The Loom

Fall Foliage–The Backstory

i-b7090c64e10af27f948678221fb077e5-autumnleaf.jpg For those who may have come to the Loom after seeing me talking about autumn leaves on ABC News this evening, you can learn more about the science in these posts (plus this article I wrote for the New York Times).

Scientists continue to investigate why leaves change colors–check out this new post yesterday from Voltage Gate.

(I should also clarify that the damage leaves suffer in the fall can come from charged atoms within the leaf, rather than directly from the sun’s photons.)

Update: Link to the ABC news segment added.


  1. #1 Homie Bear
    October 30, 2007

    I read an essay by Stephen Jay Gould once where he talks about some fossilized leaves that are basically pressed in shale (slate? something) and when they are exposed to air, their pristine green color rapidly turns brown. I thought from oxidation. Anyways, that would be cool to watch.

  2. #2 Scott
    October 30, 2007

    Are the (red/yellow) chemicals present in the ground or roots prior to being pumped out to the leaves?
    A blanket of fallen leaves serves as a mulch, choking out weeds under the tree, as well as being one stage in nutrients being broken down and returned to the soil. Could some property (antioxidant? something else?) of the colorful compounds serve to either better choke out competing ground foliage, or promote certain fungi to break down the leaves, or the colored leaves just don’t decay as fast which again would help cut down on the competition.
    Many forms of life receive benefits (like getting other creatures to care for and feed them) simply as a reward for the energy they put into being aesthetically pleasing. Oh and those trees that provide us with yummy maple syrup, if they didn’t taste so good, we wouldn’t regularly drain them of their precious bodily fluids. But then again, its those same fluids that make them so great to have around.
    Has there been much research put into comparing the weather that led to known good or bad fall colors? This year, fall came on gradually with no real early freeze and the colors here left much to be desired, while last year the colors were just amazing, but I wasn’t really tracking the weather and even with that would be hard pressed to find a pattern with just that which yields especially good or bad colors – but I bet someone has thought of looking for a correlation there already.

  3. #3 Kilian Hekhuis
    October 31, 2007

    As I asked the first time this topic was discussed here, I wonder if any of the theories account for the difference between European trees and North American ones. Over here in Europe, three do not turn red. Some turn yellow or yellow brown, most just turn brown. The only red leaves here are from imported trees like the American oak. We have sun, and we have aphids. But no red trees.

  4. #4 Dave S.
    October 31, 2007


    There are in general four chemicals (or classes of chemical) responsible for leaf colouration – chlorophylls (greens), carotenoids (yellow, brown, orange), anthocyanins (red, purple), and tannins (brown). Brown oak leaves laying on the ground are coloured courtesy of tannins. All these pigments are produced in the leaves, and don’t travel from the roots to the leaves via stems and trunk.

    During Spring and Summer, leaves have both chhlorophylls and carotenoids in them. The amount of cholorophylls usually swamp the carotenoids though, so green is usually the dominating colour. As Autumn arrives and chlorophyll is reduced, the yellows and oranges of the carotenoids show through. It’s at this time that anthocyanins are also produced, inside the leaves. This is done in response to a diminishment of nutrients to the stems (especially phosphorus) which causes changes in the sugar cycle and leads to production of anthocyanins.

    Once on the ground its been my experience the colour fades quickly. There might be some chemical pro-fungal advantage, but the weeds are probably tamed by the leaves physically blocking light, and perhaps acids leaching as they decay. Pine needles are especially efficient at taking out other plants. The offspring have usually been sown by this time, so there’s probably little point in trying to help them out now. But I could be wrong there.

    As far as weather is concerned, what you need for good Fall colouration is sunny cool days and chilly (not freezing) nights. Vacillating weather tends to smear out the colour signal, making it not so impressive. In North America, peak colour starts in the north, and moves south over a few weeks. Here in Nova Scotia the peak is already been and gone.

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