first stands on the edge of a bluff at Johnson South Sound Preserve, on the west side of the KP, just north of Devil's Head.  It’s a nice tree, maybe 40 inches in diameter, tall but nothing extraordinary about it other than the vista its high limbs must have over Case Inlet. When the borer extracts a fragile rod of banded wood, we are amazed. The sample did not reach the pith, and we count 170 rings. This is old growth, alive before logging began on Puget Sound. It’s a dry spot, exposed to the worst windstorms, and the tree’s hard-packed rings reflect a brave existence.

Then last month we are in Rocky Creek Preserve, north of Vaughn, side-hilling down from an attempted foray over a ridge that turns out to have a massive wetland perched on top. A very large tree looms above the woods. It has the deep bark of old growth. I measure it at 5 feet in diameter and 180 feet tall. Very few trees on the peninsula are that tall. We take a core. We are shocked. The tree is only about 120 years old.

Not old growth at all. Some of its rings are as wide as my fingertip. Between the roots of the cedar next to it, water trickles. We realize there are springs all over this hillside. The perched wetland must release water all year. Nearby, we find the largest western hemlock I have yet measured on the Key Peninsula, over 4 feet in diameter and 164 feet tall.

I asked Matt Provencher, service forestry program manager for the Washington Department of Natural Resources, how confident he is guessing a tree’s age, and he told me that over his 21 years as a forester, he has become much more cautious. There are many variables.

There is basic math behind a tree’s growth rate, and when you look at a stand of trees, especially if it’s an even-aged Douglas-fir stand, you can make a rough guess of its age. You just need to calibrate your guess with an understanding of the site’s quality.

The forest industry has researched this extensively. The productivity of a forest is dependent on many things: water, soil quality and depth, slope, aspect, elevation, precipitation, temperature, latitude. Instead of attempting to test and mesh all of these variables, foresters turn to the site’s real-world expression of those variables: the trees themselves.

The result is a series of growth charts, called site index curves, that function much like pediatric growth charts. Each stand is assigned a site index number, which is the height of a typical Douglas-fir on that site at 50 years old. In a stand with a site index of 110, which is right about average for the Key Peninsula, a 50-year-old Douglas-fir will be 110 feet tall. You can slide your finger back and forth along that curve to various ages and heights.

Foresters determine a site’s index by measuring and coring a few representative trees. For those of us without fancy tools, it’s a little less precise. But site index numbers are grouped into broad classes that can be generally mapped in a region. Class I sites are the best, with deep soil and the power to grow towering trees. Class V sites are the poorest. Most of the Key Peninsula is Class III, with some pockets shading into Class II. You can grow great trees here, but our shallow, rocky soils with random hardpan mean we’re far from a premier timber-producing region.

Follow the index curves for Class III sites, and you’ll see that our Douglas-firs may reach 170 feet after 120 years. In practice, I have found that the trees on most properties top out a bit shorter than that.

A fir’s growth continues after its first century of life, but it’s far slower. A 200-foot tree on the Key Peninsula, if it exists — and I’m determined to find one — is likely to be centuries old. The tallest I’ve yet measured is about 185 feet tall. It is a magnificent ancient tree. Its neighbor, a fir of similar girth that fell in the 1990s, was over 400 years old.

I hope I live to 100 so I can see just how big that spring-fed Rocky Creek fir gets.

Growth curves are all well and good, but trees are plants, and plants grow differently than animals. Their growth is nonspecific: in addition to following a certain blueprint — adding a whorl of branches each year around a central stem, in the case of our firs and pines — their forms also react to and reflect the spaces where they grow. It’s less like the physical growth of a child, I suppose, than a child’s emotional growth, dependent as it is on conditions.

Especially light. Recently, a landowner showed me two grand firs he had planted a few decades ago. One was just inside the woods; the other just outside. The one under forest canopy was a compact, thin-foliaged thing of maybe 10 feet. The one in the sun was easily three times as tall. It looked like a completely different species.

The DNR guide to identifying old growth has a photograph of a totally light-starved 6-foot-tall western hemlock. It has a flat, spider-webby crown and is known to be 210 years old.

Examine the cut end of a log, and sometimes you will see a sudden change in its rings, from tightly packed to widely spaced. This usually represents a sudden influx of light. Maybe the tree’s neighbor fell, and it shot up into the light.

One of the distinctive features of an old-growth fir is the irregular, fat, twisted quality of its limbs. Writes Robert Van Pelt in the guide, “As a Douglas-fir tree ages, it transforms itself from a simple, whorl-based growth form, into a highly individualistic shape.” Such a shape is a reflection of the slew of forces that act upon a tree and how they shift over time — turns out the slings and arrows of outrageous fortune really add up when you’re in your mortal coil for centuries.