If you take a close look at New Hampshire’s Mount Moosilauke, or the high peaks of the Franconia and Presidential Ranges, you can see gray bands of dead balsam fir trees within the green swath of live balsam firs below the summits.
On these mountains, the bands of dead trees are clearly crescent shaped and ripple across the side of mountain like a snapshot of waves lapping a beach. The bands are called “fir waves,” and they are not as still as they appear.
Describing a fir wave is tricky. There’s a chicken-and-egg problem of where to begin.
Let’s start here: a fir tree, exposed to excessive wind and rime ice, declines and dies. This exposes its neighbor to high wind and ice. And so on. After these trees die, new firs grow back where the dead trees fell. The cycle repeats itself again and again.
The tallest trees are on the downwind side of the wave. They are either dead (and not yet fallen) or dying. As each rank of firs falls, it exposes the next rank of trees to the brunt of the weather. Then new trees grow in the debris of the fallen old trees, sheltered from the wind by the deadfall and taller trees around them.
You can think of it as being something like the wave done in sports stadiums, where people stand up, throw their hands in the air then sit down in a synchronized pattern around the stadium. In the case of fir waves, the wave moves in the super slow-mo that is the life of a tree.
A fir wave moves across a slope, away from prevailing winds, at the pace of three to nine feet a year. One particular spot of ground will go through the entire fir wave cycle, from fir seedling to decayed tree, in about 60 years.
The phenomenon was first studied in Japan, where it is called “Shigamare,” after Mount Shigamare, “the mountain with dead stripes of trees.” Later it was observed in the northeastern U.S. and Newfoundland. These are the only places in the world where fir waves occur.
Of course there are patches of dead evergreens on mountainsides everywhere. But few of those show the distinctive wave pattern across large areas. Fir waves only occur when large tracts of unbroken fir forest are found at high elevations and are subject to high winds. In the Northeast, the firs are balsam firs. Two other fir species are involved in Japan.
Fir waves are found at about 11 sites in New Hampshire; they are also found in Maine and in New York’s Adirondack Mountains. There don’t seem to be any fir waves in Vermont.
Paul Schaberg, a plant physiologist with the U.S. Forest Service, says Vermont may have the occasional fir ripple, but no major waves. He’s seen pockets of fir decline that are reminiscent of small fir waves but doesn’t know their cause.
Charlie Cogbill, a freelance ecologist who has surveyed fir waves in New Hampshire, says that fir waves on Esther Mountain in New York and Mount Lafayette in New Hampshire can be seen from Vermont, but that is as close as Vermont gets to fir waves. Vermont mountains lack the elevation and the extensive, unbroken tracts of fir forest that the phenomenon requires.
Scientists are still debating the details of what causes the fir trees to die, says Cogbill. Clearly, wind plays a big role. But does the death blow come from wind-driven rime ice, which is cloud droplets that freeze on contact? Or does the wind itself rock the firs’ roots, loosening their grip on the earth, then toppling them? Both factors are probably at work.
Whatever their causes, fir waves create diverse habitats in what would otherwise be a uniform forest of fir trees. Plants and animals take advantage of this diversity, most notably birds.
Fir waves provide important habitat for Bicknell’s thrush, which has one of the most restricted breeding ranges of any bird in North America. These birds nest in dense thickets of small evergreens near the tops of mountains in the Northeast. Fir waves provide them with a never-ending supply of young firs. Magnolia warblers also nest in the fir waves’ re-growth.
The standing dead firs provide a nesting place for black-backed woodpeckers. Black-backed woodpeckers are uncommon and in our area and are at the southern edge of their range. After a black-backed moves on, red-breasted nuthatches and boreal and black-capped chickadees will move in and nest in the cavities hollowed out by the woodpecker.
Once scientists believed that when left alone, a forest reached a climax state, then remained mostly unchanged until it was disturbed. Fir waves helped amend the notion of forests as fixed systems. It seems that in a fir wave, the only constant is change.