Fir Waves

Fir_SmallBy Gus Goodwin

I suspect there is a positive correlation between one’s appreciation for fir waves and one’s distance from them.  From a distance, fir waves etch a pleasing pattern on the landscape, pose interesting ecological questions, and remind us that turmoil can be a form of stability.  Up close, they inflict scrapes and puncture wounds, incite expletives, and remind us to plan the next vacation to California, where the mountains have no trees (and it hardly ever rains).

For now, let’s keep a respectful distance.  Fir waves are a pattern of forest regeneration (sometimes also described as wave-regeneration) found in sub-alpine forests.  Unlike more familiar disturbances, such as micro-bursts or insect outbreaks, fir waves leave a striking pattern of mortality—dead trees lying in a network of undulating rows separated by patches of living trees.

As the name implies, this phenomenon affects forests dominated by fir (locally, Balsam Fir, Abies balsamea).  However, despite the fact that the forest is nearly a monoculture of fir, the forest between the waves of dead trees is far from uniform.   The diagram below depicts the linear, predictable variation in age and height in a forest experiencing wave regeneration.  To understand the forces that drive them, we’re going to have to grit our teeth, roll down our sleeves, and wade in.

Diagram2

Figure excerpted from Sprugel, 1977.

The primary culprit behind this dynamic process is wind.  When a tree falls, it no longer protects its downwind neighbors from damaging winds.  These newly exposed trees now bear the brunt of the weather and quickly succumb, exposing yet another cohort of trees to damage.  This mortality wave advances through a forest following the direction of the prevailing winds and is trailed by a wave of regeneration.  To return to the above diagram, the wave is moving from right to left and it can be inferred that the prevailing winds also move in the same direction.

The speed that the wave advances through the forest is dependent upon the average speed of prevailing winds and is commonly between 1 and 2m a year.  This bracket is relatively strict, as faster average winds would cause enough damage that the forest would be replaced by krummholtz—gnarled, stunted trees—and tundra, while slower winds would not provide enough disturbance to initiate wave regeneration.

The short lifespan of balsam fir, coupled with the with advancing fir waves, set the average maximum age for an individual tree in one of these forests at about 60-70 years.  Although the individual trees in the forest may be young, that does not necessarily mean that the forest itself is young.  This process can be maintained for hundreds of years.

Stepping back to the comfortable distance enjoyed by academics, fir waves take on even greater intrigue.  In 1977, D. Sprugel published the first scientific paper describing the ecological processes that form fir waves.   In this award-winning paper, he introduces fir waves not just as an ecological curiosity, but as a direct challenge to prevailing forest succession models.  At the time, the steady-state climax community model held a lot of weight in the scientific community.  In this model, forests follow a predictable pattern of species dominance following a disturbance, with pioneer species giving way to climax species that result in a terminal, stable community.  Fir-waves are anything but stable.

They are also anything but common.  Fir waves are found only on a handful of mountains in New England, Newfoundland, and strangely enough, Japan.   Unfortunately, Vermont doesn’t make this list.  If you want to take a first-hand look at a fir wave, you’ll have to leave the Green Mountain State for the nearby Adirondacks in New York or the White Mountains in New Hampshire.  Just don’t get too close…you might find yourself wallowing through a fir-wave, contemplating that move to California.  If you do, remember that in some ways this experience is as quintessentially New England as white-steepled churches, Puritans, and maple syrup. That’s what we’re here for, right?

Sprugel, D. 1977.  Dynamic structure of wave-regenerated Abies balsamea forests in the north-eastern United States. Journal of Ecology 64: 889-911.

In an episode from his life prior to the FNEP program, Gus Goodwin lead a group of hearty 12-year-olds on a bushwhack through countless fir waves on a remote Adirondack mountain.  After crawling on hands and knees for hours, the group arrived at the summit with big grins, proving the fact that you don’t always need to have fun to have fun.