Conspiracy of Leaf Color-Change

[Paul J Hetzler]

Fall Color Conspiracy

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 project implemented 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 kills chlorophyll, exposing underlying colors. But reds and purples are definitely not hiding beneath green chlorophyll.

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 here, 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.

Anthocyanins are large, complex organic molecules which take a lot of energy for a plant to synthesize. While relatively few tree species produce red fall colors, sugar and soft maples are renowned for their ruddy foliage. Some oaks produce deep scarlets, and dogwood and white ash can make intense red-purple hues.

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 dying chlorophyll while they’re hard at work making abscission layers to kill said chlorophyll. “Fall suntan lotion” seems an absurd explanation.

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 just grateful for our autumn display.

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