Monday, September 20, 2021

Fall Color Conspiracy

fall foliage

Conspiracy hypotheses, or theories as we like to call them, since “hypotheses” cannot be uttered without a lisp, seem to multiply unfettered these days, so I feel awkward birthing yet another. But you may be intrigued to learn that the wide spectrum of color in the region’s fall foliage is largely the result of a Depression-era stimulus project ushered in by the Hoover Administration.

We live in one of the few places on Earth where trees produce such a phantasmagoria of color. If you’ve been to Europe in autumn, or even out West, you know that the range of color is much more limited than here. Most green plants have varying amounts of yellow (xanthophylls) and orange (carotenoids) molecules, but you’d have to visit northern China to see anything close to the mélange of anthocyanins – that is to say, the burgundy, coral, crimson, raspberry, ruby, salmon, and scarlet hues – that the Northeast offers.

We’re taught in school that green chlorophyll masks pigments already within leaves. In fall, trees deposit wax between twigs and leaves to seal the vascular links. This destroys chlorophyll, exposing underlying colors.

But reds and purples are definitely not hiding beneath green chlorophyll. Anthocyanins are large, complex organic compounds which take a lot of energy for plants to synthesize. While relatively few tree species produce red fall colors sugar and soft (red) maples are renowned for their ruddy foliage. Some oaks produce deep scarlets, and dogwood and white ash can make intense red-purple hues.

Here’s my theory or hypotenuse or whatever: For millennia, fall leaves were mainly orange, with little red or yellow. During the Depression, President Herbert Hoover tried to attract wealthy European tourists, tasking the National Science Foundation with augmenting the leaf-color palette of New England. This partly successful initiative was called the Hoover Omnibus Anthocyanin and Xanthophyll project, or HOAX.

OK, to my knowledge, governments haven’t manipulated leaf color. But anthocyanins – reds and purples –don’t lurk inside green leaves, waiting for the big reveal when chlorophyll croaks.

Plants often invest in these compounds to protect emerging leaves in spring, as chlorophyll is vulnerable to UV-light damage in cool conditions. It’s chlorophyll suntan lotion. As foliage matures, plants quit making these expensive molecules. Early-season outlays make sense. But why do some trees spend energy now when they should be hoarding it for springtime?

Notorious for being frugal and pragmatic, trees don’t dip into their savings without good cause. Few hypotheses (theories are evidence-based, e.g. the theory of gravity; hypotheses are what jam the Internet) exist on why trees use precious reserves to shield chlorophyll while they’re hard at work making abscission layers to snuff the stuff. “Fall suntan lotion” seems an absurd explanation for the appearance of red color after chlorophyll is gone, because there’s nothing to protect.

Another idea is that a maple’s (for instance) red leaves change soil conditions to favor its species. Certain plant-made chemicals can inhibit growth rates or seed germination of competitors, something known as allelopathy. Problem is, anthocyanins aren’t very good at this. A truly convincing rationale has yet to be found.

I don’t know why trees make red in the fall, and an honest biologist will admit they’re not quite sure either. Conspiracy or mystery, I’m grateful for our autumn display.

An ISA-Certified Arborist since 1996, Paul Hetzler cooks up wild hypotheses in his spare time.

Above photo: Almanack file photo

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Paul Hetzler

Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.

You can read more of his work at PaulHetzlerNature.org or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World




5 Responses

  1. Boreas says:

    I wonder if the red pigments break down in the soil after leaf decomposition and this helps the tree roots or fungal network under ground to produce compounds for overwintering or sap production.

  2. Tim says:

    Sorry, Paul, but the NSF wasn’t founded until 1950. We all know the conspiracy occurred during the Truman administration.

  3. Wally Elton Wally Elton says:

    Certain maps seem to do the same thing.

  4. Kevin mckiernan says:

    Maybe they turn red because they’re angry or embarrassed…

  5. ADKresident says:

    I think it’s actually a right wing conspiracy and Trump colluded w/ both China & Russia to Make the Forests Red Again.

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