Autumn display

The color of leaves

Autumn is many people’s favorite time of year, including mine. There is something exhilarating about the scent of decaying leaves, the crunch of leaf litter, and oh, the fiery colors. Every year I marvel at the kaleidagraph of scarlet reds, buttery yellows, and fiery oranges. Then there’s the leaves with fractal green explosions in the midst of the hot colors. Often I have read about how and why leaves turn these majestic hues, but I never remember the details. I guess that helps keep me easily amused. Perhaps writing about the magic of fall colors will help to cement the details in my brain.

Leaves are usually green. This is because they contain a pigment which selectively captures sunlight. The main pigment in leaves, chlorophyll, captures all the visible light except the green light which it reflects back towards our eyes. Leaves thus appear to us as green. So in fact, leaves are exactly not green, but that’s an argument I’ll never win.

One of the great mysteries of plant biology is why plants reflect green light since this is exactly the part of the solar spectrum with the most energy. Leaves are designed to collect light - why not collect the wavelengths with the most energy? Most theories suggest that leaves reflect green photons to protect themselves from heat scorching, but it’s not accepted as an established fact. While we’re on the topic of sunlight peaking in green, you may be thinking I’ve completely lost my rag as the sun is quite obviously yellow. Well, in fact, the sun is actually white - it contains all the visible wavelengths and when viewed from space it is white, certainly from a scientific perspective. But here on Earth, we humans perceive old Helios as yellow. This is because although the sun’s spectrum peaks in green, our eyes are more sensitive to yellow light.

Irritatingly, all the internet explanations I could find about why we perceive sunlight as yellow say this is because the atmosphere alters the distribution of light. But this doesn’t square with what I learned in grad school, nor with an examination of the solar spectrum. If we plot the energy of sunlight at a given wavelength versus that that wavelength, we get a sort of skewed bell curve, or more precisely a Planck curve. We can plot the solar spectrum as observed at the top of the atmosphere and at the surface of the Earth, as seen in figure 5.3 here - try clicking on the mini figure for a bigger version. The curve for the sun at the top of our atmosphere clearly shows that green light is indeed dominant in the sun’s spectrum above the atmosphere. But similarly, green light dominates the solar spectrum at the earth’s surface even after all the absorption in the atmosphere. Thus - the sun, if any color, would be green. And yet, internet article after internet article states that the sun appears as yellow because of this atmospheric absorption. Wondering if I was missing something, I reached out to my friend Phil Plait, aka The Bad Astronomer, and he concurred that it is our eyes which make us see the sun as yellow, not an atmospheric effect. The jury is still out on this issue of why we see the sun as yellow, but I don’t buy the atmospheric interference argument yet.

But back to leaves. Remember, we see leaves as green because the main pigment in leaves, chlorophyll, reflects green light. The absorbed red, yellow and blue photons are used to power photosynthesis. The process of photosynthesis, you may recall, converts water and carbon dioxide, in the presence of light, into oxygen and glucose. Most of the oxygen thus produced escapes into the atmosphere. The glucose produced in photosynthesis is transported around the plant to the stems, the trunk and the roots to power growth, maintenance and reproduction. We can think of glucose as a crystal of sunshine energy which has solved the transportation and storage issues associated with running on solar energy. When photosynthesis is going full tilt, and chlorophyll is abundant, leaves appear green. Much of the energy that we humans use, from plants, animals, and even fossil fuels, is tied to this humble non-green-light-capturing-molecule chlorophyll.

In the autumn, as the length of the day decreases, the amount of chlorophyll in the leaves begins to decay because there is less light to power photosynthesis. Temperature and other factors play a role in the timing as well, but day length is the main determinant in when the chlorophyll levels start to drop off. Other pigments which have been in the leaves all along, begin to shine through as the chlorophyll abundance decreases. This includes the visual emergence of carotenoids, which reflect yellow and orange light. Carotenoids are what give carrots, pumpkins, tomatoes, canaries, flamingos, salmon, lobster and even daffodils their delightful glowing yellow colors. Like chlorophyll, carotenoids are a light capturing pigment that aids in photosynthesis but the carotenoids also help to protect the plant from too much light. In some plants, carotenoids are so prevalent they give the leaves a yellowish hue even at the height of summer. So it is the reduced dominance of chlorophyll, as light levels fade, which allows the yellows to shine through in autumn. But what about the reds?

When the chlorophyll content in the leaves has dropped to about 1/2 of its usual levels, a third pigment starts to be produced within the leaves. This is the pigment anthocyanin and it reflects most red light. It aids in the changing sugar regime and works to break down sugars, rather than to produce them. Typically 10% of trees in temperate regions produce anthocyanin but in northern New England the fraction of anthocyanin producing trees climbs to 70%. These include maples, oaks, sweet gums, dogwoods, tupelos, and cherry trees. Anthocyanin is familiar to us in plums, cranberries, red applies, blueberries and strawberries and even young new leaves as they unfurl. I note its beauty often in fresh new rose leaves. Anthocyanin also gives that luscious purple hue to smoke bush, many heucheras, and other purple foliage plants - long live anthocyanins!

So we have three pigments that give us our beloved autumnal display of leaf colors. Chlorophyll, carotenoids and anthocyanins. But what makes a particular autumnal display dazzling? A variety of factors of course. To get the lovely reds and purples, we need loads of anthocyanins. These need bright and warm days so there is lots of sugar in the leaf for the anthocyanin to break down. But if the night time is warm, the sugars will be passed into the rest of the plant and little anthocyanin will be left in the leaves. So to get bright reds - the trees need warm sunny days and cold, but not freezing, nights. On the other hand, carotenoids abundance doesn’t change much from year. However, climatic factors can have other effects on the leaf display. A late spring or a severe summer drought can delay fall colors and a warm period in the autumn can reduce the display’s intensity. Likewise, an early killing frost or a wind storm can take down the leaves before they turn color or before they reach their peak. Here’s a fun map tool to see a prediction of when the fall foliage will be at its best across the US.

There is no consensus on why leaves go through this color changing process. One idea is that the changing color protects against various non-biotic factors - for instance anthocyanins may protect the leaf from the harmful effect of too much light when temperatures are low. Another theory is that the changing color of autumn leaves co-evolved with insect species. One known example is an aphid that avoids apple leaves that are red. The tree pays for the red leaves, having smaller fruit, but avoids aphid infestation. And bizarrely enough, the red avoiding aphids are more numerous than the red leaf eating ones. So the aphid and red leafed apple tree seem to have to come to a mutually beneficial arrangement. A final observation that may explain the evolutionary advantage of producing anthocyanins is that leaf litter which contains red leaves poisons other species’ seedlings thus reducing competition.

Not to leave you hanging, here’s a chore for the coming weeks. Start considering what you are going to do with all the glorious free compost that is about to fall out the sky. You can make a leaf compost pile. You can add fallen leaves to your regular compost pile in layers with green compost. Or you can mow leaves straight into your lawn or collect them with the mower to use as a mulch. Myself, I do a combination of the latter two. Through out the autumn I run the mower over the lawn and collect the cuttings. These I use to make a leaf mulch 2-3 inches thick on my flower beds. Then for the last 2 or 3 of the mowings of the year, which I do with the mower on it’s highest possible setting to leave the grass long, I remove the collection bag from the mower and allow the cut up leaves to be blown on the lawn. My lawn is no mono-culture and I don’t mind a bit of brown which quickly settles into the earth. This provides my lawn plants with nutrients and a bit of organic material to open up the air spaces in the soil allowing water and gases to move more freely.

It seems only fitting to close with Robert Frost’s In Hardwood Groves

The same leaves over and over again!

They fall from giving shade above

To make one texture of faded brown

And fit the earth like a leather glove.

Before the leaves can mount again

To fill the trees with another shade,

They must go down past things coming up.

They must go down into the dark decayed.

They must be pierced by flowers and put

Beneath the feet of dancing flowers.

However it is in some other world

I know that this is way in ours.