theories of causation, and more photos
reposted from Summitpost:
[Summitpost member] BobSmith noticed, in one of my photographs, a peculiar wavelike pattern of alternating bands of living and dead evergreens (spruce or fir, I wasn't sure which at the time). I also spotted it in some (but by no means all) of my other photos of New England evergreen forests.
The photos that best show the phenomenon are
here,
here, and
here.
I Googled around and discovered that the phenomenon is called "fir waves", and seems to exist only in New England (and upstate New York)
and Japan. At first I found only passing hints about theories of of causation.
The first theory posited that the wind caused blowdowns in a pattern like ripples on sand. The ripple anology appealed to me, but this theory was obviously wrong, since you can tell even from my photographs that the dead trees die while still standing.
The second theory said that rime ice buildup killed the trees. I liked this since it makes sense that rime ice buildup would be dependent on wind.
Here's BobSmith's response to that idea:
That explanation doesn't seem to make a whole lot of sense to me. Why would tree mortality move in such an even, linear fashion?
It's no surprise that tree death and regrowth are "patchy". There is room for new trees to grow only when space is made by storm, fire, disease, etc., and it's rare for such things to affect one tree in isolation. Furthermore, one dead or falling tree
can affect its neighbors by making a hole in the "canopy", changing the amount of sunlight, wind, etc they are exposed to.
Fir trees are extremely tall and skinny, to compete for height in a crowded forest; suddenly being exposed at the side can weaken and kill them. So any disturbance has a (slight) tendency to spread outward. Meanwhile, regrowth will begin within the clearing, creating a "wave" of regeneration such as you sometimes see in Conway's Game of Life (aka cellular automata).
What needs explaining is why these disturbances sometimes take the form of parallel lines, instead of just random patchwork such as you see
here. For that, you need some sort of external influence on the direction of wave propagation. In other words, you need some external factor that can cause a canopy hole to spread, but only in a particular direction. (Furthermore, this factor must affect some areas of forest more than others, to explain why North Twin has clear fir waves and South Twin doesn't.) Prevailing winds are a good candidate. (North Twin presumably has more exposure to the north wind.)
This background info from an online ecology course nicely summarizes a 1976 paper by Sprugel (which I can't find online), which states that the direct cause of tree death in these waves is rime ice. Trees exposed at the downwind edge of a hole experience 50% more wind, and correspondingly more ice gets deposited on their needles (causing mechanical damage). (Summer wind exposure plays a role too: evaporation is higher, so the trees can be starved for water.) The hole travels in one direction, because only the trees downwind of the hole are exposed. The hole also spreads sideways as it travels, forming a broadening wave. The result over time (a wave represents a 60-year lifespan of tree growth) is a series of "ripples" such as you see in the photos.
A rather more technical paper touching on the subject is
here.
