When we talk about old-growth trees our minds may generate an image of a tree like this western red-cedar (above) or one of the mighty drive-thru California redwoods. The definition of old-growth varies depending on such factors as the species of the tree, the latitude at which it grows, geographical region, plant associations, soil productivity, elevation, and which federal or state agency you ask.
If you ask, as I did one morning, a slightly hung-over college student to guess how big an old-growth tree is, you might get an answer like this: "Big. Really big. As big as my brain is right now."
In Rare Bird, I spent a lot of time in the old-growth coastal forests of the Pacific Northwest tagging along with biologists who were studying the marbled murrelet, a seabird that nests in these forests. It was a constant challenge to avoid saying "big," "really big," and "wow." Which is understandable because an old-growth tree is awesome and we are appropriately left speechless or nearly so. But this does not help the marbled murrelet whose survival depends on our understanding of its habitat--the forests we describe in shorthand as "old-growth." Few of us--slightly hungover or not--are not likely to provide any details.
Until the 1980s, scientists were hard-pressed to come up with a solid definition of an old-growth forest. During the controversy over the protection of the northern spotted owl and the logging of its habitat (old growth? second growth?), a seven-page definition emerged and was then distilled into this working definition: a forest that "has been largely unmodified by timber harvesting, and whose larger trees average over 200 years old or greater than 31 inches (80 cm) in diameter at breast height." This gets distilled down even further to 80 cm dbh.
To complicate matters, for the purposes of a major scientific study on marbled murrelet nesting habitat on Washington state trust lands, marbled murrelet nesting trees were defined as being >48 cm dbh ( >19 inches dbh (2008 Science Team Report). Scientists have discovered that marbled murrelets will nest in trees younger than old-growth--trees described as "mature"--if these trees have the right size nesting platforms. The right size is 4 inches or greater and that size may occur in younger trees where nest branches are deformed/enlarged by mistletoe.
For the generalist-naturalist, 80 cm dbh is a good rule of thumb.
What does 80 cm dbh even look like? Does 32 inches dbh make it any easier? Sure--imagining a tree with a trunk 4 inches shy of a wooden yardstick creates a visual impression--but not one as huge as I expected. Was the Douglas-fir in my backyard old-growth? How was I going to measure the diameter of this living tree? For that matter, why do scientists measure a tree's diameter instead of its circumference? I wasn't going to wait for an answer. Nor was I going to wait for a blow-down and a chainsaw.
Seems like a lot of work to get an accurate measurement of a tree's diameter.
I converted 80 cm dbh to 251 cm circumference and then cut a piece of yellow flagging tape to measure that girth. FYI: 251 cm=98.9 inches, and, because we are friends, let's call it 100 inches. I went outside and wrapped by 100-inch tape around my Douglas-fir. A juvie! It was only 75 inches around. I put the yellow tape into my coat pocket and whipped it out last week during a snow-shoe foray in the foothills of the Cascade Range at White Pass (southeast of Mt. Rainier).
Tie a yellow ribbon 'round the old-growth tree--it's been 200 long years, will you still hug me? This hemlock's 100-inch circumference qualifies it as an old-growth tree--but is this tree habitat? Photo by N.D. Ruth
This magnificent hemlock meets the old-growth standard of 80 cm dbh--or 100 inches in girth. This tree is in a lovely but it is not in an old-growth forest. To be an old-growth forest, this tree would need to be accompanied by other similarly big living trees, big dead standing trees (snags), fallen trees (nurse logs), a multi-level canopy, and an understory of younger trees and shrubs. This tree above is in a stand of younger trees and is bordered by Hwy 12 and a cross-country ski trail on side and a lake on the other. Because this stand is in an area used for recreation, the forest here has been managed for public access and safety--not as wildlife habitat. This tree is 100 miles from salt water and too far inland to be marbled murrelet habitat (they will nest as far as 50 miles inland).
The forest below is within the 50-mile breeding range of the marbled murrelet but is it marbled murrelet habitat?
This is an old-growth stand of western red-cedar, western hemlock Douglas-fir, and Sitka spruce in Rainbow Falls State Park, in the Chehalis River Valley southwest of Olympia, Washington. ( Photo my MM Ruth)
This is Rainbow Rock State Park--a 139 acre park along the Chehalis River. Once surrounded by thousands of acres of old-growth forest, this is the remnant gem. The cedars, hemlocks, firs, and spruces all meet the 100-inch standard and the other qualifications in terms of snags, nurse logs, multi-level canopy, etc. But is this nesting habitat for a marbled murrelet? Unless scientific surveys of this forest document certain marbled murrelet behaviors (such as flying below the canopy height, landing on branches), this forest cannot be declared "occupied" habitat. But based on other factors, this forest could be declared "potential" habitat--a place where marbled murrelets could nest. Marbled murrelet surveys are expensive, time consuming, and labor intensive. And, they make people grouchy because surveys start well before dawn, which is really really early in the summer.
Because I am interested in getting to know the forests where I live, I have started taking my yellow ribbon with me when I go on a walk or hike. When I remember to also take my camera, I will start posting photographs here on my blog and and here.
Meanwhile, recommended reading: Old Growth in a New World" A Pacific Northwest Icon Reexamined, Thomas A. Spies and Sally L. Duncan, eds. Island Press, 2009.
Not recommended viewing unless you have 3'06" to fritter away on Tony Orlando and Dawn: This stunning "live" performance from the 1970s--the taste-free decade.