October 11, 2006

A Festival of Leaves

Bundle up and step outside, into the forest. Kick some leaves. Gather your thoughts. Breathe. The leaves have changed.

Poets and artists have pondered the autumn palette, and there is no better place in the country, perhaps, to take in the beauty of October.

It is fleeting, however, as Frost notes:

Nature's first green is gold,
Her hardest hue to hold,
Her early leaf's a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.

But exactly why leaves change color in the fall is a mystery deeper than any poem has grasped. It also could change the way we think of fall.

For most of the twentieth century, it was believed that autumn leaves were nothing more than a biological wastebasket of sorts. As the light and temperature in our hemisphere begins to decrease, the tree begins to shut down, reabsorbing nutrients from its leaves. A woody cork membrane grows between branch and leaf, cutting off the circulation of nutrients, and the trees stops producing chlorophyll. (Retaining leaves during the winter is not energy efficient for broad-leafed trees.)

Think back to BIO 101. Chlorophyll is a pigment not only responsible for the green color of plants, but also for the production of sugars from captured sunlight. Chlorophyll is also a very unstable compound, constantly broken down by the energy from the sun. So when the trees stops circulating nutrients, the chlorophyll breaks down completely, allowing the tree's other pigments to reflect reds (anthocyanins) or yellows (carotenoids; see sidebar).

Carl Zimmer says it best: "In other words, autumn leaves were a tree's gray hair."

Zimmer says "were" because that notion has changed somewhat over the last decade. Nature, specifically natural selection, allows very little waste, and most traits are expressed through environmental pressures. Colorful fall leaves are no exception. New evidence has birthed two new theories, both provocative in their own right, both controversial among evolutionary biologists.

When biologists discovered that the red pigment anthocyanin was actually produced in autumn, and not merely unmasked by the decay of chlorophyll, this brought up an interesting question: Why would trees waste energy to produce a pigment that would only last for a few weeks, and decay with the falling leaf?

The "leaf signal" hypothesis (or "coevolution theory")seems to provide an answer. The bright yellows, oranges and reds are warning signs to potential aphid invaders, much like the bright colors of poisonous tree frogs.

Trees have poisons and sappy snares of their own for defending against insects, and the bright colors of autumn leaves warn insects not to lay eggs in the trees, unless they want their larvae in the spring meet an untimely demise.

There is preliminary evidence to support the theory. A study of 262 tree species revealed that the trees with the brightest foliage were only subject to more specialist aphids, illustrating that the generalist population of aphids were warded off by the colors. Trees that were duller or remained green until abscission were subject to the generalists. Another study at the University of Oslo studied birches specifically and also found supportive results.

But the leaf-signal hypothesis has its critics. Another group of scientists, pointing out that some trees with bright leaves are not attacked by insects, are suggesting that bright autumn leaves serve another purpose: a natural "sunblock."

As we stated before, trees are dismantling the structures supporting chlorophyll in order to store the nutrient from such structures, and allowing the pigment to decay. There is extra radiation hitting the leaf, with potentially nothing to soak it up. This is where the synthesized anthocyanins come in. They can still absorb light in the absence of chlorophyll, reducing the radiation damage to the leaf, and subsequently, the tree.

Both theories are relatively new, and the evidence is preliminary, but thought provoking. All the beauty of an autumn forest might just be an organic neon sign reading "Back Off."

Originally published here.


  1. Very interesting. My fiancee and I were just talking about leaves the other day, wondering about the pigments. What was in the sidebar mentioned ("...carotenoids; see sidebar")?

  2. I love this theory of trees warning insects to back off.

    Some of the more intense "red" trees certainly could offer that opinion!

  3. I should have cut that. The sidebar only appeared in the print version of the article...

    It was just a brief description of each of the pigments involved.

  4. The warning hpothesis is interesting. But can aphids see red. Wonder if any one has studied the visual system of aphids. Also usually warning signals require that the animl involved be able to learn, and I am not too sure of the mental cpacities of aphids.

  5. I'm surprised you don't mention the "foliar fruit flags" hypothesis. Trees, vines and shrubs with high-lipid, autumn-ripening fruits often change color early -- just when the main pulse of fall migrant songbirds is passing through. Of course, this wouldn't contradict the other hypotheses. Different trees may change color for different reasons. Yet another line of inquiry I'm sure you're aware of is the way some species of trees appear to use their fallen leaves allopathically -- if that's the right word -- to inhibit competition from the same or other species. The concentration of certain tannic compounds, such as anthocyanin, might help in this regard.

  6. This is fascinating. I used to study Botany full time and sometimes I miss it! So, thanks for a botany lesson.