Friday, November 2, 2012

Autumn Contrasts

Once in a while, deference to the season is good for us…

I just returned from the Manchester, NH, ISCC meeting, and from some leaf-peeking in the windy (in both senses) corridor from Saratoga Springs, NY to Manchester. Based on the season and on the picturesque meeting venue, I thought it might be time to talk about autumn colors, in a slightly more speculative vein than Mark Fairchild’s Metameric Blacks column two autumns ago (see ISCC News # 448 p. 5). The dance of autumn leaf colors as a kind of co-evolution among species seemed sufficiently provocative. Consider the words of Michael Pollan [1], "We’re prone to overestimate our own agency in nature. […] In a coevolutionary relationship every subject is also an object, every object a subject. That’s why it makes just as much sense to think of agriculture as something the grasses did to people as a way to conquer the trees."

How do autumn leaves participate in co-evolution? For one thing, there’s foliar fruit flagging [2]: bright leaf colors call attention to berries containing seeds that birds and other animals can disperse (by the usual “indoor” route). And there seems to be a code whereby insects recognize which trees will tolerate them as parasites through the winter.  That code is mediated by chemicals called anthocyanins (typically red), which unlike yellow and orange carotenoids, are not present in a leaf throughout the year but appear in particular plants at the time of chlorophyll depletion. The balance between anthocyanins and carotenoids leads to quite a large palette of colors that could guide an insect or other color-perceiving animal. In addition to waving a “red flag” at aphids and other parasites to repel them, the anthocyanins in maple trees also seem to stunt the growth of nearby shrubbery, hence adding still more survival potential to the maple [3]. 

The trouble with this interesting dance is that it is hard to prove the cause-and-effect relationship of co-evolution. Oddly, one counter-question is “Do the dancers mean it?”  It doesn’t take much metabolic expense for maples to make anthocyanins, because the pigments evolve from direct reaction between sunlight and other leaf chemicals when the chlorophyll depletes and with it the phosphate supply. So maybe the trees aren’t intending the color code, and are therefore the signal isn't honest [4]---or, one might say, is insincere. In counterargument, the maples are paying an opportunity cost in shedding their chlorophyll early, thereby allowing the red to happen when there is a maximal contrast with surrounding green plants.

Faced with learned arguments about the sincerity (or lack of it) in plants, I have to retreat to the more familiar territory of vision science. Fortunately, my Manchester trip rewarded me with a vision science epiphany in a steam bath in Saratoga Springs. Upon entering the steam bath, I was unable to see anything but white steam. But in a minute or so, some ghostly shapes emerged and I could see other occupants and even a vacant place to sit down. The light was such that I might even have been able to read a newspaper---or maybe just the headlines!  Such adaptation to fog (low contrast) was not exactly a textbook visual effect. It even raised some interesting questions about a paper I was to co-author with Rob Carter, in Manchester (“The incredible lightness of the power law”).  The Weber-law behavior we were describing had some interesting experimental quirks, but none exactly like the fog-adaptation I was now experiencing.

Upon returning to New Jersey, I found a 2005 CIC13 paper by Mark Fairchild (yes, the same Mark Fairchild) called “On the salience of novel stimuli: adaptation and image noise.” Adaptation to persistent features of an image (such as blur, horizontal striations, or point-like noise) seems to be similar to chromatic adaptation, but it affects spatio-temporal as well as color channels. Could this kind of mechanism be responsible for fog adaptation?  I must defer any answer, or risk missing the deadline of this column.

Meanwhile, the rest of my trip (after the steam room) amply confirmed that fog-adaptation is not a universal law. After a spectacular sunlit view of fall foliage in Saratoga Springs, we drove across Vermont and New Hampshire in a dismal, fog-beset drizzle. The brightest anthocyanin in the world could not have provided signal or cheer in that miasma. And it didn’t get better with the passing hours.

After a while I gave up, and---lo and behold---the leaves had fallen. I had missed my opportunity. If I had been a bug or a squirrel, I would have fallen all over the dance-floor of co-evolution, unable to reap its benefits but completely sincere.

Michael H. Brill

1. Pollan M, The Botany of Desire: A Plant’s-Eye View of the World. Random House, 2001, p. xxi.
2.  Stiles, EW, Fruit flags: two hypotheses. The American Naturalist 120, 500-509 (1982). See also
4. Archetti M, Brown SP, The coevolution of autumn colors. Proc. Roy. Soc. Lond. B 271, 1219-1223 (2004),

No comments: